Stress! This word is probably all too familiar to most people. Think about the last time you were under a lot of stress. Perhaps you were getting ready to take a hard math test, arguing with a friend or family member, making a difficult decision, or waiting for your turn to perform in a musical or athletic competition. You may have felt as if you were being pulled in many directions at once. Or you may have felt so tense inside that you thought something might snap.

The rocks of the Earth's crust may be carved into strange and beautiful forms by the action of wind, water, and weather. The Needle's Eye is found in the Black Hills of South Dakota (left). The red-orange pinnacles are found in Bryce Canyon National Park in Utah. (right).

Like you, the Earth also experiences stress. This kind of stress, however, is not the result of emotionally difficult situations. Rather, it is caused by forces within the Earth itself. These forces push and pull on the part of the Earth known as the crust. As you know, the crust is the surface, or outermost, layer of the Earth. Because this is where stress occurs, it might be helpful to review its composition.

There are two major sections of the crust. One section is called continental crust. Continental crust makes up the Earth's land masses, such as the North American continent. In most places, continental crust is about 32 kilometers thick. The other section, called oceanic crust, is found under the ocean floor. It is thinner than continental crust. Oceanic crust is usually about 8 kilometers thick.

As you have just read, stress pushes and pulls on the Earth's crust. As the rocks of the crust undergo stress, they slowly change shape and volume. (Volume is the amount of space an object takes up.) They also move up or down or sideways. The movement causes the rocks to break, tilt, and fold. The breaking, tilting, and folding of rocks is called deformation. The prefix de- means undo; the root word form means shape or configuration. Can you explain why the term deformation is appropriate?

There are three basic types of stress, each of which deforms the crust in a different way. The three types of stress are compression, tension, and shearing. Refer to the graphic as you read about these different types of stress.

Each of the different forms of stress deforms the crust in a different way. The large arrows show the directions of the forces acting on the rocks.

Compression squeezes the rocks of the crust. This often causes the particles in the crustal rocks to move closer together, making the rocks denser and smaller in volume. In this case, compression is acting rather like a trash compactor, squeezing a large amount of matter into a smaller amount of space. As crustal rocks are compressed, they are pushed both higher up and deeper down. To understand this movement, imagine you are squeezing clay in your hand. As you squeeze the clay, some of it is pushed out of the opening at the top of your fist and some of it is pushed out of the opening at the bottom.

Tension pulls on the rocks of the crust, causing them to stretch out over a larger area. Like a piece of warm taffy being pulled, a rock under tension becomes thinner in the middle than at the ends. In addition, as the volume of the rock increases, its density decreases. Shearing pushes rocks of the crust in two opposite directions. This causes the rocks to twist or tear apart. During shearing, then, rocks are not compressed or stretched. They simply bend or break apart.

These rocks on the coast of New Zealand have been deformed by stress

Compression, tension, and shearing can change a rock's volume, its shape, or both. These stresses can also cause the rocks to fracture, or crack. If the rocks fracture along numerous flat surfaces which show no displacement, the cracks are called joints. Joints are generally parallel to one another. Some rocks have joints that form in more than one direction. Such rocks may break into blocks. The blocks form where the different sets of joints cross one another.
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Faulting

Stress sometimes causes rocks to break. A break or crack along which rocks move is called a fault. The rocks on one side of the fault slide past the rocks on the other side of the fault. Movements along a fault can be up, down, or sideways. Earthquakes often occur along faults in the Earth's crust. What are some other possible results of movements along a fault?

Look at the cross sections of faulted rocks in the picture. As you can see, there are two blocks of rock, one on top of the other. The block of rock above the fault is called the hanging wall. The block below the fault is called the foot wall.

Stress can cause either the hanging wall or the foot wall to move up or down along a fault. If tension is acting on a fault, the hanging wall will move down relative to the foot wall. If this occurs, the fault between the two blocks is called a normal fault. If compression is acting on a fault, the hanging wall will move up relative to the foot wall. This type of fault is called a reverse fault.

A thrust fault is a special kind of reverse fault in which the hanging wall slides over the foot wall. How does this fault effect the order of rock layers in an area?

