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                              _Geology of_
                              YELLOWSTONE


A review of the geologic processes and events responsible for the
spectacular natural wonders of the Yellowstone country, commemorating
the 100th anniversary of the oldest and largest of our national parks.

 For sale by the Superintendent of Documents, U.S. Government Printing
                                 Office
                   Washington, DC. 20402--Price $1.25
                         Stock Number 2401-1209

    [Illustration: "_* * * and behold! The whole country beyond was
    smoking with vapor from boiling springs, and burning with gases
    issuing from small craters, each of which was emitting a sharp,
    whistling sound. * * * The general face of the country was smooth
    and rolling, being a level plain, dotted with cone-shaped mounds. On
    the summit of these mounds were small craters from four to six feet
    in diameter. Interspersed among these on the level plain were larger
    craters, some of them four to six miles across. Out of these
    craters, issued blue flames and molten brimstone._"

    Description credited to Joseph Meek, 1829; quotation from page 40 of
    the book "The Yellowstone National Park" by Hiram Martin Chittenden
    (as edited and published by Richard A. Bartlett, University of
    Oklahoma Press, Norman, Oklahoma, 1964). Photograph is of Midway
    Geyser Basin.]

    [Illustration: "_Be it enacted by the Senate and House of
    Representatives of the United States of America in Congress
    assembled_, that the tract of land in the territories of Montana and
    Wyoming lying near the headwaters of the Yellowstone River is hereby
    reserved and withdrawn from settlement, occupancy, or sale under the
    laws of the United States, and dedicated and set apart as a public
    park or pleasuring ground for the benefit and enjoyment of the
    people * * *"

  Approved March 1, 1872--signed by:
  James G. Blaine, Speaker of the House
  Schuyler Colfax, Vice-President of the United States and President of
              the Senate
  Ulysses S. Grant, President of the United States]




                            _The Geologic Story of_
                                YELLOWSTONE
                               NATIONAL PARK


                          By William R. Keefer

                     _Illustrated by John R. Stacy_

  Based on a planned series of technical reports resulting from
  comprehensive geologic studies in Yellowstone National Park by the
  author and his colleagues, H. R. Blank, Jr., R. L. Christiansen, R. O.
  Fournier, J. D. Love, L. J. P. Muffler, J. D. Obradovich, K. L.
  Pierce, H. J. Prostka, G. M. Richmond, Meyer Rubin, E. T. Ruppel, H.
  W. Smedes, A. H. Truesdell, H. A. Waldrop, and D. E. White.

                    GEOLOGICAL SURVEY BULLETIN 1347


                UNITED STATES DEPARTMENT OF THE INTERIOR
                    ROGERS C. B. MORTON, _Secretary_

                           GEOLOGICAL SURVEY
                       V. E. McKelvey, _Director_

             Library of Congress catalog-card No. 79-169200

    [Illustration: U. S. DEPARTMENT OF THE INTERIOR · March 3, 1849]

                       First printing 1971 (1972)
                          Second printing 1972




                                Foreword


In the aftermath of the Civil War, the United States expanded the
exploration of her western frontiers to gain a measure of the vast lands
and natural resources in the region now occupied by our Rocky Mountain
States. As part of this effort, the Geological and Geographical Survey
of the Territories was organized within the Department of the Interior,
and staffed by a group of hardy, pioneering scientists under the
leadership of geologist F. V. Hayden. During the summer of 1871, these
men, accompanied by photographer William H. Jackson and artist Thomas
Moran, made a reconnaissance geological study of the legendary and
mysterious "Yellowstone Wonderland" in remote northwestern Wyoming
Territory. The scientific reports and illustrations prepared by Hayden
and his colleagues, supplementing the startling accounts that had been
published by members of the famous Washburn-Doane Expedition a year
earlier, erased all doubts that this unique land was eminently worthy of
being set aside "for the benefit and enjoyment of the people." By Act of
Congress on March 1, 1872, our first National Park was established.

During the past century, 50 million people have toured Yellowstone
National Park, marveling at its never-ending display of natural wonders.
No doubt many have paused to wonder about the origin of these unusual
and complex geological features--a question, needless to say, that has
intrigued and challenged scientists from the very first days of the
Hayden Survey. During the past decade a group of U. S. Geological Survey
scientists, in cooperation with the National Park Service and aided by
the interest of the National Aeronautics and Space Administration in
remote sensing of the geologic phenomena, has been probing the depths
and farthest corners of the Park seeking more of the answers. Some of
the results of this work, and those of earlier studies, are described in
this book to provide a better understanding and enjoyment of this great
National Park.

    [Illustration: V. E. McKelvey]

                        V. E. McKelvey, Director
                        U. S. Geological Survey




                                Contents


                                                                     Page
  Foreword                                                            VII
  Yellowstone country                                                   1
      A geological preview                                              4
  Geologic history of the Yellowstone region                            7
      The nature of the rocks reveals their origins                     7
      The oldest rocks                                                  7
      The deposits of the shifting seas                                13
      The first mountain-building episode                              19
      Volcanic activity                                                23
      A quiet period                                                   31
      More mountain building and deep erosion                          32
  Formation of the Yellowstone caldera                                 34
      The eruption                                                     38
      The collapse                                                     39
      The outpouring of lava                                           44
  Final sculpturing of the landscape                                   53
      Glaciation                                                       53
      Running water--canyons and waterfalls                            63
      Grand Canyon of the Yellowstone                                  64
  Hot-water and steam phenomena                                        71
      How a thermal system operates                                    71
      Hot-spring deposits and algae                                    75
      Hot springs and geysers                                          79
      Mudpots                                                          82
      Fumaroles                                                        85
      Thermal explosions                                               85
      Faulting and its control of thermal activity                     86
  Earthquakes                                                          87
  The Park and man                                                     89
  Acknowledgments                                                      90
  Selected additional reading                                          91




                                Figures


                                                                     Page
  Frontispiece Midway Geyser Basin                                     II
  Plate 1. Geologic map of Yellowstone National Park                   36
  Figure 1. Geographic map of Yellowstone National Park                 2
  2. Index map showing photograph localities                            3
  3. Skyline of the Gallatin Range in northwestern Yellowstone
          National Park                                                 6
  4. View north along the Yellowstone River and Hayden Valley toward
          the Washburn Range                                            6
  5. The rocks of Yellowstone National Park                             8
  6. The geologic time scale                                           10
  7. View downstream along the Lamar River and closeup view of
          Precambrian gneiss                                           12
  8. Positions of seaways and landmasses during the middle part of
          Permian time                                                 14
  9. Crowfoot Ridge in the southern Gallatin Range                     15
  10. Mount Everts                                                     16
  11. The faunal succession in sedimentary rocks                       17
  12. Beds of limestone along Pebble Creek and closeup views of
          outcrop and fossils                                          18
  13. Common kinds of geologic structures produced by deformation of
          the earth's crust                                            20
  14. Geologic structures in Yellowstone National Park                 22
  15. Intrusive and extrusive igneous rock bodies                      24
  16. The Absaroka volcanoes and their rocks                           25
  17. Massive beds of volcanic breccia of the Absaroka volcanic
          rocks and closeup view of outcrop                            26
  18. Massive layered breccias of the Absaroka volcanic rocks at
          Barronette Peak                                              28
  19. Giant petrified tree trunks in Yellowstone's fossil forest       29
  20. Closeup view of a specimen of intrusive igneous rock             30
  21. Bunsen Peak, a body of intrusive igneous rock                    31
  22. Outline of the Yellowstone caldera                               35
  23. Various stages in the development of the Yellowstone caldera     40
  24. Extent of the rhyolite welded tuffs that once covered
          Yellowstone National Park                                    42
  25. The Yellowstone Tuff at Golden Gate and closeup views of tuff
          specimens                                                    43
  26. Cross section through the Mount Washburn-Canyon area, showing
          relationships along north edge of the Yellowstone caldera    44
  27. View southeast across Yellowstone Lake toward the Absaroka
          Range                                                        45
  28. Radar image of lava flows in southwestern Yellowstone National
          Park                                                         46
  29. Obsidian Cliff                                                   47
  30. Thick rhyolite lava flow along Firehole River and closeup view
          of specimen                                                  48
  31. Brecciated lava flows                                            49
  32. Outcrop and closeup view of glassy rhyolite lava                 50
  33. Basalt flows at Tower and closeup views of outcrop and
          specimen                                                     52
  34. Giant glacial boulder of Precambrian gneiss at Inspiration
          Point                                                        54
  35. Glacial terrain along the Northeast Entrance Road                56
  36. Typical profiles of canyons cut by stream erosion and
          glaciation                                                   57
  37. Aerial oblique view of Electric Peak                             58
  38. Extent of ice in Yellowstone National Park during the maximum
          spreading of the Pinedale glaciers                           60
  39. Beds of sand, silt, and clay deposited in a glacially dammed
          lake in Hayden Valley                                        61
  40. Waterfalls in Yellowstone National Park                          62
  41. Grand Canyon and Lower Falls of the Yellowstone River            65
  42. Various stages in the development of the Grand Canyon of the
          Yellowstone                                                  66
  43. Common kinds of thermal features in Yellowstone National Park    70
  44. Norris Geyser Basin, showing solid floor of hot spring
          deposits                                                     72
  45. Diagram of a thermal system                                      72
  46. Infrared image of a portion of Upper Geyser Basin                73
  47. Mound of geyserite (sinter) at Castle Geyser                     75
  48. Terraces of travertine at Opal Springs and closeup of specimen   76
  49. Algal-colored terraces lining the west bank of the Firehole
          River                                                        78
  50. A geyser in action                                               80
  51. Rock rubble surrounding Seismic Geyser in Upper Geyser Basin     83
  52. Old Faithful in full eruption                                    84
  53. Mud volcano near Pocket Basin in Lower Geyser Basin              86
  54. Reactivation of a fault during the Hebgen Lake earthquake of
          August 17, 1959                                              88




                          Yellowstone Country


The vivid descriptions brought back from the Yellowstone country by the
early explorers and trappers (see frontispiece), whose reputations for
telling tall tales were widely accepted if not altogether deserved, fell
upon the disbelieving ears of the nation for more than half a century.
Yet the intriguing rumors persisted, and during the years 1869-71
several expeditions staffed partly by scientists and engineers
rediscovered this unique region atop the backbone of our nation. We now
know that the earliest visitors, even if prone to exaggerate, could not
do justice to the long-hidden secrets of Yellowstone, for none of them
saw all of the fascinating features that occur within this great
National Park.

By the time the modern-day visitor enters Yellowstone National Park
through any of its five entrances, he probably will have traveled
through many parts of the Rocky Mountains and grown somewhat accustomed
to the "lay of the land." But this will in no way lessen the exciting
impact of viewing the natural wonders of Yellowstone for the first time.
Immediate attention, of course, is still drawn to the remarkable array
of geysers, hot springs, and other thermal phenomena which in sheer
numbers and variety are unsurpassed throughout the world. But, as if
these were not enough of an attraction, nature has also provided an
incredible setting of sparkling rivers and lakes, thundering waterfalls
and cataracts, awesome canyons and gorges, and lofty glaciated mountain
peaks and extinct volcanoes. Truly this is a land apart, a spectacular
masterpiece of nature that fully deserves the accolade of "wonderland"
bestowed long ago by early explorers and trappers. (See figs. 1 and 2.)

    [Illustration: YELLOWSTONE NATIONAL PARK AREA, showing rivers,
    lakes, landforms, roads, towns, settlements, and major geyser basins
    (stippled). The Park embraces 3,472 square miles (2,221,770 acres),
    and its boundaries traverse a distance of nearly 300 miles.
    Yellowstone Lake, with an irregular shoreline of 110 miles and a
    surface area of 137 square miles, is one of the largest natural
    mountain lakes in the United States. (Fig. 1)]

    [Illustration: INDEX MAP showing localities where photographs (and
    one sketch, fig. 35) were taken to illustrate this bulletin. For
    photographs of distant views, arrows point in direction of view.
    Numbers refer to figure numbers in text. (Fig. 2)]

Beyond the first stirring impressions derived from the grandeur of the
vast Yellowstone wilderness and its myriad wildlife, assuredly shared by
people of all ages and from all walks of life, the various aspects of
the Park take on a very different meaning for different individuals. The
artist sees grand vistas to be painted, the naturalist delights in the
flower-laden meadows and the native habitats of many kinds of birds and
animals, the engineer visualizes the amount of energy stored in the
waterfalls and steaming geysers, and so on. To the geologist, in
particular, who studies rocks and fossils and all of the natural
processes involved in shaping the surface of the land, and to all those
who would share such interests, Yellowstone takes on a very special
meaning. For the Park is foremost a geological Park, created by an
extraordinary sequence of natural processes and events that have
combined to produce an immense outdoor laboratory for studies that have
contributed to a fuller knowledge and a better understanding of the
earth itself. The geological aspect of the Yellowstone country is
reflected by its very name, given long ago to the river that issues from
the great canyon of the "yellow rocks."[1] This report, borrowing from a
century of scientific study within and around the Park area, describes
the geological "how, why, and when" of this unique and fascinating
region.


                          A geological preview

Some 600,000 years ago the rumblings of an impending volcanic eruption
sounded ominously across the Yellowstone country. Suddenly, in a mighty
crescendo of deafening explosions, tremendous quantities of hot volcanic
ash and pumice spewed from giant cracks at the earth's surface. Towering
dust clouds blackened the sky, and vast sheets of volcanic debris spread
out rapidly across the countryside in all directions, covering thousands
of square miles in a matter of minutes with a blanket of utter
devastation. Abruptly, a great smoldering pit--a caldera 30 miles
across, 45 miles long, and several thousand feet deep--appeared in the
central Yellowstone region, the ground having fallen into the huge
underground cavern that was left by the earth-shaking eruptions. Lava
then began oozing from the cracks to fill the still-smoking caldera.

Thus, in one brief "moment" of geologic time there was launched that
incredible chain of events which led to the creation of many of the
natural wonders of Yellowstone National Park. Heat from the enormous
reservoir of molten rock which produced the massive eruption still
remains deep within the earth beneath Yellowstone, sustaining the
spectacular hot-water and steam phenomena for which the Park is so
justly famous. The formation of the caldera and the eruption of lavas
profoundly influenced the shape of the present-day landscape. Once a
land covered almost entirely by mountains, the part that
collapsed--nearly one-third of the total Park area--is now characterized
by low rolling plateaus formed from the thick lava flows that filled the
caldera (figs. 1 and 2; see fig. 22 for the outline of the Yellowstone
caldera). Moreover, the carving of the spectacular Grand Canyon of the
Yellowstone (fig. 41) and the fashioning of the large interior basin now
occupied by beautiful Yellowstone Lake (fig. 27) were closely related to
this mighty volcanic event.

