Roadside Geology of Yellowstone Country: Second Edition

ROADSIDE GEOLOGY OF

YELLOWSTONE COUNTRY

William J.Fritz and

Robert C.Thomas

2011

Mountain Press Publishing Company

Missoula, Montana

© 2011 by William J. Fritz and Robert C. Thomas

First Printing, September 2011

All rights reserved

Photos © 2011 by William J. Fritz and Robert C. Thomas

unless otherwise credited

Cover map modified from Christiansen, R. L. 2001. The Quaternary and Pliocene Yellowstone Plateau volcanic field of Wyoming, Idaho, and Montana. U.S. Geological Survey Professional Paper 729-G.

Geologic road maps and many of the illustrations constructed by

Mountain Press Publishing based on original drafts by authors.

Roadside Geology is a registered trademark

of Mountain Press Publishing Company

Library of Congress Cataloging-in-Publication Data

Fritz, William J., 1953-

Roadside geology of Yellowstone country.—2nd ed. / William J. Fritz and Robert C. Thomas.

     p. cm.—(Roadside geology series)

Includes bibliographical references and index.

ISBN 978-0-87842-581-5 (pbk. : alk. paper)

1. Geology—Yellowstone National Park Region—Guidebooks. 2. Yellowstone National Park—Guidebooks. I. Thomas, Robert C. (Robert Curtiss), 1962- II. Title.

QE79.F75 2011

557.87'52—dc23

2011024722

PRINTED IN HONG KONG

P.O. Box 2399 • Missoula, MT 59806 • 406-728-1900

800-234-5308 • info@mtnpress.com

www.mountain-press.com

I am convinced that, at its best, science is simple—that the simplest arrangement of facts that sets forth the truth best deserves the term scientific. So the geology I plead for is that which states facts in plain words—in language understood by the many rather than by the few.

—George Otis Smith, 1921, director,

U.S. Geological Survey, 1907–1930

This book is dedicated to Roderick (Rick) A. Hutchinson (1947–1997). Rick was a friend, geologist, geyser-gazer extraordinaire, scientist, and true friend of Yellowstone who always knew how many days it was to Christmas. Rick was tragically killed in a snow avalanche near Heart Lake while studying the geysers he loved. Rick's research and insights on Yellowstone's thermal features shaped much of the thinking presented in this book. He was a reviewer of the first edition of this book and greatly helped improve its readability and content. May his spirit live on in Yellowstone Country.

Location map of roads covered in this book.

Contents

Preface to the Second Edition

Introduction

Overview

Mountain Building

Fossil Forests

The Track of the Yellowstone Hot Spot

Quaternary Volcanism

Glaciation

Road Guides

Livingston—Gardiner (US 89)

Bozeman—Big Sky Junction (US 191)

Big Sky Junction—West Yellowstone (US 191)

West Yellowstone—Norris

Norris—Mammoth Hot Springs

Gardiner—Tower Junction

Tower Junction—Cooke City

Cooke City—Red Lodge (US 212)

Junction of US 212 and Wyoming 296—Cody

Tower Junction—Canyon Village

Norris—Canyon Village

Canyon Village—Fishing Bridge

Madison—West Thumb

West Thumb—Fishing Bridge

West Thumb—South Entrance

Fishing Bridge—East Entrance

East Entrance—Cody (US 14/16/20)

West Yellowstone—Earthquake Lake Visitor Center (US 191 and US 287)

West Yellowstone—Ashton (US 20 and Idaho 47)

Glossary

Outside Reading

Index

Stratigraphic column for Yellowstone Country.

Stratigraphic list of selected volcanic units of the Yellowstone Plateau volcanic field with their approximate ages. Each unit may contain one to numerous flows. The three tuffs related to the caldera-forming eruptions are shown in boldface type. Note that each volcanic cycle starts with pre-caldera lava flows, climaxes with pyroclastic-flow deposits (tuffs) and the formation of a caldera, and concludes with post-caldera eruptions of rhyolite and basalt lava flows. Compiled from Christiansen (2001).

