Chapter I: Part 1

_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

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.]

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

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.

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.)

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.

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_.

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.

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).

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).

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.

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

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).

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.

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_.

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).

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).

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.

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.

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).

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.

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.

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).]

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.

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.]

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).

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).

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.

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 24×20×18 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).

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.