North, east, and south of the central plateaus are extensive mountain
ranges and other highlands which provide much of the Park's scenic
beauty (figs. 3 and 4). Formed by many episodes of intense mountain
building and ancient volcanism, these uplands bear the lasting imprints
of a wide variety of geological activities that date back approximately
2.7 billion years. Indeed, as we study all the features of the
Yellowstone landscape, we find in them a most impressive and fascinating
story of that ageless conflict between the internal forces of nature
that raise the land through the upheaval of mountains and the eruption
of volcanoes, and the external forces of erosion that wear the land
down. It is this vast relentless interplay of giant forces that
determines the appearance of any given place upon the earth's surface.
And, in few other places around the globe can the processes of both
building up and tearing down the landscape be illustrated more
dramatically than in Yellowstone National Park.

    [Illustration: SKYLINE OF THE GALLATIN RANGE in northwestern
    Yellowstone National Park, as viewed from a point on the road
    between Canyon Village and Norris Junction. The range consists
    chiefly of Paleozoic and Mesozoic sedimentary rocks and Precambrian
    metamorphic rocks that were uplifted by folding and faulting of the
    earth's crust. The dark-gray rocks along the roadcut in the left
    foreground are rhyolite lava flows of the Solfatara Plateau. (Fig.
    3)]

    [Illustration: HAYDEN VALLEY. View north along the Yellowstone River
    and Hayden Valley toward the Washburn Range. Mount Washburn, part of
    an ancient Absaroka volcano, is the highest prominence (elevation,
    10,293 feet) on the skyline to the right, and Dunraven Pass is in
    the notch in the center of the skyline. The foot of the range marks
    the north edge of the Yellowstone caldera. Hayden Valley is cut in
    glacial lake sediments that overlie thick lava flows covering the
    caldera floor. (Fig. 4)]




               Geologic History of the Yellowstone Region


             The nature of the rocks reveals their origins

Geologists believe that "the present is the key to the past." After
observing lava erupting from a present-day volcano or limestone forming
in marine waters, we infer that similar types of ancient lavas or
ancient limestones formed in virtually the same ways. This kind of
reasoning is used to interpret the origins of all types of ancient
rocks, for all the known geological processes that form rocks seem to
have been operating since the earth's beginning.

Figure 5 shows the many different rock units that have been recognized
in Yellowstone National Park. Arranged in a vertical column according to
the geologic time intervals in which they formed, these rocks represent
a large part of total earth history (fig. 6). A generalized geologic map
(plate 1) shows the distribution of the various units (or groups of
closely related units) exposed at the surface throughout the Park area.
This map and figure 5 summarize much of the information that is
necessary to interpret the Park's geologic history--in essence, to
provide answers to these two important questions: What were the geologic
events that formed the rocks? When did these events occur?


                            The oldest rocks

If we were to walk backward in time at the rate of one century per step,
the first step would return us to 1872, the year that Yellowstone
National Park was established. But to return to the oldest recorded
event in its geologic history, we would have to walk (at 3 feet per
step) some 15,000 miles, or three-fifths of the way around the world!
Occurring far back in the antiquity of the Precambrian
Era--approximately 2.7 billion years ago according to radiometric dating
(fig. 6)--the oldest event resulted in rocks so crumpled and changed by
heat and pressure that their original character is obscure. These rocks,
having been transformed from still older ones, are called _metamorphic
rocks_. Considered to form part of the very foundation of the continent
itself, they are also commonly referred to as _basement rocks_.

    [Illustration: THE ROCKS of Yellowstone National Park, separated
    into individual units or formations and arranged according to their
    geologic ages (see fig. 6). A formation is a body of rock that
    contains certain identifying features (such as composition, color,
    and fossils) which set it apart from all other rock units. The
    identifying features of each formation provide valuable clues
    bearing on its origin. Most formations are given formal names, and
    usually each formation is thick and widespread enough to be
    recognized over broad areas. Some, however, change character from
    place to place, and different names may be used in different areas
    even though the rocks represent the same geologic time interval.
    (Fig. 5)]

    AGE, IN THOUSANDS OF YEARS        ROCK FORMATION OR UNIT

          40± to present              Stream sand and gravel
                                       Hot-spring deposits
            9 to 250±                    Glacial deposits
            60 to 600                    Plateau Rhyolite
               600                 Upper Unit, Yellowstone Tuff
           600 to 2,000           Rhyolite and basalt lava flows
              2,000                Lower Unit, Yellowstone Tuff
              2,000+              Rhyolite and basalt lava flows

                    KINDS OF ROCKS SHOWN IN COLUMNS
                        Sandstone or stream sand
           Conglomerate, glacial moraines, or stream gravels
                            Volcanic breccia
                                 Shale
                               Limestone
                                Dolomite
                               Lava flows
                              Welded tuff
                        Travertine or geyserite

             ROCK FORMATIONS               AGE, IN    PERIOD       ERA
                                          MILLIONS
                                          OF YEARS
  Northern part of     Southern part of
         park                 park

 Thick lava flows, welded tuffs, glacial            QUATERNARY _CENOZOIC_
   deposits, and hot-spring deposits
                                             2-3
 Pliocene, Miocene, and Oligocene rocks
   not known to be present
                                            37-38
 Absaroka volcanic   Absaroka volcanic              TERTIARY
   rocks               rocks (Eocene)
                                            53-54
 Volcanic and        Pinyon Conglomerate
   sedimentary         (Paleocene and
   rocks (largely      Cretaceous)
   eroded away
   before Absaroka
   volcanic rocks
   were deposited)
                                             65
 Landslide Creek Fm  Harebell Formation             CRETACEOUS _MESOZOIC_
 Everts Formation    (Eroded away before
                       Harebell was
                       deposited)
 Eagle Sandstone     Bacon Ridge
                       Sandstone
 Telegraph Creek Fm     "
 Cody Shale          Cody Shale
 Frontier Formation  Frontier Formation
 Mowry Shale         Mowry Shale
 Thermopolis Shale   Thermopolis Shale
 Kootenai Formation  Cloverly Formation
                                             136
 Morrison Formation  Morrison(?) Fm                 JURASSIC
 Swift Formation     Sundance Formation
 Rierdon Formation      "
 Sawtooth Formation  Gypsum Spring Fm
                                           190-195
 Woodside &          Chugwater Formation            TRIASSIC
   Thaynes(?)
   Formations
 Dinwoody Formation  Dinwoody Formation
                                             225
 Shedhorn Sandstone  Phosphoria Fm and              PERMIAN    _PALEOZOIC_
                       related rocks
                                             280
 Quadrant Sandstone  Tensleep Formation             PENNSYLVANIAN
 Amsden Formation    Amsden Formation
 Mission Canyon      Madison Limestone              MISSISSIPIAN
   Limestone
 Lodgepole Limestone    "
                                             345
 Three Forks Fm      Darby Formation                DEVONIAN
 Jefferson Formation    "
 Bighorn Dolomite    (Not exposed,                  ORDOVICIAN
                       except for
                       isolated outcrops
                       of some
                       formations in
                       Falls River area,
                       in southwestern
                       part of park)
                                             500
 Snowy Range Fm         "                           CAMBRIAN
 Pilgrim Limestone
 Park Shale
 Meagher Limestone
 Wolsey Shale
 Flathead Sandstone
                                             570
 Gneiss and Schist   (Not exposed)                  PRECAMBRIAN
                                            2,700


  A
    CENOZOIC
      50 M.Y.
    MESOZOIC
      200 M.Y.
    PALEOZOIC
      500 M.Y.
    PRECAMBRIAN
      4.5 B.Y.
  B
    Quaternary--Early man
      2
    Tertiary
      65
    Cretaceous
      140
    Jurassic
      190
    Triassic
      220
    Permian
      280
    Pennsylvanian
      310
    Mississippian
      340
    Devonian
      390
    Silurian
      440
    Ordovician
      500
    Cambrian
      575 First abundant fossils
    Precambrian
      2700  Oldest rocks in Yellowstone
    Beginning of the earth
      4,500 M.Y.
  C
        PRINCIPAL EVENTS
    Holocene
        Glaciation, canyon cutting, thermal activity, eruption of
              Plateau Rhyolite
    Pleistocene
        Eruption of Yellowstone Tuff and associated lava flow; collapse
              of Yellowstone caldera; normal faulting
  2
    Pliocene
        Regional uplift; large-scale normal faulting and uplift of
              mountain ranges; deep erosion
      12
    Miocene
        Moderate erosion; possibly some volcanic activity
      26
    Oligocene
           "
      37
    Eocene
        Eruption and deposition of Absaroka volcanic rocks
      54
    Paleocene
        Laramide Orogeny--folding, faulting, uplift and erosion of
              mountain ranges; deposition of sand and gravel in
              subsiding basins
      65
    Cretaceous
        Deposition of sediments in oceans and along beaches and river
              flood plains
      ...
    Cambrian
      570 M.Y.


    [Illustration: THE GEOLOGIC TIME SCALE--the "calendar" used by
    geologists in interpreting earth history. Column A, graduated in
    billions of years (B.Y.) and subdivided into the four major geologic
    eras (Precambrian, for example), represents the time elapsed since
    the beginning of the earth, which is believed to have been about 4.5
    billion years ago. Column B is an expansion of part of the time
    scale in millions of years (M.Y.), to show the subdivisions
    (periods--Cambrian, for example) of the Paleozoic, Mesozoic, and
    Cenozoic Eras; column C is a further expansion to show particularly
    the subdivisions (epochs--Paleocene, for example) of the Tertiary
    and Quaternary Periods. The principal events in the geologic history
    of Yellowstone National Park are listed to the right of column C,
    opposite the time intervals in which they occurred. The ages, in
    years, are based on radiometric dating. Many rocks contain
    radioactive elements which begin to decay at a very slow but
    measurable rate as soon as the parent rock is formed. The most
    common radioactive elements are uranium, rubidium, and potassium,
    and their decay ("daughter") products are lead, strontium, and
    argon, respectively. By measuring both the amount of a given
    daughter product and the amount of the original radioactive element
    still remaining in the parent rock, and then relating these
    measurements to their known rate of radioactive decay, the age of
    the rock in actual numbers of years can be calculated. The decay of
    radioactive carbon (carbon-14) to nitrogen is especially useful for
    dating rocks less than 40,000 years old. (Fig. 6)]

Gneiss, a coarsely banded rock (fig. 7), and schist, a finely banded
rock, are the most common kinds of metamorphic rocks in Yellowstone.
Originally, the gneiss probably was granite, and the schist was a shale
or sandstone. Outcrops of the gneisses and schists occur only in the
northern part of the Park (pl. 1), where they form the central cores of
some mountain ranges such as the Gallatin Range (fig. 3). They also lie
buried beneath younger rocks in many other areas of the Park.

From the time of the metamorphic event, when the gneisses and schists
were formed, until the deposition of sediments of the Cambrian Period
(figs. 5 and 6), there is virtually no record. It is reasonably certain,
however, that several times during this 2.1-billion-year interval the
region was intensely squeezed and uplifted into high mountains and then
deeply eroded. By the end of Precambrian time, approximately 570 million
years ago, the ancient Yellowstone landscape had been reduced by erosion
to a flat, stark, almost featureless plain, which was soon to be flooded
by a shallow sea encroaching from the west. This very old surface is now
partly exposed in some places across the Buffalo Plateau, at the north
edge of the Park (fig. 1).

    [Illustration: LAMAR RIVER. View downstream (west) along the Lamar
    River in Lamar Canyon. The rocks along the river banks are coarsely
    banded Precambrian gneisses more than 2.5 billion years old, some of
    the oldest rocks in Yellowstone National Park. (Fig. 7)]

    [Illustration: Closeup views show coarse banding and texture of the
    gneiss; minerals include quartz, feldspar, and biotite (black
    mica).]


                   The deposits of the shifting seas

From the appearance of the rugged, mountainous terrain of Yellowstone
National Park, it is difficult to visualize a time when this region lay
close to sea level, at times even below sea level. Yet the evidence is
clear that from the Cambrian Period to the latter part of the Cretaceous
Period, a span of about 500 million years, vast stretches of western
lands were flooded repeatedly by broad shallow seas that often reached
from Canada to Mexico (fig. 8). During these great floodings, widespread
horizontal beds of sand, silt, clay, limy mud, and other sediments were
deposited on the ocean floors, along the adjoining beaches and wide
tidal flats, and across the broad flood plains of large rivers that
emptied into the seas. All of these ancient sediments have now hardened
into compact well-layered sandstones, shales, and limestones (figs. 9
and 10). These _sedimentary rocks_ have been divided into 25 or more
distinct formations in the Yellowstone region (fig. 5), where they
locally attain a combined thickness of more than 10,000 feet.

The first Paleozoic sea to reach the Yellowstone region, some 550
million years ago, brought with it the earliest abundant signs of life
on earth. Small hard-shelled animals that lived mainly on the shallow
sea bottom are now preserved as fossils in rocks deposited during the
Cambrian Period. Many of these animals were _trilobites_, long-extinct
organisms resembling today's crabs and spiders. Each younger set of
rocks or formations contains a different group of dominant fossils, each
diagnostic of that period of geologic time in which they lived (fig.
11).

    [Illustration: MIDDLE PERMIAN SEAS. Distribution of sea (blue) and
    land (red) during the middle part of Permian time (approximately 250
    million years ago). Only a part of the Yellowstone National Park
    area (black) was flooded during this period. (Fig. 8)]

Fossils indicate the kind of environment in which the animals lived
(fig. 12). Some species thrived in the open oceans; others thrived only
along the beaches and in nearby lagoons. Still others, such as the
incredibly large dinosaurs of the Jurassic and Cretaceous Periods, could
survive only on the land or in swamps. From studies of the fossils and
of the physical characteristics of the rocks in which they are now
found, the shoreline patterns of the shifting seas can be determined.
Studies show that the seas advanced and retreated across the Yellowstone
Park region at least a dozen times during the Paleozoic and Mesozoic
Eras.

Toward the end of the Mesozoic Era (in the latter part of the Cretaceous
Period), the metamorphic basement rocks of Yellowstone lay covered by
the vast blanket of flat-lying sediments. Today, these sedimentary rocks
are exposed along the Snake River and its tributaries in the
south-central part of the Park, over much of the Gallatin Range in the
northwest corner, and at several places in the north-central and
northeastern parts (pl. 1). Elsewhere, either they are hidden from view
beneath volcanic debris--ash and lava--that later buried them, or they
have been removed by erosion. But wherever exposed, the original
horizontal layers of sedimentary rocks have been severely twisted and
broken by later mountain-building movements.