Preface to the Second Edition

We wrote this book to introduce the nongeologist to some of the interesting history written in the rock of Yellowstone Country. We hope to help residents and visitors better appreciate the countryside and to provide a more meaningful trip to some out-of-the-ordinary areas. This book is not for our professional colleagues. It assumes no previous knowledge of geology or science. However, we also hope that some of our colleagues will find this a useful guide for their first visit to the area or for leading students on field trips and for geology students of all ages studying the fascinating area known as Yellowstone.

Because this is a general book that touches on many aspects of the local geology, we have summarized the published work of many geoscientists. We hope that we have done justice to their interpretations and thank them for their work. In the Outside Reading section, we list some of the more important references to the geology of Yellowstone Country. Because this book is intended for the general reader, the list is far from complete for a research scientist and does not acknowledge all outside papers that we read while writing this book. If you are really interested in researching the geology of Yellowstone Country, much of the technical literature to date is cited in this book.

This second edition is greatly expanded over the 1985 first edition. For starters, the notion of a track of the Yellowstone hot spot was not well understood in the early 1980s. Most of the work showing the track of the Yellowstone hot spot over the past 16.5 million years, with nested calderas along the Snake River Plain and the profound impact that the hot spot had on the modern basins and ranges north and south of the track, are recent developments in geologic thinking. The main reason for this new edition is to bring the book into line with current geologic thought.

One criticism of the first edition was that it said very little about tectonics and orogenies (mountain building events), which are central parts of the Yellowstone story. We have tried to remedy these deficiencies by writing several new sections, adding new road guides, and revising all of the old text to weave in these topics. We hope that you will find this a useful update. Of course, readers of the old version will notice new color photos along with maps and diagrams in full color!

Both of us have led students and professionals on field trips through Yellowstone Country throughout our careers and have mentored graduate and undergraduate research projects on the geology of the region. We learned a great deal about the geology of Yellowstone Country from these experiences and are thankful for the perceptive questions of the participants because they forced us to rethink what we were sure we knew about the geology.

We generalized the geology of the road guide maps from numerous publications, road logs, field guides, and technical maps so that you may more easily interpret the general geologic history without getting bogged down in details. There are two important generalizations you should keep in mind when using the road guide maps. First, rather than differentiating individual formations, we combined many similar units of rock. Examples include some of the volcanics of Quaternary age, which we combined into a much smaller number of units, and all the rock formations of Paleozoic and Mesozoic age, which we combined into two units. Second, we excluded most of the thin skim of recent stream, lake, glacial, and soil cover. Thus the map units refer to underlying bedrock; overlying deposits are shown only when very thick, extensive, and obscuring all bedrock.

To get the most out of the book, we suggest that you first read the introductory sections, and then read the entire road guide for your planned trip. As you drive, keep track of the geology on the road guide maps. If you carefully read the entire book, you will find some repeated information and descriptions, which was intentional. We tried to make each road guide as self-contained as possible, so that it can be used without a lot of searching for a previous description of a term, date, or process. Thus, by necessity, we repeat information in some sections. A glossary of technical terms is included at the end of the book, but we attempted to explain as many terms within the text as possible in order to save the reader time in looking them up.

Bill Fritz wishes to thank the staff of Yellowstone National Park for their cooperation both in his past research in Yellowstone and for reviewing the first edition of this book; however, the park is not responsible for errors contained in this volume. Bonnie Fritz has edited many versions of the manuscript throughout the years, endured long hikes in the Yellowstone backcountry, and asked probing questions that sharpened his understanding of the rocks. Dave Alt provided inspiration and help in writing the first edition, and Lanny H. Fisk first introduced him to the wonders of Yellowstone Country.

