Yellowstone Geology

by Susan McCloskey

illustrated by Adair Payne

YellowstoneYellowstoneGeology at Work

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Photo Credits: Cover Spread, Jeff Henry; p. 2-3, 16, Jeff Henry; p. 4, Jeff Henry;p. 7, Barbara Magnuson/Visuals Unlimited; p. 8, Jeff Henry; p. 9, L. Linkhart/Visuals Unlimited;p. 11, Jeff Henry; p. 12, The National Park Service; p. 13, The National Park Service; p. 14, Jeff Henry/RocheJaune Pictures; p. 15, The National Park Service

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Yellowstone is huge. Its 3,472 square miles make it big-ger than the states of Delaware and Rhode Island combined.Yellowstone is wild. It has a greater concentration of animalsthan any other area in the United States except Alaska.

Yellowstone has many wonders less fearsome than steam-ing volcanoes. You can walk and hike on its mountains ormeander through its valleys. You can fish in its streams,brooks, rivers, and lakes.

If you are feeling lazy, you can just sit and wait for someof Yellowstone’s animals to show themselves. If you’re lucky,you might see elk, deer, or bison. (Be careful, though—youmight also see bear’s and mountain lion’s!) While your wait-ing, you can relax, enjoying some of the most spectacularscenery in the world.

What caused the wonders of Yellowstone? Change—change that is still going on at this very moment!

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by Susan McCloskey

illustrated by Adair Payne

Yellowstone:Yellowstone:Geology at Work

OldFaithfulGeyser

ContinentalDivide

MONTANAWYOMINGMommoth

Hot Springs

GallatinRange

RooseveltLodge Yellowstone

River

Mt. WashburnLower FallsUpper Falls

MorningGlory Pool

Devil’sPunchbowl

ShoshoneLake

YellowstoneLake

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CONTENTSThe Wonders of Yellowstone. . . 3The Battle of Yellowstone . . . . . 5Volcanoes . . . . . . . . . . . . . . . . . 6Glaciers . . . . . . . . . . . . . . . . . . 9Geysers. . . . . . . . . . . . . . . . . . 10Hot Springs . . . . . . . . . . . . . . 12Mud Pots . . . . . . . . . . . . . . . . 13Yellowstone’s Future . . . . . . . 14Glossary . . . . . . . . . . . . . . . . . 16


THE WONDERS OF YELLOWSTONESteaming volcanoes! Shaking, quaking ground!

Plopping, slopping mud! Hissing, scalding jets of watershooting sky-high! No, theseare not scenes from somestrange planet in a farawaygalaxy. They are some of thefeatured attractions of aplace that millions of peoplevisit every year—Yellowstone National Park.

President Ulysses S. Grant set Yellowstone aside as anational park in 1872. It is the oldest national park inthe world. Most of Yellowstone is in Wyoming, but itswildest and remotest areas are in Idaho and Montana, inthe Rocky Mountain range.

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•YellowstoneNational Park


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Yellowstone is huge. Its 3,472 square miles make it big-ger than the states of Delaware and Rhode Island combined.Yellowstone is wild. It has a greater concentration of animalsthan any other area in the United States except Alaska.

Yellowstone has many wonders less fearsome than steam-ing volcanoes. You can walk and hike on its mountains ormeander through its valleys. You can fish in its streams,brooks, rivers, and lakes.

If you are feeling lazy, you can just sit and wait for someof Yellowstone’s animals to show themselves. If you’re lucky,you might see elk, deer, or bison. (Be careful, though—youmight also see bear and mountain lion!) While you’e waiting,you can relax, enjoying some of the most spectacular sceneryin the world.

What caused the wonders of Yellowstone? Change—change that is still going on at this very moment!


THE BATTLE OF YELLOWSTONEAbout 570 million years ago, large areas of what we now

call North America were flat and low. There was nothing tohold back an ancient inland sea, which flooded theYellowstone area many times. At last the sea retreated,revealing a gently rolling plain. It must have looked as if thesea had sculpted it.

Then, about 75 million years ago, a great battle began.The battle was between powerful forces in the earth’s outerlayer—its crust.