    [Illustration: CROWFOOT RIDGE in the southern Gallatin Range, as
    viewed from the road along the Gallatin River near the northwest
    corner of Yellowstone National Park. The rocks, chiefly Paleozoic
    limestone, sandstone, and shale, were deposited in broad shallow
    seas that covered all of the Yellowstone region several hundred
    million years ago. The original layers were horizontal, but they
    have since been tilted and broken by giant mountain-building forces
    originating deep within the earth. (Fig. 9)]

    [Illustration: MOUNT EVERTS, as viewed toward the northeast from the
    road south of Mammoth Hot Springs. The mountain, about 1,500 feet
    high above the plain, is formed by gently tilted sedimentary rocks
    of Cretaceous age, chiefly sandstone and shale of the Frontier,
    Cody, and Everts Formations (fig. 5). The conspicuous rimrock at the
    top of the mountain to the right is composed of the Yellowstone
    Tuff. When the tuff was deposited (by explosive eruptions from the
    south), there was no valley along the edge of the mountain. (Fig.
    10)]

    [Illustration: FAUNAL SUCCESSION in sedimentary rocks. The different
    animals are now preserved as fossils, which are diagnostic of the
    period in which the animals lived. (Fig. 11)]

                                                 Man
      CENOZOIC        QUATERNARY and TERTIARY    Mammals
                      CRETACEOUS
      MESOZOIC        JURASSIC                   Dinosaurs
                      TRIASSIC
                      PERMIAN                    Reptiles
                      PENNSYLVANIAN              Amphibians
                      MISSISSIPPIAN
      PALEOZOIC       DEVONIAN                   Fishes
                      SILURIAN                   Sea scorpions
                      ORDOVICIAN                 Nautiloids
                      CAMBRIAN                   Trilobytes
      PRE-CAMBRIAN                               Soft-bodied
                                                 creatures

    [Illustration: LIMESTONE OF MISSISSIPPIAN AGE along Pebble Creek at
    the Pebble Creek campground, northeastern Yellowstone National Park.
    (Fig. 12)]

    [Illustration: Closeup A shows one of the highly fossiliferous
    layers within the limestone.]

    [Illustration: Closeup B shows some of the fossils and their casts.
    Most of the fossils are of a variety of shelled sea animals
    (brachiopods) that lived on the ocean floors approximately 300
    million years ago.]


                  The first mountain-building episode

Near the close of the Mesozoic Era the earth was subjected to a series
of intense crustal disturbances that geologists call the Laramide
orogeny (orogeny means mountain-building). The origin and nature of the
forces that bent and cracked the crust are unknown, but current theories
being developed about sea-floor spreading and continental drift may shed
light on this major upheaval that began about 75 million years ago. A
significant effect of the Laramide orogeny was the uplift and contortion
of many of the mountain ranges within what we today call the Rocky
Mountains.

At the onset of the crustal disturbance, the gently rolling landscape of
the Yellowstone region began to warp and flex into large upfolds
(_anticlines_) and downfolds (_synclines_) (fig. 13). Gradually the
mountain-building pressures increased, finally reaching such magnitude
that the limbs of the folds could bend and stretch no further;
thereupon, the rock layers broke and were shoved over one another along
extensive _reverse faults_. The severely crumpled rocks within the Park
area can now be seen only along the north edge and in the south-central
part along the Snake River. In both places, the folds and faults are
especially well displayed by the layered Paleozoic and Mesozoic
sedimentary formations (fig. 9).

One of the most prominent Laramide structural features is a large
anticline in the north-central and northeastern parts of the Park (fig.
14, section B-B'); the road from Mammoth to the Northeast Entrance
crosses much of this feature (pl. 1). Although originally forming a high
mountain mass, the anticline has been eroded so extensively that it no
longer appears mountainous (fig. 18). It displays a broad core of
Precambrian gneisses and schists and is bounded along its southwest
margin by a large reverse fault. Along the fault, the ancient gneisses
and schists have been shoved over rocks as young as Late Cretaceous, a
movement amounting to 10,000 feet or more. The Cretaceous rocks are
those that are now exposed at Mount Everts (fig. 10).

    [Illustration: COMMON KINDS OF GEOLOGIC STRUCTURES produced by
    deformation of the earth's crust. An original horizontal rock layer
    may be upfolded into anticlines, downfolded into synclines, and
    broken by either reverse or normal faults. A fault is a fracture or
    a zone of fractures within the earth's crust along which movement
    has taken place. A reverse fault is one generally produced by
    compression (squeezing together), and the hanging-wall block has
    moved up with respect to the footwall block. A normal fault is one
    generally produced by tension (pulling apart), and the hanging-wall
    block has moved down with respect to the footwall block. All these
    kinds of structures are present in Yellowstone National Park. (Fig.
    13)]

  HORIZONTAL (undeformed)
  REVERSE FAULT (compressional)
    Hanging wall
    Footwall
    Forces
  ANTICLINE (upfold)
    Crest
    Limb
  NORMAL FAULT (tensional)
    Footwall
    Hanging wall
    Forces
  SYNCLINE (downfold)
    Limb
    Trough


During the Laramide orogeny, many folds and faults formed in the
northwestern part of the Park, in the area now occupied by the Gallatin
Range (fig. 14, section A-A'). In south-central Yellowstone, the
Paleozoic and Mesozoic sedimentary rocks were tightly folded into three
anticlines separated from one another by synclines and faults (fig. 14,
section C-C'). Movement along one reverse fault in this area was locally
more than 10,000 feet.

As the lands were uplifted and contorted, they came under vigorous
attack by the ever-present agents of erosion. Tremendous quantities of
rock were stripped from the highlands, and the debris was carried by
streams into the adjacent lowland basins and deposited mostly as sand
and gravel. As the highlands continued to rise, the basins continued to
sink, and in a short period of time great thicknesses of basin-fill
sediments accumulated locally. One such deposit, the Harebell Formation
of latest Cretaceous age in south-central Yellowstone (fig. 5), is more
than 8,000 feet thick.

Other similar anticlines, synclines, and reverse faults no doubt extend
far into the interior of Yellowstone National Park, and perhaps entirely
across it in places, but they lie buried beneath a thick capping of
volcanic rocks. Nevertheless, it seems safe to conclude that none of the
Park area escaped the effects of the great forces of the Laramide
orogeny. These forces, regardless of how they originated deep within the
earth, seem to have been compressional (fig. 13), pushing the upper
layers of the earth's crust from the east and northeast toward the west
and southwest. This interpretation is based on the style of the
structural features just described, which shows that the steep limbs of
folds, as well as the direction of movements along reverse faults, point
toward the west or southwest (fig. 14).

By early Eocene time, about 20 million years after they had begun, the
deformational forces relaxed. But the effects of the giant earth
movements were to last for a very long time. Crustal disturbances of
such magnitude commonly produce conditions deep within the earth which,
in places, gives rise to intense volcanic activity; one such place was
Yellowstone.

    [Illustration: CROSS SECTIONS SHOWING GEOLOGIC STRUCTURES in
    Yellowstone National Park. These illustrate the possible rock
    relationships that might be seen along the faces of vertical slices
    of the earth's crust, if it could be cut and pulled apart (much like
    slicing a cake and looking at the different layers). The locations
    of the sections are shown on the geologic map, plate 1. Reverse
    faults and most folds originated during the Larimide orogeny, and
    normal faults originated chiefly during Pliocene and later times.
    The arrows indicate the relative movements of fault blocks. Geologic
    symbols: Qs, Quaternary superficial deposits; Qb, Quaternary basalt
    flows; Qy, Quaternary Yellowstone tuff; Tav, Tertiary Absaroka
    volcanic rocks; Mzr, Mesozoic sedimentary rocks; Pzr, Paleozoic
    sedimentary rocks; pCr, Precambrian metamorphic ("basement") rocks.
    (Based partly on information supplied by E. T. Ruppel and J. D.
    Love.) (Fig. 14)]


                           Volcanic activity

In early Eocene time, between 55 and 50 million years ago, several large
volcanoes erupted in and near Yellowstone National Park. This volcanic
activity resulted in the accumulation of the vast pile of Absaroka
volcanic rocks (fig. 5) which now makes up most of the Absaroka and
Washburn Ranges and part of the Gallatin Range, and which covers several
other smaller areas in the Park (pl. 1).

What special geologic conditions would cause these spectacular eruptions
of molten rock at the earth's surface? Measurements taken in deep mines
and oil wells show that the normal increase in the earth's temperature
with depth is about 1°F per 100 feet. This heat is generated by the
decay of radioactive elements--chiefly uranium, thorium, and
potassium--which are present in at least small amounts in virtually all
rocks of the earth's crust. Ordinarily, enough heat is conducted to the
earth's surface so that the deeply buried rocks do not become hot enough
to melt. In some places, however, the heat is not carried off fast
enough, and the temperature rises slowly toward the melting point of the
rock. Such hot spots may develop (1) because the rocks in those places
contain more than an average amount of radioactive elements; (2) because
hotter material moves upward from still deeper levels in the earth; or
(3) because drastic changes in pressure are brought about by the
alternate squeezing and relaxing of mountain-building forces, which in
turn substantially affect the melting point of the rocks. Whatever the
cause, the eventual result is the accumulation of a huge body of molten
rock, called _magma_, enclosed in a deep underground chamber.

Magma, being a mixture of hot liquids and gases that is lighter in
weight than the solid rocks surrounding it, tends to rise toward the
earth's surface. Forcing its way upward, some of the molten material
solidifies before reaching the surface and forms bodies of various kinds
of _intrusive igneous rocks_ (fig. 15). Some of the magma, however,
reaches the surface and either pours out as lava or is blown out
explosively as rock fragments, ash, and pumice to form _extrusive
igneous rocks_.

    [Illustration: INTRUSIVE AND EXTRUSIVE IGNEOUS ROCK BODIES.
    Extrusive rocks solidified above ground, and intrusive rocks
    solidified below ground. All features shown occur in Yellowstone
    National Park. Extrusive rocks are the predominant rock type seen
    along the Park roads, and the table lists the three principal kinds
    that are present. (Fig. 15)]

  Rock name   Principal rock-forming                 Color
                     minerals

  Rhyolite   Quartz,[a] feldspar[b]    Light to medium shades of gray
             (sanidine).               and brown.
  Andesite   Feldspar[b]               Medium to fairly dark shades of
             (plagioclase),            brown, red, purple, and gray.
             pyroxene[c] (augite).
  Basalt     Feldspar (plagioclase),   Nearly black.
             pyroxene, olivine,[c]
             magnetite.[d]


[a]Clear to light-colored silicon dioxide.

[b]Light-colored aluminum silicate minerals.

[c]Dark-colored iron and magnesium silicate minerals.

[d]Very dark colored iron oxide mineral.


The magmas which formed the Absaroka volcanoes erupted mainly through
large central vents (fig. 16). Most of the eruptions were fairly quiet,
with the molten rock welling up to the surface and cascading down the
sides of the volcanoes chiefly as viscous lava flows and breccias. Rain,
seeping into these porous rocks, caused huge landslides of mud and
broken rock to stream down the mountainsides. Hence, many of the rocks
seen today are volcanic breccias--jumbled but crudely layered deposits
of large and small angular blocks embedded in a sandy matrix, much like
man-made concrete except that the rock fragments are considerably
coarser (fig. 17). Viewed from a distance, however, most of the breccia
deposits have a distinct layered appearance (fig. 18). The predominant
extrusive igneous rock in the Absaroka volcanic sequence is andesite,
but basalt also occurs in places (fig. 15).

    [Illustration: ABSAROKA VOLCANOES and their rocks. Lava (mostly
    andesite) poured from central vents and formed volcanoes, some steep
    sided and others broad and relatively flat. As the lava spilled out,
    much of it quickly solidified, broke up into large angular blocks
    (breccia), and then either tumbled down the slopes of the volcanoes
    as individual boulders or slid down in mudflows and landslides. Some
    of the material was also explosively blown out as rock bombs,
    cinders, and ash. The more fluid lava (mostly basalt), on the other
    hand, flowed quietly down the volcanic slopes and onto the
    surrounding lowlands. The rocks near the volcanic centers therefore
    include thick crudely layered coarse breccias, thin fine ash and
    dust falls, and thin to thick lava flows. The volcanoes were
    repeatedly attacked by erosion, and the eroded material was
    redeposited by streams and mudflows in widespread layers of volcanic
    conglomerate and sandstone across the flat-floored valleys and
    plains between the volcanoes. Forests, which grew luxuriantly in
    these lowland areas, were repeatedly buried by volcanic eruptions
    and are now preserved (see inset) as the fossil forests of
    Yellowstone. (Based on information supplied by H. W. Smedes and H.
    J. Prostka.) (Fig. 16)]

                  Chiefly lava flows of shield volcano
      Chiefly volcanic sandstone and conglomerate of lowland areas
   Chiefly volcanic breccias and thin lava flows of cone-type volcano

At times the Absaroka volcanic eruptions were violently explosive,
showering the countryside with rock bombs, cinders, and ash. The finer
debris that reached the lower slopes of the volcanoes was reworked and
carried by streams into the intervening valleys, where it was deposited
as sand and gravel (fig. 16). Eventually the entire Yellowstone region
was choked with volcanic debris, the material from one volcano mixing
with that from neighboring volcanoes. Even the mountain masses uplifted
during the preceding Laramide orogeny were covered by the vast
accumulation (fig. 18).

    [Illustration: MASSIVE BEDS OF BRECCIA of the Absaroka volcanic
    rocks along the road north of Dunraven Pass. This breccia formed
    part of a steep-sided volcanic cone, of which Mount Washburn is a
    remnant. (Fig. 17)]

    [Illustration: Closeup view shows very coarse character of the
    breccia, with large rock fragments imbedded in fine ash, dust, and
    sand. Nearly all the rocks are of andesitic composition, consisting
    chiefly of feldspar and pyroxene. Most common colors are medium to
    fairly dark shades of brown, red, purple, and gray.]

Absaroka volcanism, however, was not a simple, continuous process--the
eruptions were intermittent, the many volcanoes were not always active
at the same time, and between eruptions there were long periods of
quiescence during which the erupted material was deeply eroded. The
repetitive nature of the eruptions is best illustrated by the famous
fossil forests of Yellowstone. Here is striking evidence that enough
time elapsed between eruptions for widespread forests to become
established on the lower slopes of the volcanoes and in the broad
valleys between them. Judged from the great size of some of the
now-petrified logs (fig. 19), several hundreds of years must have passed
before another volcanic outburst smothered the forest. Many different
forest layers have been recognized in the Specimen Ridge area as well as
in several other places throughout the Park.