Rob Thomas wishes to thank the participants of the many Geological Society of America GeoVenture trips he co-led in Yellowstone Country. They helped him to learn the regional geology and hone his communication skills. In addition, the students and faculty of the Yellowstone-Bighorn Research Association geology field camp and The University of Montana Western have been instrumental in providing him with incentives to better understand the geology of the region. Discussions with Dick Berg, Gillian Foulger, Marc Hendrix, Dave Malone, Marli Miller, Lisa Morgan, Ken Pierce, Sheila Roberts, William Sager, David Schwartz, Jim Sears, Robert B. Smith, Mike Stickney, Charles Wicks, Don Wise, and Huaiyu Yuan were invaluable. They are not, however, responsible for any errors. Bud Burke, Don Winston, and Jody Bourgeois inspired his passion for field geology during his career. He owes them all a debt of gratitude for helping him to discover the great life of a field geologist. The largest sacrifices of such a project ultimately come from family, so his deepest gratitude goes to Anneliese Ripley, Abbey Thomas, and Haley Thomas.

The first edition was improved by thoughtful criticism and advice from Dave Alt, Norman A. Bishop, Robert L. Christiansen, Lanny H. Fisk, Wayne Hamilton, Sylvia Harrison, Jennifer Hutchinson, Roderick A. Hutchinson, Timothy E. La Tour, Timothy R. Manns, and Jana Morman. Since the second edition incorporates color, we sought out geoscientists and Yellowstone enthusiasts for images that would help illustrate the geology. The photographic talents of Pete Bengeyfield, Hans Giersberg, Tom Hauge, Wade Johnson, Alexandre Lussier, David H. Malone, Marli Miller, Jim Peaco, J. Schmidt, Richard Tollo, and Lyudmila Zinkova greatly improved the visual quality of the book. Dr. Ronald C. Blakey generously provided access to his widely recognized and respected paleogeographic maps.

BILL FRITZ,

College of Staten Island,

The City University of New York,

Staten Island, New York, and the

CUNY Graduate Center, March 2011

ROB THOMAS,

The University of Montana Western,

Dillon, Montana, March 2011

Introduction

The Earth is slightly older than 4.5 billion years, and the rock exposed in Yellowstone Country provides an incomplete record of more than two-thirds of this history. The record is incomplete because there are many unconformities, places in the rock record where chunks of time are not accounted for. The geologic story of Yellowstone Country begins with rocks that are at least 3.5 billion years old, some of which are mashed remnants of even older rock bodies that have been dated to nearly 4 billion years old, and continues with processes that are still shaping the land's surface today. In the following sections we will attempt to summarize this history, hitting the highlights and concentrating on features you can actually see in Yellowstone Country. We have provided background information about mountain building, fossil forests, the track of the Yellowstone hot spot, Quaternary volcanism, and glaciation to help you understand the processes that formed the rock units and landforms you will see as you follow the road guides. We wrote this book for people with no training in geology. Nevertheless, we use some technical words, so definitions are available in the glossary.

Nobody can safely appreciate and understand the geology of Yellowstone Country while driving, so throughout the book we have tried to point out pullouts and rest areas where you can safely get out of your car and take a closer look at rocks and landforms. Collecting is prohibited in Yellowstone National Park. If you want to collect specimens, visit localities outside of the park. Nearly all rock units exposed in the park also occur along the roads outside of the park. Please note that you are required to have a permit to collect certain fossils even outside of the park. It is, however, always appropriate to take only pictures so that others can also enjoy the natural treasures of Yellowstone Country. When hiking in the park, it is always advisable to first check with a park ranger regarding weather conditions, level of streams to be crossed, recent bear sightings, and so on. Also, please stay out of all hot springs and thermal features, and remain on the boardwalks and paths around them—even relatively cool thermal features can scald. We have included side trips on dirt roads outside of the park. Check with authorities before taking any of these because snow and impassable muddy conditions can remain well into the late spring and early summer.