The earth’s crust is broken up into huge pieces calledplates. Plates can drift apart, or they can bump together. Thebumping causes layers of rock to slowly bend and crumple. Itcan also cause cracks called faults. Along these faults largechunks of earth can be pushed up—and up and up—untilthey become mountains. The Rocky Mountains were raisedduring this ancient battle between opposing plates.

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Mountains can form when huge chunks of earth arepushed up along a fault.


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VOLCANOESAfter the Rocky Mountains were raised, an

enormous pool of melted rock formed underYellowstone. About 2 million years ago, faults in the earth’s surface enabled melted rock toexplode from the ground. Melted rock is calledmagma when it is underground. When it comesto the surface, it is called lava.

These early eruptions of magma built upinto huge volcano cones. The cones eruptedeasily. Each time they erupted, they spewedtons of ash into the air. The ash sifted throughthe surrounding forest’s thick canopy and fell tothe ground. Trees and plants were suffocated ina layer of ash up to fifteen feet deep. Gradually,buried tree trunks petrified, or turned to stone.

Hundreds and even thousands of yearspassed between eruptions. After each one thecycle would begin again. A new layer of soilwould build up and a new forest would grow,only to be destroyed by a new eruption.Specimen Ridge in Yellowstone Park revealsproof of this cycle. Twenty-seven petrifiedforests can be found there, one on top of theother.


About 600,000 years ago, there was more volcanicactivity. Magma surged upward to form a large under-ground pool. It made the ground above it rise in a domeabout sixty miles across. The rocks at the dome’s surfacecracked because of the pressure from beneath. Thecracks reached down to the magma. A huge eruptionfollowed as gas in the magma exploded through thecracks, spewing ash fifty miles into the air. The explo-sion destroyed the surrounding countryside. Nothingwas left but smoking embers.

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So much magma exploded from the dome that itcollapsed. A huge crater was formed. After many yearswater filled the crater, creating a lake. Finally the wateroverflowed, pouring from the crater at its lowest point.It joined water from the Yellowstone River.

Over time, the river cut a canyon in the rock. It iscalled the Grand Canyon of the Yellowstone River. It isabout fifteen miles long. In some places it is more than1,000 feet deep.

Part of the canyon lies over a pool of magma. Waterheated by the magma seeps into the rock of the canyon.Slowly, over thousands of years, the rock has been bakedand boiled. Because of this slow cooking, its color haschanged from brown and gray to bright yellow.


GLACIERSSome time after the canyon was formed, the weather

turned colder. It got so cold in the mountains that wintersnow never melted in the spring. Instead, it formed sheets ofice, called glaciers. Some of the glaciers were more than3,000 feet thick.

It was not the area’s first ice age, and it would not be thelast. During each ice age the glaciers inched down the moun-tains. They scraped and scoured the landscape. They dug outvalleys and blocked rivers and streams.

The glaciers finally got to the canyon. If they had veeredoff and followed the path of the river, they would have dugout a U-shaped valley. Instead, they scraped across thecanyon, flattening the land around its edges.

Eventually, glaciers covered most of the park. When theymelted, they released boulders they had picked up duringtheir journey. They also released tons of water, which filledhollows in the ground, creating glacial lakes.

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GEYSERSThe glaciers are gone. However, the

magma pool beneath the park is still there.Heat from it causes one of Yellowstone’s mostexciting features—It’s more than 200 geysers.

What makes a geyser? Besides heat, itneeds water. Yellowstone gets a lot of water as rain or melted snow. Some of the waterruns off into rivers, streams, and brooks.Much of it seeps into the ground. The deeperit goes, the closer it gets to rocks heated bymagma, and the hotter it gets.

The water gets so hot that it should boil.However, the water is so deep and the pres-sure on it so great that it can neither boil nor turn to steam. All it can do is rise towardthe surface, and as it does, the pressure on itlessens. The water begins to boil, turning tosteam. The steam erupts from the groundthrough a small opening.

This reduces the pressure on the hotwater even more, and it begins to boil furi-ously. A column of steam bursts from theground. The explosion can be so strong thatthe ground trembles. Finally, steam stopsforming. The eruption appears to have dwin-dled and then finally stops. Then the cyclebegins again.


Old Faithful erupts every 33 to 96 minutes.Its spray sometimes goes nearly 200 feet.