As the Absaroka magma rose from deep underground, some of it squirted,
like toothpaste, into the layered Paleozoic and Mesozoic sedimentary
rocks through which it passed. These relatively small masses of molten
rock material slowly cooled and crystallized to form intrusive igneous
rocks such as diorite (fig. 20). The resulting intrusive bodies, called
_sills_, _dikes_, _stocks_, and _laccoliths_, depending on their form,
are most abundant in the Gallatin Range and in the vicinity of the East
Entrance (pl. 1). At the conclusion of volcanic activity, the last of
the rising magma solidified in the main conduits to form slender,
somewhat cylindrical bodies of rock called _volcanic necks_ that
probably conform closely to the shape of the original conduits. The
circular intrusive rock body at Bunsen Peak (fig. 21), now exposed to
view because erosion has stripped away the lava and volcanic breccia
that once completely buried it, represents either a volcanic neck or a
small stock that solidified directly beneath a volcano.

    [Illustration: MASSIVELY LAYERED BRECCIAS, conglomerates, and
    sandstones of the Absaroka volcanic sequence at Barronette Peak, as
    viewed from the road near the Northeast Entrance; the ridge is 3,000
    feet high. These rocks, deposited as part of an alluvial plain
    between volcanoes, once filled the Yellowstone region to a level
    higher than the top of Barronette Peak, but erosion since late
    Tertiary time has stripped the volcanics from much of the Park area.
    The volcanic rocks (Eocene in age, fig. 5) rest directly on
    Paleozoic sedimentary rocks along the line indicated. During the
    Laramide orogeny, in Late Cretaceous and early Tertiary times, the
    region was folded and uplifted into mountains. Thousands of feet of
    Mesozoic and Paleozoic sedimentary rocks were then eroded off the
    rising mountains before the Absaroka volcanic rocks were deposited,
    (Fig. 18)]

    [Illustration: GIANT PETRIFIED TREE TRUNKS in Yellowstone's fossil
    forest. The enclosing rocks, part of the Absaroka volcanic sequence
    that forms Specimen Ridge, are approximately 50 million years old.
    Many of the tree trunks are still upright, having been smothered and
    buried in their original positions by breccia, ash, and dust from
    nearby volcanoes. It is evident that more than one "forest" is
    represented in this view. Prof. Erling Dorf, of Princeton
    University, counted a total of 27 different forest layers in the
    rocks now exposed at Specimen Ridge. He also determined that the
    most common kinds of trees were sycamore, walnut, magnolia,
    chestnut, oak, redwood, maple, and dogwood. The nearest living
    relatives of many of these trees are now found in the warm temperate
    to subtropical forests of the southeastern and southern United
    States. (National Park Service photograph.) (Fig. 19)]

Mount Washburn is the north half of one of the ancient Absaroka
volcanoes (fig. 26), and many of the rocks and other features related to
this volcano, which characterized this great period of volcanism, can be
seen along the road between Canyon Village and Tower. In roadcuts just
south of Dunraven Pass several thin igneous dikes cut through volcanic
breccias. These dikes radiate outward from the nearby central core of
the volcano, which lies east of the highway in the vicinity of Washburn
Hot Springs. From Dunraven Pass northward for 2-3 miles, the road is
lined with lava flows and very coarse breccias that accumulated close to
the volcanic neck (fig. 17). Farther north toward Tower Falls, breccias
and conglomerates predominate, but the average size of individual rock
fragments decreases gradually northward away from the center of
eruption. Beds of sandstone then begin to appear in the sequence, having
been deposited mainly by streams that drained the north slope of the
volcano.

    [Illustration: IGNEOUS ROCK. Closeup view of intrusive igneous rock
    (diorite) from the Electric Peak stock in the Gallatin Range;
    Electric Peak is pictured in figure 37. The rock is composed chiefly
    of light-colored quartz and feldspar and dark-colored iron and
    magnesium silicate minerals. (Fig. 20)]

At the end of Absaroka volcanism, approximately 40 million years ago
(fig. 6), all of Yellowstone lay buried beneath several thousand feet of
lavas, breccias, and ash (fig. 18). The landscape must have appeared as
a gently rolling plateau, drained by sluggish, meandering streams and
dotted here and there by volcanoes still rising above the general level
of the ground. This plateau surface, however, probably stood at a
maximum of only a few thousand feet above sea level, for animals and
plants now found as fossils in the Absaroka volcanic rocks indicate that
warm-temperature to even subtropical climates existed during the
volcanic period (fig. 19).

    [Illustration: BUNSEN PEAK, a roughly circular body of intrusive
    igneous rock, is the eroded remnant of either the "neck" of an
    Absaroka volcano or a small stock that solidified directly beneath a
    volcano. The peak rises approximately 1,200 feet above a flat plain
    (foreground) that is covered by flows of younger basalt. The
    Yellowstone Tuff, formed by volcanic ash and dust exploded from the
    central Yellowstone region to the south, underlies the basalt. When
    erupted, the volcanic debris (as well as the basalt lava) flowed
    around this high-standing peak. (Fig. 21)]


                             A quiet period

Little is known in detail of the geologic events in Yellowstone during
Oligocene and Miocene times. Rocks of these ages have not been
recognized within the Park; if ever deposited there, they have since
been removed by erosion or buried by younger volcanic rocks. Thus, we
can only speculate as to what events took place during this
25-million-year period. No doubt the broad Absaroka volcanic plateau was
eroded, but not deeply, because the topographic relief and stream
gradients of the region remained low. There are also hints that some
volcanic activity took place, for volcanic rocks representing parts of
this time interval occur south of the Park, and some of these rocks may
have originated within the Park area. Little transpired, however, to
significantly alter the existing geological makeup of the Park; it was
indeed a quiet time, particularly when compared with the extremely
dynamic periods which immediately preceded and followed it.


                More mountain building and deep erosion

Many features of the present-day landscape of Yellowstone stem from
Pliocene time, about 10 million years ago. At that time the entire
region--in fact, much of the Rocky Mountain chain--was being uplifted by
giant earth movements to heights several thousand feet above its
previous level. This episode of regional uplift accounts in large
measure for the present high average elevation of the Yellowstone
country. Although the precise cause of the uplift is unknown, the uplift
assuredly reflects profound changes that were taking place deep within
or beneath the earth's crust.

Great tensional forces, operating during Pliocene time, pulled the
Yellowstone region apart and partially broke it into large steep-sided
blocks bounded by _normal faults_ (fig. 13). Some blocks sank while
others rose, commonly on the order of several thousand feet. The
Gallatin Range, in the northwest corner of the Park, for example, was
lifted as a rectangular mountain block along north-trending 20-mile-long
normal faults that border it on each side (fig. 14, section A-A'; pl.
1). In the south-central part of the Park, the differential movements
between several adjacent fault blocks totaled more than 15,000 feet
(fig. 14, section C-C'). Farther south, the Teton Range moved up and the
floor of Jackson Hole moved down along a normal-fault zone that
stretches along the east foot of the range. An enormous offset of about
30,000 feet developed between the two crustal blocks, accounting in
large part for the now incredibly steep and rugged east face of the
Teton Range.

The pronounced rise in elevation of the general ground surface and the
chopping of the region into many mountainous fault blocks caused a
profound increase in the rate of erosion. Once-sluggish streams turned
into vigorous, fast-moving rivers that began to cut deeply into the
Absaroka volcanic plateau. Huge quantities of rock debris were stripped
off and carried out of the area, and at the end of the Pliocene, the
Yellowstone region must have been very highly dissected mountains and
table- and canyon-lands. Much of the landscape may have resembled the
rugged terrain now seen in the Absaroka Range along the east side of the
Park. These mountains (fig. 27), and the Washburn Range in the interior
of the Park (fig. 4), today represent but small remnants of the vast
pile of Absaroka volcanic rocks that once covered all of Yellowstone and
the surrounding regions.

    [Illustration: Moose]




                  Formation of the Yellowstone Caldera


We have now approached that point in geologic time--the beginning of the
Quaternary Period between 2 million and 3 million years ago--when the
stage was set for the triggering of those all-important events that
culminated in the development of the 1,000-square-mile Yellowstone
caldera and ultimately gave rise to the world-renowned hot-water and
steam phenomena. Involved were some of the earth's biggest explosions,
which have had no apparent counterpart in recorded human history. A few
extremely explosive eruptions have occurred historically, however, such
as the one that took place on the uninhabited island of Krakatoa,
between Java and Sumatra in the East Indies, during the latter part of
August 1883. For several days this island had been shaken by a series of
violent explosions. Then, on August 27, it was ripped by an explosion
that was heard as far away as Australia, a distance of about 3,000
miles. Fifty-mile-high dust clouds became windborne around the globe,
producing colorful sunrises and sunsets in all parts of the world for
several years. When the air around Krakatoa finally cleared, it was
found that two-thirds of the island, some 12 square miles, had collapsed
and vanished into the sea. Though the Krakatoa eruption resulted in a
caldera that is only a small fraction of the size of the one in
Yellowstone, it provides a mental picture to help us understand what has
been discovered about the great volcanic holocaust in Yellowstone
National Park that was described briefly in an early part of this
report.

    [Illustration: OUTLINE OF THE YELLOWSTONE CALDERA produced by the
    enormous volcanic eruption 600,000 years ago. The two oval-shaped
    areas are resurgent domes that arched the caldera floor over twin
    magma chambers after the eruption. The margins of the resurgent
    domes are surrounded by ring fracture zones which extend outward
    toward the edge of the caldera. Numerous fractures in these zones
    provided escape routes through which lavas of the Plateau Rhyolite
    oozed to the surface and poured out across the caldera floor. Today
    these zones also provide underground channels for the circulation of
    hot water in the Yellowstone thermal system. The area outlined by
    the dotted line shows the smaller and younger inner caldera now
    occupied by the West Thumb of Yellowstone Lake. (Based on
    information supplied by R. L. Christiansen and H. R. Blank, Jr.; the
    existence of a caldera in Yellowstone National Park was first
    recognized by F. R. Boyd in the late 1950's.) (Fig. 22)]

[Illustration: GEOLOGIC MAP OF YELLOWSTONE NATIONAL PARK (PLATE 1)

    Generalized from detailed mapping by R. L. Christiansen and H. R.
    Blank, Jr. (Quaternary volcanic rocks); H. W. Smedes and H. J.
    Prostka (Absaroka volcanic rocks); E. T. Ruppel (sedimentary and
    metamorphic rocks, northern part of park); and J. D. Love and W. R.
    Keefer (sedimentary rocks, southern part of park).]

    [Illustration: EXPLANATION]

  CENOZOIC
    QUATERNARY
      Stream sand and gravel, glacial and landslide debris, hot-spring
            deposits, and lake beds
      Basalt flows
      Plateau Rhyolite
      Yellowstone Tuff and related lava flows
    TERTIARY
      Absaroka volcanic rocks
      Intrusive igneous rocks
      Tertiary formations
  Mesozoic formations
  Paleozoic formations
  Precambrian gneiss and schist
  Contact


  FAULT AND FOLD SYMBOLS
      Dotted where concealed beneath younger unfaulted rocks
    Reverse fault
      _Sawteeth on side that moved up_
    Normal fault
      _Symbol on side that moved down_
    Reverse fault, along which there was later normal-fault movement
    Anticlinal axis
  _B_ _B'_ Line of cross section shown in figure 14
  (_D-D'_ is figure 26)


Near the beginning of the Quaternary Period a vast quantity of molten
rock had again accumulated deep within the earth beneath Yellowstone.
This time, in contrast to Absaroka volcanism, the magma was charged with
highly explosive materials which eventually caused two caldera-making
eruptions, one 2,000,000 years ago and the other 600,000 years ago.
Because both eruptions affected the central part of the Park, the
features related to the older one were largely destroyed by the activity
associated with the younger one. Thus, the outline of the volcanic
caldera we now see in the Yellowstone landscape is chiefly the one that
formed 600,000 years ago (fig. 22). The sequence of events described in
the following pages, and illustrated diagrammatically in figure 23, is
based on studies of this later eruption; the pattern for the
2,000,000-year-old eruption probably was similar.


                              The eruption

The giant reservoir of molten rock that built up beneath the Park area
fed two large magma chambers that rose to within a few thousand feet of
the surface. As the pressures increased, the overlying ground arched,
stretched, and cracked (fig. 23A). Small amounts of lava began to flow
out through the cracks in places, but finally, in a great surge of
rapid, violently explosive eruptions, first from one chamber and then
the other, mountains of hot pumice, ash, and rock debris spewed from the
earth (fig. 23B). The dense, swirling masses of erupted material spread
out across the countryside in extremely fast moving _ash flows_, swept
along by hot expanding gases trapped within them. Large quantities of
ash and dust were also blown high into the air and dispersed by the
wind. Thin layers of airborne volcanic ash from Yellowstone are now
found throughout much of the central and western United States.

The ash flows (fig. 23B), as they sped across the Yellowstone
countryside, first filled the old canyons and valleys that had been
eroded into the Absaroka volcanic pile and older rocks during Pliocene
time. Eventually much of this older landscape was buried by ash. Some of
the larger highlands, such as Mount Washburn and adjacent ridges and
Bunsen Peak, however, stood well above the level of the sweeping ash
flows; so the debris flowed around them rather than across them (fig.
21). Finally coming to rest, the hot pumice, ash, and rock particles
settled down in vast horizontal sheets (fig. 24). Upon cooling and
crystallizing, the particles welded together to form a series of compact
rocks with the composition of rhyolite (figs. 15 and 25). The term
"ash-flow tuff" (also, the term "welded tuff") is commonly used to
describe these rocks, which now make up the Yellowstone Tuff (fig. 5).


                              The collapse

With the sudden removal of hundreds of cubic miles of molten rock from
underground, the roofs of the twin magma chambers collapsed. Enormous
blocks of rock fell in above each of the chambers, and a great crater,
or _caldera_, broke the ground surface in central Yellowstone (fig.
23C). The exact depth to which the original surface collapsed is
unknown, but it must have been several thousand feet. The subsidence
took place chiefly along large vertical, or normal, faults in the ring
fracture zones above the margins of the magma chambers (fig. 22).
Abundant, though less extensive, normal faults also formed outside the
caldera proper, as the surrounding areas adjusted to the staggering
impact of the explosive eruptions and subsequent collapse.