There are plentiful online resources available for Yellowstone National Park, including a great deal of information about the geology. The National Park Service's official Web site (www.nps.gov/yell) provides beneficial information to help you plan your visit, and the Nature & Science link provides background information about Yellowstone geology, including online videos of presentations by professional geoscientists and microbiologists who are conducting research in the park. Some of the other beneficial not-for-profit Web sites for park geology include the Yellowstone Association (www.yellowstoneassociation.org); the Yellowstone Volcano Observatory (volcanoes.usgs.gov/yvo), sponsored by the U.S. Geological Survey, Yellowstone National Park, and The University of Utah; the Research Coordination Network, a National Science Foundation site that provides information about the park's thermal features (www.rcn.montana.edu/?pg=about&nav=1); and a virtual 3D tour available through the U.S. Geological Survey (3dparks.wr.usgs.gov). The historic 3-D tour of Yellowstone on this site includes original stereographs purchased as souvenirs by Emma Sanor of Minerva, Ohio, while visiting Yellowstone in 1916. The tour is well worth your time.

Old Faithful Geyser erupting in winter. —Courtesy of Pete Bengeyfield

On a final note, the capitalization of the names of rock units depends on whether they are formally recognized; therefore, you may see rhyolite or basalt lowercased in a name like Snake River Butte rhyolite alongside a rock name capitalized differently, such as Mount Jackson Rhyolite.

The Lower Falls of the Yellowstone River. —Courtesy of Pete Bengeyfield

Overview

Our blue planet is an old and seemingly unique place. It formed about 4.56 billion years ago from the dust and gas left over from the formation of the sun, and since its infancy, Earth has undergone tumultuous change. More so than many places on Earth, Yellowstone Country records a large percentage of Earth's history, and geologists have learned to read the record by learning how to read the story in the rocks. This is a relatively easy process that requires only a few basic principles, some of which were developed nearly 350 years ago by Danish anatomist and geological pioneer Nicolas Steno.

The rock record is read like a book, only from the back to the front. The oldest rock exists at the bottom of the stack, unless disrupted by forces (plate tectonics) that disturb the order. Although there are numerous exceptions, many layered rocks were originally horizontal, so if they are tilted or folded, it likely occurred after the layering formed. If other rocks or faults (fractures in the rock with significant displacement) cut across the layered rocks, they must be younger than the rocks they cut. In addition, layered rocks are somewhat laterally continuous, which means that you can follow a layered rock in all directions until it thins and changes to another type of layered rock or is cut by an intrusive rock (igneous rock) or a fault.

It is important to understand that the sequence of rocks in any given place on the planet represents only a part of the Earth's long history, because in no one place is there a continuous accumulation of rock. The geologic record is riddled with gaps called unconformities, and in any one place there are more unconformities than geologic record. Most of the time, the unconformities represent times when a part of the Earth was being eroded. Other times, unconformities exist simply because no rock record was accumulating. Regardless of how they form, geologists learn as much about Earth history from the gaps in the record as they do from the record itself! Ultimately, Earth history is reconstructed from piecing together all of the rock sequences studied from every corner of our planet.

Geologists are able to correlate rocks from one place to another through a variety of methods, but one of the most common ways is to use the fossilized remains of animals and plants that are preserved in the rocks. Because of biological evolution, different life existed on the planet at different times. If you know the sequence of life, you can correlate rocks around the globe by correlating fossils of similar organisms that lived at similar intervals of time. This method not only allows geologists to determine what was happening spatially across the Earth at any particular time interval in the past, but it also provides a way to understand how the Earth has changed over time.

The first geologic timescale was not based on time at all, because hundreds of years ago humans did not understand how to determine the numerical ages of rocks. The early timescale was a relative timescale built upon the relative sequence of fossils found in the rocks around the world. The names on the time-scale were derived from a variety of sources, with some based on the first or best places to study rocks of a particular span of Earth history. For example, the Devonian Period was named after the English county of Devon, where rocks from this period were first studied, and the Permian Period was named after Perm, Russia, for similar reasons. The boundaries of the time periods were typically placed at major changes in the fossils found in the rocks, some of which were the result of mass extinctions, which are times when many different types of organisms go extinct over relatively short periods of time across the globe.