A special type of reverse fault is a thrust fault. A thrust fault is formed when compression causes the hanging wall to slide over the foot wall. Thrust faults are special because they are almost horizontal, whereas regular reverse faults and normal faults are almost vertical. Thrust faults usually carry rocks many kilometers from their original position. Rocks are usually severely bent at the same time that thrust faulting occurs. In addition, thrust faults mix up the order of the layers in rock. Normally, older rock layers are found under younger rock layers. But a thrust fault pushes older rocks on top of younger rocks. The Lewis Overthrust Fault in Glacier National Park, Montana, is an example of a thrust fault. Here very old rocks have slid eastward more than 48 kilometers and now rest on top of younger rocks.

Stress does not cause blocks of crustal rock to move only up and down. Shearing will cause the blocks of rock to slide horizontally past each other. One block moves to the left or right in relation to the other block. The fault along which the blocks move horizontally past each other is called a lateral fault. (Right) In a lateral fault, which is also known as a strike-slip fault the blocks of rock move horizontally past each other

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Faulted Mountains and Valleys

When there are many normal faults in one area, a series of mountains and valleys may form. Mountains formed by blocks of rock uplifted by normal faults are called fault-block mountains. A vast region in western North America called the Cordilleran Mountain region contains many fault-block mountains. The region extends from central Mexico to Oregon and Idaho and includes western Utah, all of Nevada, and eastern California.

A fault block mountain (left) is formed when a block (or blocks) of rocks between two normal faults is pushed up. A rift valley (right) is formed when a block of rock between two normal faults slides down.

Valleys also form when mountains form. Some valleys are formed when the block of land between two normal faults slides downward. Valleys created in this way are called rift valleys. One example of a rift valley is Death Valley in California. It is a long, narrow valley 87 meters below sea level. Scientists believe that the valley was formed by a series of small movements along two faults at either side of the valley. They estimate that the land along the eastern fault of Death Valley will move another 3 meters during the next 1000 years.

Another example of a rift valley is the Great Rift Valley in the Middle East and Eastern Africa. This valley contains the Dead Sea, in Israel, the lowest spot on Earth, and Victoria Falls on the Zambizi River, in Zimbabwe.

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Folding

Sometimes when stress is applied to the rocks of the crust, the rocks bend but do not break. The rocks bend in much the same way a rug wrinkles as it is pushed across a floor. A bend in a rock is called a fold. As you can see, a rock can fold either upward or downward. An upward fold in a rock is called an anticline (AN-tih-klighn). A downward fold in a rock is called a syncline (SIHN-klighn).

Folds vary in size. Some folds are so small that you need a magnifying glass to see them clearly. Others are large enough to form mountains. Layered rocks with large folds often have smaller folds within the layers. The Appalachian Mountains in the eastern United States are made up of many anticlines and synclines. This folded mountain chain extends from Canada to Alabama.   Anticlines are upward folds in rocks. Synclines are downward folds in rocks. Some folds are quite large. the speck at the top of the English hill is a person!

Even though an anticline is an upward fold, it is not always higher than the surrounding land. An anticline can be under hills, valleys, or flat areas. An anticline may be hidden by layers of rock that build up in the low-lying areas around it after it forms. Or the stress may not have been great enough to bring the fold to the Earth's surface.

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Fault or Fold?

A number of factors determine whether rocks will fault or fold. One factor is temperature. If the rocks become extremely hot during compression, they are more likely to fold than to fault. Do you know why? If you ever left a box of crayons in the sun when you were young, you may have firsthand experience with the effect of temperature on folding and faulting. At normal temperatures, crayons snap in two when stress is applied. In other words, they fault. But warm crayons can bend without breaking-they fold.

Another factor that affects whether rocks will fault or fold is pressure. The greater the pressure applied to the rocks, the more likely they are to fold rather than fault.

Rock type is yet another factor that determines whether rocks will fault or fold. Some types of rocks break easily when stress is applied. Such fragile rocks are said to be brittle. Sandstone is one example of brittle rock. Other rocks, such as rock salt, bend easily under stress. Rocks that bend easily are said to be ductile. Ductile rocks are more likely to fold, whereas brittle rocks are more likely to fault.