Because the Yellowstone caldera now lies partly buried by thick lava
flows, the appearance of the caldera today is not nearly as impressive
as it must have been when the caldera was first formed. Many of the
important features, however, are particularly well exposed in the
vicinity of Canyon Village (fig. 26). The steep south slope of the
nearby Washburn Range (fig. 4) marks the north edge of the caldera, and
the range itself stands high because it was not involved in the
collapse. Canyon Village, on the other hand, lies at a much lower
elevation within the caldera proper. Turnouts on the road just south of
Dunraven Pass provide especially fine views of the northern part of the
caldera, and on a clear day Flat Mountain and the Red Mountains, which
mark the south edge of the caldera, south of Yellowstone Lake, can be
seen 50 miles away. As might be expected, the large basin occupied by
Yellowstone Lake owes its existence in part to caldera collapse. The
south edge of the caldera cuts across the south-central part of the
lake, along Flat Mountain Arm and the north tip of the Promontory; the
east edge coincides approximately with the east edge of the lake north
of Southeast Arm (fig. 27). Also, the prominent bluffs north of the
Madison River near Madison Junction mark part of the north rim of the
caldera.

    [Illustration: CALDERA DEVELOPMENT. Schematic diagrams showing
    idealized stages in the development of the Yellowstone caldera
    600,000 years ago. The scales shown in Diagram A are approximately
    the size of the features in Yellowstone. Although only one magma
    chamber is pictured in the diagrams, two chambers were involved in
    the Yellowstone eruption. (Based on information supplied by R. L.
    Christiansen and H. R. Blank, Jr.) (Fig. 23)

    A, A large magma chamber formed deep within the earth, and the
    molten rock began to force its way slowly toward the surface. As it
    pushed upward, it arched the overlying rocks into a broad dome. The
    arching produced a series of concentric fractures, or a ring
    fracture zone, around the crest of the dome. The fractures extended
    downward toward the top of the magma chamber.]

    [Illustration: B, The ring fractures eventually tapped the magma
    chamber, the uppermost part of which contained a high proportion of
    dissolved gases. With the sudden release of pressure, tremendous
    amounts of hot gases and molten rock were erupted almost instantly.
    The liquid solidified into pumice, ash, and dust as it was blown
    out. Some of the dust and ash was blown high into the air and
    carried along by the wind, but much of the debris moved outward
    across the landscape as vast ash flows, covering thousands of square
    miles very rapidly.]

    [Illustration: C, The area overlying the blown-out part of the magma
    chamber collapsed to form a gigantic caldera. The collapse took
    place mostly along normal faults that developed from the fractures
    in the ring fracture zone. The depth of the collapse was probably
    several thousand feet.]

    [Illustration: D, Renewed rise of molten rock domed the caldera
    floor above the magma chamber. A series of rhyolite lava flows
    poured out through fractures in the surrounding ring fracture zone
    and spread across the caldera floor.]

    [Illustration: ORIGINAL EXTENT OF THE YELLOWSTONE TUFF (ash-flow
    tuff) that covered most of Yellowstone National Park about 600,000
    years ago. The tuff was erupted explosively from the ring fracture
    zones of the Yellowstone caldera. The outline of the caldera is
    shown by the dashed line. (Based on information supplied by R. L.
    Christiansen and H. R. Blank, Jr.) (Fig. 24)]

    [Illustration: YELLOWSTONE TUFF AT GOLDEN GATE. The rocks consist of
    layered ash-flow tuff; the height of the cliff is about 200 feet.
    (Fig. 25)]

    [Illustration: Closeup B shows typical characteristics of the tuff
    in most outcrop areas. Of the light-colored materials, the larger
    masses are compressed pumice fragments and the smaller masses are
    pumice, feldspar, and quartz. The dark grains are chiefly magnetite
    and pyroxene. Closeup A is of a coarse-grained specimen from Tuff
    Cliff. The large fragments are mostly crystallized pumice, and the
    light-colored matrix is composed of very fine particles of volcanic
    ash and dust.]

    [Illustration: GEOLOGIC CROSS SECTION showing generalized
    relationships along the north edge of the Yellowstone caldera in the
    Mount Washburn-Canyon area (line of section labeled D-D' on pl. 1).
    The caldera subsided along normal faults in the ring fracture zone,
    and the Plateau Rhyolite (lava flows) poured out across the caldera
    floor between 600,000 and 500,000 years ago. The faults cut across
    the central intrusive igneous core of the 50-million-year-old
    (Eocene) Washburn volcano; the north half of the volcano is still
    preserved, but the south half subsided as part of the caldera and is
    now buried by lava flows. (Based on information supplied by H. J.
    Prostka and R. L. Christiansen.) (Fig. 26)]

  Grand Canyon
  Plateau Rhyolite (lava flows)
  Edge of caldera
  Intrusive igneous rocks of Washburn volcano
  Mount Washburn
  Absaroka volcanic breccias


                         The outpouring of lava

The final violent eruption 600,000 years ago, although releasing much of
the explosive energy of the gases contained in the magma, did not quell
all potential volcanic activity in the twin chambers. Molten rock again
rose in both of them, and in a few hundreds or thousands of years the
overlying caldera floor was domed over the two chambers. One of these
prominent domes lies near Old Faithful and the other east of Hayden
Valley (figs. 22 and 23D). Soon, too, the magma found its way upward
through the wide ring fracture zones encircling the caldera. Pouring out
rather quietly from many openings (fig. 23D), the lavas flooded the
caldera floor and began to fill the still-smoldering pit. The first
lavas appeared soon after the collapse 600,000 years ago, and the latest
ones only 60,000-75,000 years ago. The flows were confined chiefly to
the caldera proper, but here and there they spilled out across the rim,
particularly toward the southwestern part of the Park (fig. 28). Some
flows also erupted along fractures outside the caldera, the most
prominent flow being the very famous one at Obsidian Cliff (fig. 29).

    [Illustration: YELLOWSTONE LAKE. View southeast across Yellowstone
    Lake toward the western foothills and crest of the Absaroka Range.
    The Absaroka Range is an erosional remnant of a vast pile of
    volcanic lavas and breccias (Absaroka volcanic rocks) that once
    covered all of Yellowstone; the lake occupies part of the
    Yellowstone caldera. (Fig. 27)]

The chief rock type in the lava flows is rhyolite, similar in
composition to the welded tufts erupted earlier but different in other
major characteristics. The rock, for example, shows much contorted
layering as evidence of having flowed as a thick liquid across the
ground (fig. 30). A coarse brecciated texture is also a common feature,
well shown by lavas along the Firehole Canyon drive (fig. 31). Locally,
some parts of the flows cooled so rapidly that few crystals formed, and
the lava solidified mainly into a natural glass (fig. 32).

    [Illustration: RADAR IMAGE of a part of southwestern Yellowstone
    National Park. The lobate landforms are the edges of a lava flow of
    the Plateau Rhyolite that forms the Pitchstone Plateau (fig. 1). The
    low concentric ridges that parallel the toe of the flow are pressure
    ridges produced by the wrinkling of the nearly solidified crust of
    lava along the edge of the flow. (Image courtesy of National
    Aeronautics and Space Administration.) (Fig. 28)]

    [Illustration: OBSIDIAN CLIFF, Jim Bridger's famous "mountain of
    glass." The rock is rhyolite lava which contains a high proportion
    of obsidian, a kind of black volcanic glass. Note columnar jointing
    along the sides of the cliff, similar to that shown by the basalt
    flows at Tower (fig. 33). The cliff is approximately 200 feet high.
    (Fig. 29)]

    [Illustration: THICK RHYOLITE LAVA FLOW along west bank of Firehole
    River. (Fig. 30)]

    [Illustration: Closeup view is of a cut surface of rhyolite, showing
    the striking banding that results from the flowage of viscous molten
    rock. The dark bands are chiefly concentrations of volcanic glass
    (also some cavities), and the light bands are concentrations of tiny
    crystals of feldspar and quartz.]

    [Illustration: BRECCIATED RHYOLITE LAVA FLOWS along the Firehole
    Canyon drive. As a lava flow moves outward from its center of
    eruption, a chilled crust develops along its upper surface and outer
    edges because of the cooler temperatures in those parts of the flow.
    Continued movement of the still-molten rock in the interior of the
    flow causes this crust to break up (brecciate) into angular blocks.
    The blocks are then tumbled along until the whole mass finally
    solidifies. (Fig. 31)]

    [Illustration: OUTCROP OF GLASSY RHYOLITE LAVA along the road
    between Canyon Village and Norris Junction. The conspicuous lines in
    the face of the rock outline different layers produced by lava
    flowage.  The feldspar crystals are alined parallel to the direction
    of flow. (Fig. 32)]

    [Illustration: In closeup A, dark parts of the rock are volcanic
    glass (closeup B shows glassy fracture) and light-colored crystals
    are quartz (blocky) and feldspar (tabular).]

    [Illustration: Closeup B.]

About 30 different flows have been recognized. Grouped within a major
rock unit called the Plateau Rhyolite (fig. 5), they cover more than
1,000 square miles. The gently rolling plateau surface of central
Yellowstone, broken here and there by clusters of low-lying hills and
ridges, is essentially the landscape that characterized the upper
surfaces of the lava flows soon after they cooled and solidified.
Natural valleys formed between some of the adjacent flows, and in places
streams still follow these readymade channels. Rhyolite, in both lava
flows and ash-flow tuffs, is by far the predominant rock type seen along
the Park roads.

Several basalt flows were erupted along with the more common rhyolite
flows, and in the vicinity of Tower Falls they form some of the most
unusual rock units in the whole Park area (fig. 33). As the flows
cooled, contraction cracks broke the basalt into a series of upright
many-sided columns; from a distance they appear as a solid row of
fenceposts. They are now covered by younger rocks, but if one could see
the upper flat surface of the basalt layers where just the ends of the
columns are sticking out, the pattern would be like that seen in a
honeycomb.

During the eruptions of the Plateau Rhyolite, at least one relatively
small caldera-making event occurred in the central Yellowstone region.
This "inner" caldera developed sometime between 125,000 and 200,000
years ago, forming the deep depression now filled by the West Thumb of
Yellowstone Lake (fig. 22). Like the main Yellowstone caldera, but on a
much smaller scale, it formed as a direct result of the explosive
eruption of rhyolitic ash flows and subsequent collapse of an
oval-shaped area approximately 4 miles wide and 6 miles long. West Thumb
is nearly the same size as Crater Lake, Oregon, which occupies one of
the world's best-known calderas.

With the outpouring of the last lava flows 60,000-75,000 years ago, the
forces of Quaternary volcanism finally died down. The hot-water and
steam activity, however, still remains as a vivid reminder of
Yellowstone's volcanic past. But who can say even now that we are
witnessing the final stage of volcanism? Someday, quite conceivably,
there might be yet another outburst of molten rock--only time, of
course, will tell.

    [Illustration: TWO LEDGES OF BASALT spectacularly exposed in the
    east wall of the Grand Canyon of the Yellowstone at The Narrows near
    Tower Falls. The light-colored rocks between the basalt flows are
    ancient stream gravels deposited about 1½ million years ago, when
    the channel of the Yellowstone River was farther east and not as
    deep as it is today. The hill is capped by lake sediments, sand, and
    gravel deposited when the Yellowstone River was blocked by a glacial
    dam farther downstream (to the left). The brown rocks at the base of
    the cliff are Absaroka andesite breccias. (Fig. 33)]

    [Illustration: Pronounced columnar jointing of the basalt is seen at
    close range at the edge of the road on the opposite (west) side of
    the canyon. Inset shows the dense character of the black basalt,
    which consists of microscopic crystals of feldspar, pyroxene,
    olivine, and magnetite.]




                   Final Sculpturing of the Landscape


The many episodes of mountain building and volcanism all left their
lasting and unmistakable imprints across the face of the Yellowstone
country. During the latter part of the Tertiary Period, erosion, too,
had begun to make its own deep marks. But only in the last 100,000 years
or so have the powerful exterior forces of the earth--chiefly running
water and moving ice--had a virtually free hand in shaping the Park's
landscape. Nevertheless, in this short period of time they have wrought
profound changes.


                               Glaciation

A giant boulder of Precambrian gneiss lies among the trees beside the
road leading to Inspiration Point on the north rim of the Grand Canyon
of the Yellowstone (fig. 34). This boulder, measuring approximately
24x20x18 feet and weighing at least 500 tons, is of considerable
interest, not so much for its great size but because it is completely
out-of-place in its present surroundings. The boulder rests on rhyolite
lava flows of Quaternary age, at least 15 miles from the nearest
outcrops of the ancient gneiss to the north and northeast. Obviously,
this seemingly immovable chunk of rock was pushed or carried a long way
by some very powerful transporting agent before it was finally dropped.
A natural force of such magnitude could only have been exerted by moving
ice; in fact, no further proof than this one boulder is needed for us to
conclude beyond question that glaciers once existed in Yellowstone.
There is, to be sure, much additional evidence that the Park region was
extensively glaciated. Deposits of out-of-place boulders (_glacial
erratics_), like the one mentioned above, are found nearly everywhere
(fig. 35) and the mountains and high valleys still bear the vivid scars
of ice sculpturing (figs. 36 and 37).

    [Illustration: GIANT BOULDER (glacial erratic) of Precambrian gneiss
    near Inspiration Point on the north rim of the Grand Canyon. The
    boulder, measuring 24x20x18 feet and weighing more than 500 tons,
    was dropped at this locality by glacial ice; it now rests on the
    much younger Plateau Rhyolite. The distance that the boulder was
    carried or pushed was at least 15 miles. (Fig. 34)]

The principal requirement for the formation of glaciers is simple: more
snow has to accumulate during the winter than is melted during the
summer. If this condition continues for a long enough period of time
(measured in centuries), the snow compacts to ice, and extensive
icefields grow until they finally begin to move under their own weight,
thereby becoming glaciers. Records show that the average year-round
temperature is 32°-33°F along Yellowstone Lake, 35°F at Old Faithful,
and 39°F at Mammoth. Each winter, snow accumulates to depths of 5-10
feet throughout much of the Park. If the average annual temperatures
were to decrease a few degrees or the yearly snowfall were to increase a
foot or so, either change could possibly herald the beginning of another
ice age in the Yellowstone region.

Yellowstone was glaciated at least three times. These glaciations are,
from oldest to youngest, the pre-Bull Lake, Bull Lake, and Pinedale.
Their precise age and duration are imperfectly known, but estimates
based on a few radiometric determinations are: (1) the oldest glaciation
(pre-Bull Lake glaciation) began more than 300,000 years ago and ended
between 180,000 and 200,000 years ago; (2) Bull Lake Glaciation began
about 125,000 years ago and ended more than 45,000 years ago; (3)
Pinedale Glaciation began about 25,000 years ago and ended about 8,500
years ago. The pre-Bull Lake and Bull Lake are known only from scattered
deposits of rock debris (_glacial moraines_) and other features, but the
distribution of these deposits indicates that glaciers were widespread
throughout the region and occurred both between and during eruptions of
the Plateau Rhyolite. The effects of the Pinedale glaciers, on the other
hand, are obvious in many parts of the Park, and the history of this
youngest glacial cycle (described below) is known in much greater detail
than that of the two older ones.