Late in the eighteenth century, James Hutton, a Scottish physician, naturalist, chemist, and experimental farmer published the idea of deep time. This was the idea that the Earth was very old, not 6,000 or 7,000 years old, as was the biblical view at the time. Although he didn't know the age of the Earth, he famously wrote that it has no vestige of a beginning, no prospect of an end. It was not until the discovery of radioactivity in 1896 and its application to geology in the early twentieth century that numerical ages would begin to be assigned to the geologic timescale. By 1913, British geologist Arthur Holmes estimated the Earth's age to be at least 1.6 billion years, about three times less than what we calculate today. The timescale and the ages of rocks in Yellowstone Country continue to change as new dating methods are developed and techniques are improved.

Radioactivity occurs because the forces binding protons and neutrons together in some atoms are not strong enough, and the nuclei spontaneously break apart, or decay. Over time, unstable (parent) atoms decay into stable (daughter) isotopes at a constant rate. The time it takes for half of the parent to decay is called the half-life of the isotope, and if it is known, and the parent-to-daughter ratio can be measured in a rock sample, then the age of that rock can be calculated. The clocks start ticking when unstable atoms reach certain temperatures as liquid rocks crystallize or heated rocks recrystallize.

This age-dating technique has been used to determine the numerical age of all of the rocks in Yellowstone Country. It is hard for us to conceive of billions of years, but it is easier to understand when all of Earth history is put into the context of a single year. In Yellowstone Country, the oldest rocks date from about mid-March in this theoretical year. Living things first appeared in tropical oceans in May, and plants emerged on land in late November. The Rocky Mountains were first uplifted and dinosaurs roamed the coastal margins of an interior sea around mid-December. The Absaroka Volcanic Supergroup (a major group of rocks in Yellowstone Country) was being erupted in late December, and the Yellowstone hot spot reached its current position on the evening of December 31. The most recent glacier to cover the Yellowstone area reached it maximum size about 1 minute and 30 seconds before midnight, just about the time humans arrived in the northern Rockies. Columbus landed in the Bahamas about 3 seconds before midnight, and geology was born with the writings of James Hutton just slightly more than 1 second before the end of the year.

Geologic time allows us to understand how so much change could have occurred in Yellowstone Country, but in order to understand why these changes happened, it is crucial to understand the processes of plate tectonics. This theory describes and explains why Earth's surface is so mobile and dynamic, from rising mountains to spewing volcanoes and widening ocean basins. Before the plate tectonic revolution in the 1960s, most geoscientists considered Earth to be static, with only up and down movements of the crust, which comprises the rock of the continents and seafloor. An early version of plate tectonics called continental drift was proposed in the early twentieth century by German meteorologist Alfred Wegener, based on evidence that continents were once part of a single mass, or supercontinent, called Pangaea (Greek for all lands), and had since drifted to their current positions after Pangaea broke apart. Although abundant evidence showed that the continents had drifted apart, Wegener's hypothesis was rejected because of his assertion that the continents floated like boats across solid oceanic crust, dragged along by tidal forces—a physical impossibility.

To understand the theory of plate tectonics, it is necessary to first have a basic understanding of the structure of the Earth. Analogous to a hard-boiled egg, Earth has three different layers: The crust, or eggshell, is made up of rocks of the continents and seafloor; the mantle, or egg white, composes the bulk of the planet; and at the center is the core, or yolk. The upper mantle is not homogenous. Its lower portion, called the asthenosphere, is a weak region composed of hot, mushy rock in a plastic state, meaning it can flow very slowly (think taffy). The upper portion is rigid, and together with the crust it is called the lithosphere. Basically, the theory of plate tectonics posits that the outer layer of Earth, the lithosphere, is broken into large slabs (the size of continents or larger), or tectonic plates, which move independently of one another on top of the weak asthenosphere.

Our understanding of the causes of plate motion are a work in progress, but there is general agreement that it results, in part, from convection, or movement, in the asthenosphere that is driven by Earth's internal heat. Earth gets hotter with depth, and it is still very hot mostly because of radioactivity, which is the breakdown of unstable radioactive elements such as uranium, potassium, strontium, and many others. This is the same energy harnessed for use in nuclear weapons and nuclear power plants. Earth will remain tectonically active far into the future until all of the unstable elements convert to stable ones. Then the Earth will become a dead planet like Mars. Mars was once active, like Earth, but because it is smaller it ran out of nuclear energy.