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HOT SPRINGS In some places in Yellowstone, hot water can rise freely

through openings in the rock. Often it collects in bowl-shaped hollows, forming hot springs. The springs stay atabout the same temperature winter and summer. Some aretoo hot to touch.

Emerald, Sapphire, Opal, Turquoise, Topaz—the names ofsome of the hot springs describe their beautiful colors. Thesprings can be red, blue, yellow, orange, green, brown, gray,and even a combination of these colors. Some get their colorfrom minerals or from tiny plants called algae. Others gettheir color by reflecting the blue sky. These blue springs areusually the hottest of all—too hot for algae to grow in.

Grand Prismatic is the biggest hot springin Yellowstone.

Mud pots are also called paintpots. They are usually cream,

gray, or black in color.


MUD POTSMud pots are like hot springs, except that they don’t

have much water. What little water there is mixes withgases that contain a lot of acid. The resulting liquid dissolves the rock around it, creating mud of many dif-ferent colors. Rising gases can “burp” the mud severalfeet into the air. These small explosions create differentsounds, depending on where the mud pot is. Some peo-ple call it “mud pot music.”

Mud pots can be dangerous. They can form a crustthat looks thick enough to walk on, but isn’t. Falling in can result in serious burns. Because it is the park’s policy to keep the park natural, visitors cannot count onrailings and barriers to keep them safe. They must usetheir own judgment and common sense.

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YELLOWSTONE’S FUTUREYellowstone is forever changing. Parts of it are cooling off,

while others are heating up. Trees that once were green arenow dead and dry as tinder because of scalding water that hasnewly sprung up around them.

Some of these changes may seem small. However,Yellowstone may be headed for bigger changes. This isbecause its volcanoes are not dead, its pool of magma is ashot as ever, and it still has earthquakes. What does this meanfor Yellowstone’s future? No one knows for sure. Some geolo-gists—scientists who study the earth’s rocks and soil—believethat Yellowstone is in a dying stage of a volcanic cycle.

Scalded trees show the presence of new undergroundsprings of hot water.


However, other geologists remind us that cycles repeat them-selves. Even if Yellowstone’s underground activity is dyingnow, it may well heat up again.

If Yellowstone’s volcanic cycle does begin again, whenmight there be another big blast? Not very soon, according togeologists. Don’t forget, Yellowstone’s biggest eruption hap-pened 600,000 years ago. It may be an equally long timebefore there is another big explosion.

Whatever happens, it is not likely to come as a surprise.That’s because geologists use scientific equipment to recordthe movement of the earth under Yellowstone. With thistechnology, geologists can warn people if a destructive erup-tion is coming.

No one can say what Yellowstone will look like in the dis-tant future. We only know that it might look much differentfrom the way it does today.

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Yellowstone keeps itsbeauty in the winter.


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GLOSSARY

acid (p. 13) a strong substance that can burn and dissolve

algae (p. 12) small plants that have no roots or stems

Algae grow in wet or damp places

crater (p. 8) a big hole in the ground

crust (p. 5) the hard outer layer of the earth

faults (p. 5) large cracks in the earth’s crust

geologists (p. 14) scientists who study the earth’s rocksand soil

hot springs (p. 12) hollows containing water heatedunderground

lava (p. 6) melted rock that flows from faults

magma (p. 6) melted rock underneath the ground

mud pots (p. 13) hot springs that form mud because theydon’t have much water

petrify (p. 6) to turn to stone

plates (p. 5) huge slabs of the earth’s crust that drift


Old Faithful erupts every 33 to 96 minutes.Its spray sometimes goes nearly 200 feet.

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Think and Respond1 How do the captions help you better understand

the photographs in the book?

2 Find the graphic aids in the book. How do theyhelp you understand the text?

3 What is the main idea of this book? Give detailsthat support your answer.

4 How do you think Yellowstone got its name?

5 Compare and contrast Yellowstone to anothernatural area you know. How is it similar? How is itdifferent?

6 Which part of Yellowstone did you find the mostinteresting? Why?

Letter Imagine that you have just hiked withsome friends through Yellowstone. Write aletter to a friend who couldn’t join you on the

hike. Describe some of the things you saw and heard.How did they make you feel?

School-Home Connection Share theselection with a family member. Discuss

natural wonders he or she has seen, such asmountains and waterfalls.

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Yellowstone Geology

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