In the early stages of Pinedale Glaciation, an enormous icefield built
up in the high Absaroka Range southeast of the Park area. A glacier, fed
by this icefield, flowed northward down the upper Yellowstone valley and
into the basin now occupied by Yellowstone Lake. At about the same time,
another great icefield formed in the mountains north of the Park and
sent long tongues of ice southward toward the lower Yellowstone and
Lamar River valleys. Smaller valley glaciers flowed westward out of the
Absaroka Range along the east edge of the Park, and still others formed
along the main ridges and valleys of the Gallatin Range, in the
northwestern part of the Park. Thus, many huge masses of ice from the
north, east, and southeast converged and met in the Park. At this stage,
probably about 15,000 years ago, only the west edge of the Park, and
perhaps a few of the highest peaks and ridges within the Park, remained
free of ice. It is interesting to note that although ice moved across
and buried the ancestral Grand Canyon of the Yellowstone, it did not
flow down and scour the canyon (fig. 36). If it had, the canyon would
look much different than it does today (fig. 41).

    [Illustration: GLACIATED TERRAIN along the Northeast Entrance road.
    The boulders, many of them measuring 10 feet across or more, were
    carried into the area by ice flowing down Slough Creek from
    mountains north of the Park during the Pinedale Glaciation. As the
    glaciers melted, the boulders were left stranded in hummocky,
    morainal deposits. Shallow depressions in the irregular topography
    are now commonly filled by small ponds. (Fig. 35)]

For the next 10,000 years, the ice thickened and spread out over more
and more of the Park area. The mass centered over the Yellowstone Lake
basin grew to a depth of 3,000 feet or more and dominated the entire
scene; it formed a broad "mountain" of ice which became so high that it
caused more snow to fall upon itself and was cold enough to prevent much
of this snow from melting. Eventually the Pinedale glaciers covered
about 90 percent of Yellowstone (fig. 38).

    [Illustration: CANYON PROFILES. Typical profiles of a canyon cut by
    a stream (A) and of a canyon gouged by a glacier (B). Glacial
    cirques (C) are shown at the head and high on the side of the
    glaciated valley. (Fig. 36)]

    [Illustration: GLACIAL CIRQUE on east face of Electric Peak,
    northern Gallatin Range. During several episodes of glaciation, this
    steep-walled amphitheaterlike valley was cut and filled by ice which
    fed glaciers moving downslope to the lower right. The cirque floor
    is now covered by a thick deposit of rock rubble underlain in part
    by ice, and the whole mass is still moving slowly downhill as a rock
    glacier. The dark rock at lower right is part of the Electric Peak
    stock, composed of diorite (fig. 20) and other kinds of intrusive
    igneous rocks. The rocks in the cirque walls are chiefly Cretaceous
    shales (light to moderately dark color) with thin sills of igneous
    rock (very dark color). (West-looking oblique aerial photograph,
    courtesy of William B. Hall, University of Idaho.) (Fig. 37)]

After their maximum advance, the Pinedale glaciers began to melt,
leaving behind the rock debris they had gouged from the landscape and
had pushed or carried along with them. These glacial moraines are now
found in many areas throughout the Park. In places, glacial ice and (or)
rock debris formed natural dams across stream valleys, thereby
impounding lakes. Parts of Hayden Valley, for example, contain layers of
very fine sand, silt, and clay several tens of feet thick (fig. 39) that
accumulated along the bottom of a large lake. This lake formed behind a
glacial dam across the Yellowstone River near Upper Falls. Some of the
glacial dams broke and released water catastrophically, causing giant
floods; the occurrence of one such flood is particularly evident along
the Yellowstone River valley near Gardiner, Montana.

By about 12,000 years ago the thick Pinedale ice sheet had melted
entirely from the Yellowstone Lake basin and most other areas of the
Park, although valley glaciers continued to exist in the mountains until
about 8,500 years ago. Then, following a short period of total
disappearance, small icefields formed again in the heads of some of the
higher mountain valleys. Since the melting of the Pinedale ice, however,
none has descended as a glacier into the lower stretches of the valleys.
Even though a few snowfields persist locally throughout the summers
(except during the warmest years), no glaciers exist in the Park at the
present time.

    [Illustration: EXTENT OF ICE in Yellowstone National Park during the
    maximum spreading of the Pinedale glaciers, probably about 15,000
    years ago. Long arrows indicate direction of strong flowage of ice;
    short arrows show direction of less vigorous ice flowage. The
    dark-blue area shows the main ice mass centered over the Yellowstone
    Lake basin in the southeast corner of the Park. Many of the high
    peaks and ridges such as Mount Washburn, which are here shown free
    of ice, were glaciated at least once during the past 250,000 years.
    Whether they were covered by the Pinedale glaciers, however, is
    still an unresolved question. (Based on information supplied by G.
    M. Richmond, K. L. Pierce, and H. A. Waldrop.) (Fig. 38)]

    [Illustration: FLAT-LYING BEDS of fine sand, silt, and clay near the
    mouth of Trout Creek in Hayden Valley. These beds were deposited in
    a glacially dammed lake that covered part of Hayden Valley when the
    Pinedale glaciers were melting. The height of the streambank is
    about 40 feet. (Fig. 39)]

    [Illustration: Douglas-fir branch and cones.]

    [Illustration: WATERFALLS in Yellowstone National Park. (Fig. 40)

    A, Lewis Falls on the Lewis River. The falls cascade over the steep
    edge of a rhyolite lava flow.]

    [Illustration: B, Upper Falls on the Yellowstone River. The brink of
    the falls marks the contact between dense, resistant rhyolite lava
    (which forms the massive cliff) and more easily eroded rhyolite lava
    containing a high proportion of volcanic glass immediately
    downstream, as shown in figure 42.]

    [Illustration: C, Gibbon Falls on the Gibbon River. The river
    tumbles over a scarp etched in the Yellowstone Tuff. The scarp first
    formed along faults at the north edge of the Yellowstone caldera
    600,000 years ago, at a point that now lies ¼ to ½ mile downstream.
    Continued erosion has caused the falls to recede northward to their
    present position.]

    [Illustration: D, Tower Falls on Tower Creek. The rocks at the brink
    of the falls, and in the vertical cliff beneath, are coarse breccias
    and conglomerates of the Absaroka volcanic rocks. The channel of
    Tower Creek has not been cut down rapidly enough to keep pace with
    the downcutting of the main channel of the Yellowstone River, which
    lies a short distance downstream from the base of the falls.]


                 Running water--canyons and waterfalls

Yellowstone is, among its many attributes, the source of large and
mighty rivers. Located across the Continental Divide, the Park feeds two
of the most extensive drainage systems in the nation--(1) the Missouri
River system (and ultimately the Mississippi River) on the Atlantic
side, via the Yellowstone, Madison, and Gallatin Rivers, and (2) the
Columbia River system on the Pacific side, via the Snake River (fig. 1).
These streams are fed by an annual precipitation which averages about 17
inches at Old Faithful and Mammoth, but which is considerably greater in
the mountain ranges.

Many stretches of the main river valleys in Yellowstone are broad and
flat bottomed. In these, the stream gradients range from about 10 to 30
feet per mile, and there is little erosion going on at present (Hayden
Valley is a good example, fig. 4). But here and there the gradients are
steeper, and the valleys are narrow and rugged. In some places these
streams drop 50 or even 100 feet per mile, and the fast-moving waters
have carved deep V-shaped gorges (fig. 36).

Waterfalls, features for which Yellowstone is also justly famous (fig.
40), generally result from abrupt differences in rock hardness. If a
stream flows over rocks that are more resistant to erosion than the
rocks immediately downstream, a ledge or bench will form across the
streambed at that place because the less resistant rocks are worn away
faster. And, as the ledge becomes higher, the softer downstream rocks
will erode even faster. A true waterfall is one in which there is a
free, vertical fall of water. If the ledge or ledges form only a rough,
steep runway in the streambed, then the term "rapids" or "cascades" is
more appropriate.

The existence of many waterfalls in Yellowstone today is due in large
part to the fact that, because of recent volcanism and glaciation, much
of the region's topography is very young in terms of geologic time.
Streams, even some of the largest ones, have not had enough time to wear
away all the features that may produce waterfalls, cascades, or rapids
along their channels. This is particularly true along the margins of
lava flows, where there are sharp dropoffs between the tops of the flows
and the lower ground beyond. The Grand Canyon of the Yellowstone and the
Upper and Lower Falls, well illustrate the erosive power of running
water.


                    Grand Canyon of the Yellowstone

Except for Old Faithful, the Grand Canyon of the Yellowstone is probably
the best known and most talked about and photographed feature in the
Park (fig. 41). Although not so deep or wide as some of the other great
canyons in America, its sheer ruggedness and beauty are breathtaking.
Here the aptness of the name "Yellowstone" can be fully appreciated and
understood, for the viewer is at once engulfed in a sea of yellow hues
streaked and tinted with various shades of red and brown.

    [Illustration: GRAND CANYON AND LOWER FALLS of the Yellowstone
    River, as viewed upstream (southwest) from Artists Point on the
    south rim. The yellow-hued rocks lining the canyon walls are soft,
    hydrothermally altered rhyolite lavas. The rocks at the brink of the
    falls consist of less altered and therefore more resistant
    rhyolites. The falls, 309 feet high, formed at the contact between
    the hard and soft rhyolite units. (Photograph courtesy of Sgt. James
    E. Jensen, U.S. Air Force.) (Fig. 41)]

    [Illustration: DEVELOPMENT OF GRAND CANYON. Profiles along the floor
    of the Grand Canyon of the Yellowstone as it appears today (C) and
    as it appeared at two older stages in its development (A and B).
    Note particularly the various kinds of rocks through which the
    canyon has been cut, and how rock differences have influenced the
    location of the two falls. Diagonal lines indicate unaltered
    rhyolite; large dots, rhyolite with much volcanic glass; small dots,
    hydrothermally altered rhyolite; and circles and dots, Absaroka
    volcanic rocks. (Based on information furnished by R. L.
    Christiansen and G. M. Richmond; vertical scale is exaggerated about
    10 times.) (Fig. 42)]

  C: As it appears today
    Yellowstone Lake
    Hayden Valley
    Grand Canyon of the Yellowstone
    Upper Falls
    Lower Falls
    Inspiration Point
    North edge of Yellowstone caldera
    Confluence of Lamar and Yellowstone Rivers
  B: A stage somewhat before 300,000 years ago
    Profile of Yellowstone River today
  A: A stage somewhat before 600,000 years ago
    Profile of Yellowstone River today


At first glance, the canyon may appear to be a giant crack which
suddenly opened up and into which the Yellowstone River then plunged
headlong over high waterfalls at its southwest end. This, of course, is
not the way the canyon formed. Nevertheless, it is apparent that certain
unusual conditions caused the river, after winding slowly through
flat-floored Hayden Valley for about 13 miles, to cut a precipitous
gorge 1,000-1,500 feet deep and 20 miles long (fig. 42C). A full
explanation must be based on all the many events surrounding the
eruption of the Yellowstone Tuff, the collapse of the Yellowstone
caldera, the outpouring of the Plateau Rhyolite, and the various
episodes of glaciation. Geologic studies show that all these events took
place while the canyon was being cut, and that each one played an
important role in its development. Hot-water and steam activity likewise
was a significant factor. However, despite its many complexities, the
history of the Grand Canyon can be divided into a few major stages, as
outlined below:

  1. From more than 2,000,000 years ago to about 600,000 years ago, a
  shallow canyon was gradually being cut into the Absaroka volcanic
  sequence by the ancestral Yellowstone River as it eroded headward from
  a point near the present confluence of the Yellowstone and Lamar
  Rivers (fig. 33). By the time of the climactic volcanic eruption in
  central Yellowstone 600,000 years ago, the head of the "old" canyon
  probably had been eroded southward nearly to the place where the north
  rim of the Yellowstone caldera was to form later (fig. 42A). This
  point now lies about 5 miles below Lower Falls.

  2. Ash-flow tuffs that were erupted 600,000 years ago filled the "old"
  canyon, and the river recarved its channel, chiefly along its previous
  course.

  3. A large lake formed behind (south of) the north rim of the caldera,
  the damming resulting in part from lava flows of Plateau Rhyolite that
  poured out across the caldera floor in this area between 600,000 and
  500,000 years ago. Eventually the lake rose and spilled northward into
  the head of the "old" canyon, causing additional downcutting in what
  is now the lower 15-mile stretch of the canyon.

  4. As the lake emptied, the river began to erode upstream into the
  thick rhyolite lava flows toward the present site of Lower Falls; the
  process was very similar to that of a common stream gully eroding
  headward into a hillside. At a stage somewhat more than 300,000 years
  ago, the head of the canyon probably lay near the falls, and the river
  had cut a channel 400-600 feet deep along this upper 5-mile stretch
  (fig. 42B).

  5. Approximately 300,000 years ago the canyon area was covered by ice
  during pre-Bull Lake glaciation. During and after the retreat of this
  ice, sediments accumulated in a lake that occupied the upper reaches
  of the canyon between the present site of Upper Falls and Inspiration
  Point. Subsequently, very little downcutting was accomplished until
  about 150,000-125,000 years ago, when the canyon was eroded nearly to
  its present depth.

  6. Canyon development was further interrupted by the advance and
  retreat of glaciers during Bull Lake and Pinedale Glaciations. During
  and since the melting of the Pinedale glaciers about 12,000 years ago,
  the canyon has attained its present depth, and its walls have acquired
  much of their picturesque erosional form. The Yellowstone River now
  maintains a fairly uniform gradient (60-80 feet per mile) throughout
  the 20-mile-long gorge, even though different segments of the canyon
  were cut at different times and through different kinds of rocks (fig.
  42C).

The spectacular erosional development in the upper 5-mile segment of the
Grand Canyon, which is the only part seen by most Park visitors, except
for the very lower end near Tower Falls (fig. 33), has taken place
mostly within the past 150,000-125,000 years. One reason for such a
rapid rate of erosion stems from the fact that this part of the canyon
overlies one of the wide ring fracture zones of the Yellowstone caldera
(fig. 22). The fracture zone extends to great depth, providing a ready
avenue of travel for the upflow of hot water and steam rising in the
Yellowstone thermal system, as described in the following chapter.
Through many thousands of years, the upward percolation of the hot
fluids has caused severe chemical and physical changes (known as
_hydrothermal alteration_) in the rhyolite lava flows. One spectacular
result of the alteration has been the change from the normal brown and
gray color of the rhyolites to the bright yellow and other colorful hues
now seen in the canyon walls (as well as in many other places throughout
the Park). Another significant result of alteration has been the
weakening of the rocks; that is, the altered rocks are softer and less
resistant to erosion than unaltered rocks. Hence, the river has been
able to erode these softer rocks, upstream to Lower Falls, at a very
rapid rate.