As the heat rises from the mantle and spreads outward, it causes the overlying lithosphere to move with it, moving the plates. Where rising asthenosphere diverges, the overlying lithosphere diverges, creating what's called a spreading ridge at the surface. These ridges occur along seafloors and are where new ocean floor is made as partially melted mantle rock wells up to the surface. As new ocean lithosphere forms at the spreading ridge, it pushes lithosphere farther and farther from the ridge, like a conveyor belt. This continuous process is called seafloor spreading. The theory of plate tectonics came into being in the 1950s and early 1960s as extensive exploration of the North Atlantic Ocean floor showed that there is an extensive mountain range with a deep valley, or rift, running down its center. In the early 1960s, Harry Hess of Princeton University and Robert Dietz of the Scripps Institution of Oceanography concluded that new seafloor was forming at this rift and moving away from it as more ocean floor formed.

As the lithosphere ages, it becomes cold and dense and eventually sinks, or is subducted, back into the mantle where it melts. Basically, it is recycled. Subduction plays an important role in plate motion, because as the ocean floor descends into the mantle it acts like an anchor, pulling the remainder of the plate across Earth's surface. It is now thought that the sinking slabs of ocean lithosphere may descend as deeply as the core-mantle boundary, indicating that the entire mantle is somehow involved in the convection system that recycles the plates. The total number of plates is a matter of some debate, but all geoscientists agree that there are about seven major tectonic plates, six intermediate-sized plates, and over a dozen smaller plates.

From earthquakes to volcanoes, most of the geologic action takes place at the boundaries between tectonic plates rather than within the plates. Plates can interact with one another in three ways—converge, diverge, or slide past one another. Where an oceanic plate converges with another oceanic plate or a continental plate, a deep ocean-trench, or subduction zone, forms as the one oceanic plate dives beneath the other plate and heads into the mantle. Subduction zones produce mega-earthquakes and tsunamis and chains of explosive volcanoes. Where two continental plates collide, some of the highest mountains on Earth are formed, like the Himalayas, which are uplifting at a rate of 3.9 inches (10 cm) a year because the subcontinent of India is shoving into and under Asia. Thrust faults and high-angle reverse faults develop in the rocks in these two types of collision zones, and the rocks are also intensely folded and metamorphosed (changed by heat and pressure).

Cross section of Earth's interior. The upper mantle and crust are divided into a rigid region called the lithosphere, which rests on a weak region called the asthenosphere.

The major tectonic plates of the Earth. The arrows show the relative movement at the plate boundaries. —Modified from the U.S. Geological Survey

Where plates diverge, a mid-ocean spreading ridge ultimately forms, which is where new ocean floor develops and continents are separated from each other over time. For example, the Mid-Atlantic Ridge beneath the Atlantic Ocean, which is the longest mountain range in the world, has been separating the Eurasian Plate from the North American Plate at a rate of about 1 inch (2.5 cm) per year to create the North Atlantic Ocean over the last 130 million years. Divergent plate boundaries start developing on land, causing continental lithosphere to fracture along high-angle extensional faults to form what is called basin and range topography as the lithosphere is pulled apart. The steep-sided mountain ranges that form in such extensional environments can be imposing, such as the Teton Range, which forms well over 1 mile (1.6 km) of nearly vertical relief along its faulted flank relative to the down-dropped valley that is Jackson Hole. Over time, divergence can produce a rift valley (a long, narrow valley that lies between two extensional faults) like the Great Rift Valley of East Africa, linear seas like the Gulf of California and the Red Sea, and ultimately an ocean basin like the Atlantic Ocean.

Folded metamorphic rocks in the Himalayas, near Mt. Everest, that formed as India collided with Asia.

The third type of plate boundary, called a transform boundary, is where plates slide horizontally past each other. These occur primarily on the ocean floor, where they offset mid-ocean spreading ridges into steplike patterns (if viewed from above). However, transform boundaries do occur on land, like the