Another factor that determines whether rocks will fault or fold is how the stress is applied to the rocks. If the stress is applied gradually, the rocks will usually fold. But if the stress is applied suddenly, the rocks will usually fault.

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Plateaus

A plateau (pla-TOH) is a large area of flat land that is raised high above sea level. You can get a pretty good idea of what a typical plateau looks like if you place a sandwich on a plate and look at it from the side. The flat layers of bread slices, cold cuts, cheese, lettuce, tomato, and mayonnaise (or whatever you put in your sandwich) correspond roughly to the horizontal rock layers that make up a plateau. Like a sandwich, a plateau is wider than it is tall. In addition, a plateau is often surrounded by steep cliffs that rise sharply from the surrounding land, much as a sandwich rises above the surface of the plate on which it is placed.

Although plateaus are often raised up by the same processes that form mountains, the rock layers in a plateau remain flat. (This is not the case with mountains, in which the rock layers are tilted and broken by faulting or are warped by folding.) One way a plateau may be formed is by a slow, flat-topped fold. The Appalachian Plateau, which lies just west of the folded Appalachian Mountains, was created millions of years ago by such a fold. This plateau covers much of New York, Pennsylvania, Ohio, Kentucky, West Virginia, and Tennessee.   (right) Colorado Plateau

Another way a plateau may be formed is through vertical faulting. The Colorado Plateau, which is located west of the Rocky Mountains, was uplifted when the underlying region of the inner Earth became hotter and expanded. As this region expanded, it pushed up on the crust above it. The rocks at the edge of the forming Colorado Plateau fractured, and the plateau was slowly pushed upward. The Colorado Plateau covers parts of New Mexico, western Colorado, eastern Utah, and northern Arizona. Most of the plateau is more than 1500 meters above sea level. The Colorado Plateau was formed hundreds of millions of years after the Appalachian Plateau.

Plateaus can also be formed by a series of molten rock flows on the surface of the Earth. Molten rock at the surface of the Earth is called lava. Molten rock deep within the Earth is called magma. Magma reaches the Earth's surface through long cracks in the ground. Great floods of hot molten rock periodically stream out of the cracks. The flowing lava spreads out over a large area and hardens into a sheet. The lava sometimes fills in valleys and covers hills. The flowing and spreading out of the lava is repeated over and over again. The hardened lava sheets pile up and form a raised plateau. The Columbia Plateau, which covers parts of Oregon, Washington, and Idaho, is a lava plateau. Here lava built up a large flat region covering almost 5 million square kilometers. The plateau is 1 to 2 kilometers thick.

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Domes

You know now that lava flows out onto the Earth's surface to form plateaus. Sometimes, however, magma pushes upward but does not reach the Earth's surface. The stress caused by the magma causes the rock layers above it to fold upward, forming an uplifted area. At some point, the magma cools and forms hardened rock.   Over a long period of time, the uppermost rock layers may be worn away to reveal the dome's core of hardened magma

The uplifted area created by rising magma is called a dome. A dome is a raised area shaped roughly like the top half of a sphere. The outline of a dome is oval or circular. You can think of a dome as rather like a blister on the surface of the Earth. Like a blister, a dome is formed when fluid collects beneath the surface and pushes up on overlying layers, forming a raised spot in the immediate area but leaving the surrounding regions flat and undisturbed. Domes that have been worn away in places form many separate peaks called dome mountains. The Black Hills of South Dakota and Wyoming are dome mountains. In this region, many layers of flat-lying rocks were arched up. Over a long period of time, the rocks on top were worn away. The hardened magma that caused the uplifting was then exposed.     A dome may be formed when rising magma causes the rock layers above it to fold upward.

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1. What is stress?

2. Name the two major sections of the Earth's crust. How thick is each?

3. What are the three basic types of stress?

4. What is a fault? Name the two major parts of a fault.

5. What are the four types of faults? Describe each.

6. What type of stress causes a normal fault?

7. What type of stress causes a reverse fault?

8. What type of stress causes a lateral fault?

9. Describe what happens to cause a fault-block mountain.

10. Describe what happens to cause a rift valley.

11. What is the difference between a fault and a fold?

12. Contrast anticline and syncline.

13. What factors determine whether rocks fault or fold?

14. What causes a dome?

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