The position of Lower Falls, as might be expected, coincides with a
change from highly altered to less altered rhyolite; the difference in
the erosion rates of the two kinds of rocks here is self-evident (figs.
41 and 42C). The position of Upper Falls is likewise closely controlled
by differences in rock hardnesses. The rhyolites on the upstream side
are hard and dense, whereas those on the downstream side contain a high
proportion of volcanic glass which causes them to be more easily eroded
(fig. 42C).

    [Illustration: Swan family.]

    [Illustration: COMMON KINDS OF THERMAL FEATURES in Yellowstone
    National Park. (Fig. 43)

    A, Hot springs and terraces colored by algae at Mammoth Hot
    Springs.]

    [Illustration: B, Castle Geyser erupting in Upper Geyser Basin.]

    [Illustration: C, Fountain Paint Pots in Lower Geyser Basin.]

    [Illustration: D, pool in Lower Geyser Basin.]




                     Hot-Water and Steam Phenomena


Although Yellowstone is geologically outstanding in many ways, the great
abundance, diversity, and spectacular nature of its thermal (hot-water
and steam) features were undoubtedly the primary reasons for its being
set aside as our first National Park (fig. 43). The unusual
concentration of geysers, hot springs, mudpots, and fumaroles provides
that special drawing card which has, for the past century, made the Park
one of the world's foremost natural attractions.

To count all the individual thermal features in Yellowstone would be
virtually impossible. Various estimates range from 2,500 to 10,000,
depending on how many of the smaller features are included. They are
scattered through many regions of the Park, but most are clustered in a
few areas called geyser basins, where there are continuous displays of
intense thermal activity. (See frontispiece.) The "steam" that can be
seen in thermal areas is actually fog or water droplets condensed from
steam; so the appearance of individual geyser basins depends largely on
air temperature and humidity. On a warm, dry summer day, for example,
the activity may seem very weak (fig. 44), except where individual
geysers are erupting. On cold or very humid days, however, "steam"
plumes are seen rising from every quarter.


                     How a thermal system operates

An essential ingredient for thermal activity is heat. A body of buried
molten rock, such as the one that produced volcanic eruptions in
Yellowstone as late as 60,000 to 75,000 years ago, takes a long time to
cool. During cooling, tremendous quantities of heat are transmitted by
conduction into the solid rocks surrounding the magma chamber (fig. 45).
Eventually the whole region becomes much hotter than non-volcanic areas
(fig. 46). Normally, rock temperatures increase about 1°F per 100 feet
of depth in the earth's crust, but in the thermally active areas of
Yellowstone the rate of temperature increase is much greater. The amount
of heat given off by the Upper Geyser Basin, for example, is 800 times
the amount given off by normal (nonthermal) areas of the same size. This
excess heat is enough to melt 1½ tons of ice per second! And, contrary
to popular opinion, the underground temperatures have not cooled
measurably in the 100 years that records have been kept on the thermal
activity in the Park. In fact, geologic studies indicate that very high
heat flows have continued for at least the past 40,000 years.

    [Illustration: NORRIS GEYSER BASIN, as viewed northward from the
    Norris Museum. This is one of the most active thermal areas in
    Yellowstone, but the photograph was taken on a warm dry summer day
    when little hot-water and steam activity was visible from a
    distance. Clouds of water droplets (the visible "steam" in thermal
    areas) normally form only when the air is cool and (or) moist. The
    floor of the basin is covered by a nearly solid layer of hot-spring
    deposits. (Fig. 44)]

    [Illustration: HEAT FLOW AND SURFACE WATER. Diagram showing a
    thermal system, according to the explanation that water of surface
    origin circulates and is heated at great depths. (Based on
    information supplied by D. E. White, L. J. P. Muffler, R. O.
    Fournier, and A. H. Truesdell.) (Fig. 45)]

  Water enters at ground surface and sinks in conduit formed by fault or
          fracture
  Surface (meteoric) water sinks to levels perhaps as much as 10,000
          feet below ground. Heated far above its normal boiling point,
          it begins to rise toward the surface
    Descending cool surface water
  Permeable zone allows water to flow through it
  Cooling magma chamber
  Water begins to boil near ground surface because of greatly reduced
          pressures
    Rising hot water
  Hot spring or geyser


    [Illustration: INFRARED IMAGE of a part of Upper Geyser Basin.
    Infrared instruments, sensitive to heat, are able to detect "hot"
    spots in the landscape. Note especially the sharp "image" of Old
    Faithful. (Image courtesy of National Aeronautics and Space
    Administration.) (Fig. 46)]

A second, equally essential ingredient for thermal activity is water.
Many thousands of gallons are discharged by the hot springs and geysers
in Yellowstone every minute--where does all this water come from?
Studies show that nearly all the water originates above ground as rain
or snow (meteoric water; fig. 45), and that very little comes from the
underlying magma (magmatic water).

The mechanism for heating the water, on the other hand, is a matter of
some uncertainty. Until a few years ago the heating was assumed to occur
near the ground surface and to be caused by hot magmatic gases (mostly
steam) rising from the underlying magma chamber. Deep wells drilled
recently in many thermal areas throughout the world (including research
drill holes in Yellowstone), however, suggest a better explanation.
According to this explanation, the surface water enters underground
passages (fractures and faults) and circulates to great depths--as much
as 5,000-10,000 feet in some areas (fig. 45)--there to become heated far
above its surface boiling point. Research drill holes in Yellowstone,
for example, have demonstrated that water of surface origin exists at
all depths at least to the maximum drilled (1,088 feet), and that the
water reaches temperatures up to at least 465°F. The increase in
temperature with depth causes a corresponding decrease in the weight
(density) of the water. Because of this, the hot, "lighter," water
begins to rise again toward the ground surface, pushed upward by the
colder, "heavier," near-surface water which sinks to keep the water
channels filled. Thus is set into motion a giant _convection current_
which operates continuously to supply very hot water to the thermal
areas (fig. 45). Just how deep the waters circulate in Yellowstone no
one really knows; as a guess, the depth probably is at least 1 or 2
miles.

The effect of pressure on the boiling temperature of water also plays a
vital role in thermal activity. In a body of water, the pressure at the
surface is that exerted by the weight of air above it (_atmospheric
pressure_). Water under these conditions boils at 212°F at sea level and
at about 199°F at the elevation of most of the geyser basins in
Yellowstone. However, water at depth not only is subjected to
atmospheric pressure but also bears the added weight of the overlying
water. Under such additional pressures, water boils only when the
temperature is raised above its surface boiling point. In a well 100
feet deep at sea level, for example, the water at the bottom would have
to be heated to 288°F before it will boil. Thus it follows that in the
underground "workings" of hot springs or geysers, (1) The deepest water
is subjected to the greatest pressures, and (2) these deeper waters (in
Yellowstone) must be heated well above 199°F before they can actually
begin to boil. By this same reasoning but in reverse, if the pressure is
released, which happens as the water rises toward the ground surface,
the "hotter-than-boiling" water will then begin to boil. The boiling
will be rather quiet if the pressure is released gradually, as in most
hot springs. But if the pressure is released suddenly, boiling may
become so violent that much of the water flashes explosively into steam,
expanding to several hundred times its normal volume. This expansion
provides the necessary energy for geyser eruptions.


                     Hot-spring deposits and algae

    [Illustration: MOUND OF SINTER at Castle Geyser, Upper Geyser Basin.
    Lower part of mound has well-defined layers probably deposited by
    normal hot springs. The upper, irregular part resulted from the
    vigorous eruptions characteristic of geysers and marks a change in
    the local hot-spring activity. (Fig. 47)]

Nearly all geysers and many hot springs build mounds or terraces of
mineral deposits; some are so unusual in form that descriptive names
have been given to them, such as Castle Geyser (fig. 47). These deposits
are generally made up of many very thin layers of rock. Each layer
represents a crust or film of rock-forming mineral which was originally
dissolved in hot water as it flowed through the underground rocks, and
which was then precipitated as the water spread out over the surrounding
ground surface.

    [Illustration: TERRACES OF TRAVERTINE at Opal Springs, Mammoth Hot
    Springs area. (Fig. 48)]

    [Illustration: Closeup view shows layered and porous nature of the
    travertine.]

In all major thermal areas of the Park, with the exception of Mammoth
Hot Springs, most of the material being deposited is _sinter_ (the kind
found around geysers is popularly called _geyserite_). Its chief
constituent is silica (the same as in quartz and in ordinary window
glass). At Mammoth, the deposit is _travertine_ (fig. 48), which
consists almost entirely of calcium carbonate. The material deposited at
any given place commonly reflects the predominant kind of rock through
which the hot water has passed during its underground travels. At
Mammoth Hot Springs the water passes through thick beds of limestone
(which is calcium carbonate), but in other areas the main rock type
through which the water percolates is rhyolite, a rock rich in silica.

Through centuries of intense activity, layers of sinter have built up on
the floors of the geyser basins (fig. 44); these deposits are generally
less than 10 feet thick. In one drill hole at Mammoth, deposits of
travertine extend to a depth of 250 feet. Dead trees and other kinds of
vegetation whose life processes have been choked off by the heat, water,
and precipitated minerals of hot-spring activity are a common sight in
many places (fig. 51).

Both travertine and sinter are white to gray. Around active hot springs,
however, the terraces that are constantly under water may be brightly
colored (figs. 43 and 49) because they are coated by microscopic plants
called _algae_. These organisms, which thrive in hot water at
temperatures up to about 170°F, are green, yellow, and brown. Oxides of
iron and manganese also contribute to the coloring in some parts of the
thermal areas. The delicate blue color of many pools, however, results
from the reflection of light off the pool walls and back through the
deep clear water (fig. 43). Other pools are yellow because they contain
sulfur, or are green from the combined influence of yellow sulfur and
"blue" water.

    [Illustration: ALGAL-COLORED TERRACES lining the west bank of the
    Firehole River at Midway Geyser Basin. Algae are microscopic plants
    that grow profusely on rocks covered by hot water at temperatures up
    to about 170°F. (Fig. 49)]


                        Hot springs and geysers

Hot springs occur where the rising hot waters of a thermal system issue
from the ground-level openings of the feeder conduits (fig. 45). By far
the greatest numbers discharge water and steam in a relatively steady
noneruptive manner, although they vary considerably in individual
behavior. Depending upon pressure, water temperature, rate of upflow,
heat supply, and arrangement and size of underground passages, some hot
springs boil violently and emit dense clouds of vapor whereas in others
the water quietly wells up with little agitation from escaping steam. In
some hot springs, however, the underground channels are too narrow or
the upflow of very hot water and steam is too great to permit a steady
discharge; periodic eruptions then result. These special kinds of
springs are called "geysers" (from the Icelandic word _geysir_, meaning
to "gush" or "rage"). At least 200 geysers, of which about 60 play to a
height of 10 feet or more, occur in Yellowstone National Park; this is
more than in any other region of the world.

How does a geyser work? We cannot, of course, observe the inner plumbing
of a geyser, except for that part which is seen by looking into its
uppermost "well." Deeper levels directly below the "well" can be probed
by scientific instruments to some extent, and research drilling in some
parts of the geyser basins also provides much useful information. The
available information suggests that the plumbing system of a geyser (1)
lies close to the ground surface, generally no deeper than a few hundred
feet; (2) consists of a tube, commonly nearly vertical, that connects to
chambers, side channels, or layers of porous rock, where substantial
amounts of water can be stored; and (3) connects downward through the
central tube and side channels to narrow conduits that rise from the
deepwater source of the main thermal system.

Considering a geyser system as described above and applying what is
known about the behavior of water and steam, we can understand what
causes a natural thermal eruption. Figure 50 shows diagrammatically the
succession of events believed to occur during the typical eruptive cycle
of a geyser such as Old Faithful.

    [Illustration: A GEYSER IN ACTION. Photographs of successive stages
    in the eruption of Old Faithful illustrate what probably happens
    during a natural geyser eruption. The underground plumbing is
    diagrammatic and does not reflect any specific knowledge of Old
    Faithful's system. Direction of flow of water is shown by arrows.
    (Based on information supplied by D. E. White, L. J. P. Muffler, R.
    O. Fournier, and A. H. Truesdell.) (Fig. 50)

    Stage 1 (Recovery or recharge stage). After an eruption, the partly
    emptied geyser tubes and chambers fill again with water. Hot water
    enters through a feeder conduit from below, and cooler water
    percolates in from side channels nearer the surface. Steam bubbles
    (with some other gases such as carbon dioxide and hydrogen sulfide)
    start to form in upflowing currents, as a decrease in pressure
    causes a corresponding decrease in boiling temperature. At first the
    bubbles condense in the cooler, near-surface water that is not yet
    at boiling temperature, but eventually all water is heated enough
    that the bubbles will no longer condense or "dissolve."]

    [Illustration: Stage 2 (Preliminary eruption stage). As the rising
    gas bubbles grow in size and number, they tend to clog certain parts
    of the geyser tube, perhaps at some narrow or constricted point such
    as at A. When this happens, the expanding steam abruptly forces its
    way upward through the system and causes some of the water to
    discharge from the surface vent in preliminary spurts. The deeper
    part of the system, however, is not yet quite hot enough for
    "triggering."]

    [Illustration: Stage 3 (Full eruption stage). Finally, a preliminary
    spurt "unloads" enough water (with resulting reduction in pressure)
    to start a chain reaction deeper in the system. Larger amounts of
    water in the side chambers and pore spaces begin to flash into
    steam, and the geyser rapidly surges into full eruption.]

    [Illustration: Stage 4 (Steam stage). When most of the extra energy
    is spent, and the geyser tubes and chambers are nearly empty, the
    eruption ceases. Some water remains in local pockets and pore
    spaces, continuing to make steam for a short while. Thereafter the
    system begins to fill again, and the eruptive cycle starts anew.]

No two geysers have the same size, shape, and arrangement of tubes and
chambers. Also, some geysers, such as Great Fountain, have large surface
pools not present in cone-type geysers such as Old Faithful. Hence, each
geyser behaves differently from all others in frequency of eruption,
length of individual eruptions, and amount of water discharged. Geysers
may also vary in their own behavior as their plumbing features change
through the years. The great amount of energy that builds up in some of
them from time to time creates enough explosive force to shatter parts
of the plumbing system, thereby causing a change in their eruptive
behavior. In fact, some geyser eruptions have been so violent that large
chunks of rock have been exploded out of the ground and scattered around
the surrounding area (fig. 51). With time, the precipitation of minerals
may partly seal a tube or chamber, gradually altering the eruptive
mechanism.

Despite all the variable factors involved in geyser eruptions, and all
the changes that can take place from time to time to alter the pattern
of those eruptions, several of the Yellowstone geysers function
regularly, day after day, week after week, and year after year. Within
this group of regulars is the most famous feature of all--Old
Faithful--which has not missed an eruption in all the many decades that
it has been under close observation (fig. 52). We can only conclude that
nature has provided this incredible geyser with a stable plumbing system
that is just right to trigger delightfully graceful eruptions at
close-enough time intervals to suit the convenience of all Park
visitors.


                                Mudpots

Mudpots are among the most fascinating and interesting of the
Yellowstone thermal features. They are also a type of hot spring, but
one for which water is in short supply. Whatever water is available
becomes thoroughly mixed with clay and other fine undissolved mineral
matter. The mud is generally gray, black, white, or cream colored, but
some is tinted pale pink and red by iron compounds (fig. 43); hence, the
picturesque term "paint pots" is commonly used.

Mudpots form in places where the upflowing thermal fluids have
chemically decomposed the surface rocks to form clay. Such small amounts
of water are involved, however, that the surface discharge is not great
enough to flush the clay out of the spring. Caldrons of mud of all
consistencies result, from the very thin soupy material in many mudpots
to the almost hard-baked material in the less active features. Some
mudpots expel pellets of very thick viscous mud which build up circular
cones or mounds; this type is commonly called a "mud volcano" (fig. 53).

    [Illustration: SEISMIC GEYSER, showing rock rubble blown out during
    an explosive thermal eruption. Note the trees that have been killed
    by the heat and eruptive activity. According to George D. Marler of
    the National Park Service, this geyser developed from cracks caused
    by the Hebgen Lake Earthquake of August 17, 1959. (Fig. 51)]

    [Illustration: OLD FAITHFUL IN FULL ERUPTION. The interval between
    eruptions averages about 65 minutes, but it varies from 33 to 96
    minutes. The time lapse between eruptions can be predicted rather
    closely, mainly on the basis of the length of time involved in the
    previous eruption. If an eruption lasted 4 minutes, for example,
    this means that a certain amount of water emptied from the geyser's
    chambers and that a certain length of time will be necessary to
    recharge the system for the next eruption. But if the previous
    eruption lasted only 3 minutes, less time will be needed for
    recharge, and the next eruption will occur sooner. (The above
    discussion is based primarily on many years of observation and study
    of Old Faithful by George D. Marler and other observers of the
    National Park Service; photograph courtesy of Sgt. James E. Jensen,
    U.S. Air Force.) (Fig. 52)]

Mudpot activity differs from season to season throughout the year
because of the varying amounts of rain and snow that fall upon the
surface to further moisten the mud. Accordingly, mudpots are commonly
drier in late summer and early fall than they are from winter through
early summer.


                               Fumaroles

Fumaroles (from the Latin word _fumus_, meaning "smoke") are those
features that discharge only steam and other gases such as carbon
dioxide and hydrogen sulfide; hence, they are commonly called "steam
vents." Usually these features are perched on a hillside or other high
ground above the level of the flowing springs. In many fumaroles,
however, water can be heard boiling violently at some lower, unseen
level.


                           Thermal explosions

A few features present in the Yellowstone thermal areas display evidence
that extremely violent thermal explosions occurred in the past,
particularly during Pinedale Glaciation, about 15,000 years ago. Such
explosion features, of which Pocket Basin in Lower Geyser Basin is a
good example, appear as craterlike depressions a few tens of feet to as
much as 5,000 feet across surrounded by rims of rock fragments that were
blown out of the craters. The underground mechanism causing the
explosions was similar to that of geysers, but in these special cases
the energy remained bottled-up until a very critical explosive stage was
reached.

The best explanation for Pocket Basin and related features is that the
ground above the sites of the explosions was weighted down by the water
of small lakes which had formed in melted-out pockets of glacial ice.
Such localized melting of the glaciers would occur where the ice was in
direct contact with underlying thermal features. A rapid draining of the
lake waters would then produce a sudden release of pressure over the hot
area, resulting in an unusually violent thermal eruption.

    [Illustration: MUD VOLCANO near Pocket Basin in the Lower Geyser
    Basin. The mud is formed by chemical decomposition of the rocks
    chiefly by the action of carbon dioxide and sulfuric acid. The
    splatter, 5-6 feet high, is caused by the escaping gases. (Fig. 53)]


              Faulting and its control of thermal activity

Most of the major thermal areas of Yellowstone are related to the ring
fracture zones of the Yellowstone caldera (fig. 22). Many deep-seated
faults and fractures in these zones are presumably situated above the
main source of heat of the thermal system. Thus, they provide convenient
avenues of travel for underground waters to circulate to great depths,
there to become heated and then rise to the earth's surface (fig. 45). A
few areas like Mammoth Hot Springs and Norris Geyser Basin, on the other
hand, are not within the ring fracture zones of the caldera. In these
areas, the thermal activity is commonly related to other prominent zones
of faulting which also afford readymade channelways for the circulation
of hot water and steam.




                              Earthquakes


Earthquakes occur frequently in areas of active faulting and volcanism;
they are caused by sudden movements between adjacent blocks of the
earth's crust as the crust adjusts to new conditions and pressures.
Because of its volcanic history and the fact that very recent fault
movements have occurred there, it is not surprising that Yellowstone is
an especially active earthquake area. Sensitive instruments
(_seismographs_) record an average of about five earth tremors daily in
and around the Park, and on rare occasions they may record 100 tremors
or more in a single day. Nearly all these tremors are so slight that
they cannot be felt by man, but at times, perhaps only once in a human
lifetime, one is triggered with high enough intensity to sharply draw
our attention to the very real earthquake potential that exists
constantly in this geologically active area. Such a high-intensity quake
occurred in the Yellowstone region near midnight on August 17, 1959.

The Hebgen Lake Earthquake, as it is known, was centered in the Madison
Valley along the west boundary of Yellowstone National Park about 12
miles north of the town of West Yellowstone, Montana (fig. 1). As a
result of the quake, a 200-square-mile area, occupied in part by the
Hebgen Lake reservoir, subsided a foot or more; maximum subsidence was
20 feet. Movements of several feet along old faults in the highlands
along the north side of the valley produced fresh scars several miles
long (fig. 54). Moreover, the severe vibrations that rocked the
surrounding countryside caused the loose silt, sand, and gravel of the
valley floor to slip and become "faulted" in many places. By far the
most drastic result was the shaking loose of a huge landslide in the
vicinity of the Rock Creek campground about 25 miles downstream on the
Madison River from the west boundary of the Park.

    [Illustration: EARTHQUAKE DAMAGE. Severe damage caused by
    reactivation of a fault during the Hebgen Lake Earthquake of August
    17, 1959. The building is on the Blarneystone Ranch, about 10 miles
    north of West Yellowstone, Montana, and 1½ miles west of the west
    boundary of Yellowstone National Park. (Fig. 54)]

Within Yellowstone National Park, the quake caused only slight damage to
buildings at Old Faithful, Mammoth Hot Springs, and a few other places.
Small landslides also occurred in various places, for example, at Tuff
Cliff near Madison Junction. The earthquake affected many thermal
features, particularly those in the main geyser basins near the west
side of the Park. In several places the intensity of the thermal
activity increased markedly, in fewer places the activity decreased.
Some geysers, long dormant, erupted immediately after the earthquake;
others erupted with much greater force and frequency than usual; still
others became dormant and have remained so. A general, widespread effect
was a noticeable increase in the muddiness of many pools and springs, as
if the quake had produced a giant surge of water coursing through the
underground channels of the geyser basins. Of immediate concern to
everyone, of course, was the earthquake's effect on Old Faithful.
Fortunately, the only measurable effect was a slight lengthening of time
between eruptions. After several months the time interval stabilized at
about 65 minutes.

Detailed scientific studies bearing on the Hebgen Lake Earthquake in the
weeks that followed showed that it was felt over more than 600,000
square miles of the Western United States, and that it was the strongest
shock ever recorded in this part of the Rocky Mountains. In 12 years'
time many traces of the quake have disappeared, but its frightful
aspects will not soon be forgotten. It serves as a vivid reminder, once
again, of the great restlessness that through the ages has been, and
continues to be, the very special trademark of the Yellowstone country.




                           _The Park and Man_


A hundred years ago another powerful force entered the Yellowstone
scene. Man, arriving in ever increasing numbers, came armed with the
power to choose between preserving or destroying the wonders that nature
has taken more than 2.5 billion years to create. Sensing this grave
responsibility, he took the necessary steps to insure that these
irreplaceable natural features would be preserved and protected. Today,
Yellowstone National Park indeed exists "for the benefit and enjoyment
of the people," a fitting and lasting symbol of a great national
heritage that now includes more than 275 places of natural and
historical interest. On the eve of the 100th anniversary of our first
National Park, we are again reminded of that continuing responsibility
we all share in preserving these unique places for the benefit and
enjoyment of all future generations of visitors.




                           _Acknowledgments_


The subject matter of this bulletin is based chiefly on the results of a
systematic program of geological investigations in Yellowstone National
Park, conducted by the U.S. Geological Survey during the years 1965-71.
The program, ably organized and directed by A. B. Campbell, required the
special skills and knowledge of many individuals to make a comprehensive
study of all the varied and complex features of the Park area. Without
their invaluable cooperation, assistance, and interest, this endeavor to
summarize the geologic story of Yellowstone would not have been
possible. I therefore express my sincere thanks to the colleagues listed
below, all of whom furnished unpublished information bearing on
different aspects of that story: R. L. Christiansen and H. R. Blank, Jr.
(Quaternary volcanism); H. W. Smedes and H. J. Prostka (Absaroka
volcanism); D. E. White, L. J. P. Muffler, R. O. Fournier, and A. H.
Truesdell (thermal activity); G. M. Richmond, K. L. Pierce, and H. A.
Waldrop (glaciation); E. T. Ruppel and J. D. Love (sedimentary rocks and
geologic structure); J. D. Obradovich and Meyer Rubin (radiometric
dating). W. L. Newman provided many helpful suggestions regarding the
preparation of the manuscript.

The geological studies in Yellowstone received the full support and
cooperation of former Park Superintendent J. S. McLaughlin,
Superintendent J. K. Anderson, and other personnel of the U.S. National
Park Service. In particular, the helpful advice, interest, and
enthusiasm of the entire Park Naturalist staff, especially J. M. Good
and W. W. Dunmire, former and present Chief Park Naturalists,
respectively, greatly facilitated the work in all phases of the program.




                               Footnotes


[1]The specific area about which the early-day Indians first used the
    term that is now translated as "Yellowstone" is unknown. The name
    may have referred to the yellowish rocks that line the banks of the
    Yellowstone River near its confluence with the Missouri River in
    eastern Montana and western North Dakota. However, in the opinion of
    H. M. Chittenden, who studied the question in considerable detail,
    there is little doubt that the name was taken from the striking
    yellow-hued walls of the gorge now known as the Grand Canyon of the
    Yellowstone.




                     _Selected Additional Reading_


Allen, E. T., and Day, A. L., 1935, Hot springs of the Yellowstone
National Park: Carnegie Institution of Washington Publication 466, 525
pages.

Boyd, F. R., 1961, Welded tuffs and flows in the rhyolite plateau of
Yellowstone Park, Wyoming: Geological Society of America Bulletin,
volume 72, number 3, pages 387-426.

Christiansen, R. L., and Blank, H. R., Jr., 1972, Volcanic stratigraphy
of the Quaternary rhyolite plateau in Yellowstone National Park: U.S.
Geological Survey, Professional Paper 729-B (in press).

Dorf, Erling, 1960, Tertiary fossil forests of Yellowstone National
Park, Wyoming, _in_ West Yellowstone--Earthquake area, Billings
Geological Society Guidebook 11th Annual Field Conference, 1960: pages
253-260.

Hague, Arnold, Iddings, J. P., Weed, W. H., Walcott, C. D., Girty, G.
H., Stanton, T. W., and Knowlton, F. H., 1899, Geology of the
Yellowstone National Park: U.S. Geological Survey Monograph 32, part 2,
893 pages and atlas of 27 sheets folio.

Hague, Arnold, Weed, W. H., and Iddings, J. P., 1896, Description of the
Yellowstone National Park quadrangle [Wyoming]: U.S. Geological Survey
Geologic Atlas, Folio 30.

Hayden, F. V., 1872, Preliminary report of the United States Geological
Survey of Montana and portions of adjacent Territories, being a fifth
annual report of progress--Part I: Washington, U.S. Government Printing
Office, pages 13-204.

Howard, A. D., 1937, History of the Grand Canyon of the Yellowstone:
Geological Society of America Special Paper 6, 159 pages.

Marler, G. D., 1969, The story of Old Faithful: Yellowstone Library and
Museum Association, 49 pages.

Richmond, G. M., Pierce, K. L., and Waldrop, H. A., 1972, Surficial
geologic map of Yellowstone National Park: U.S. Geological Survey
Miscellaneous Geologic Investigations Map I-710 (in press).

Ruppel, E. T., 1972, Geology of pre-Tertiary rocks in the northern part
of Yellowstone National Park, Wyoming: U.S. Geological Survey
Professional Paper 729-A (in press).

Smedes, H. W., and Prostka, H. J., 1972, Absaroka Volcanic Supergroup in
the Yellowstone National Park region: U.S. Geological Survey
Professional Paper 729-C (in press).

Smith, R. L., 1960, Ash flows: Geological Society of America Bulletin,
volume 71, number 6, pages 795-841.

U.S. Geological Survey, 1964, The Hebgen Lake, Montana, earthquake of
August 17, 1959: US Geological Survey Professional Paper 435, 242 pages.

U.S. Geological Survey, 1972, Geologic map of Yellowstone National Park:
U.S. Geological Survey Miscellaneous Geologic Investigations Map I-711
(in press).

White, D. E., 1967, Some principles of geyser activity, mainly from
Steamboat Springs, Nevada: American Journal of Science, volume 265,
number 8, pages 641-684.

                      * U. S. GOVERNMENT PRINTING OFFICE: 1972 O-467-725

    [Illustration: U. S. DEPARTMENT OF THE INTERIOR · March 3, 1849]




                          Transcriber's Notes


--Retained publication information from the printed edition: this eBook
  is public-domain in the country of publication.

--Corrected a few palpable typos.

--Included a transcription of the text within some images.

--Renumbered footnotes, and modified references to them accordingly.

--In the text versions only, text in italics is delimited by
  _underscores_.