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Movement of the Earth’s Crust. Earth’s Changing Surface. There are two major sections of the crust: the continental crust (32 km thick) and the oceanic crust (8 km thick). . Deformation.
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Earth’s Changing Surface • There are two major sections of the crust: the continental crust (32 km thick) and the oceanic crust (8 km thick).
Deformation • Stress pushes and pulls on the Earth’s crust. As the rocks of the crust undergo stress, they slowly change shape and volume. They also move up or down or sideways. The movement causes the rocks to break, tilt or fold, This is caused deformation.
Stress • 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.
Compression • 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. As the rocks are compressed, they are pushed both higher up and deeper down.
Tension • Tension pulls on the rocks of the crust, causing them to stretch out over a larger area. A rock under tension becomes thinner in the middle than at the ends. As the volume of the rock increases, its density decreases.
Shearing • 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 bend or break apart.
Joints • Compression, tension and shearing can change a rock’s volume, its shape or both. These stresses can cause the rocks to fracture or crack. If the rocks fracture along numerous flat surfaces which show no displacement, the cracks are called joints. Such rocks may break into blocks when the different sets of joints cross one another.
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. The movement can be up, down or sideways. Earthquakes often occur along fault lines.
Hanging Wall • The block of rock above the fault is called the hanging wall.
Foot Wall • The block below the fault is called the foot wall.
Normal Fault • Stress can cause hanging 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.
Reverse 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.
Thrust Fault • 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 • Thrust faults carry rocks many km from their original position. Rocks are usually severely bent also. Thrust faults also mix up the order of the layers in rock pushing older rocks on top of younger rocks. The Lewis Overthrust Fault in Glacier National Park in Montana is an example.
Lateral Fault • 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.
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 fault-block mountains. A vast region in western North America called the Cordilleran Mountain region contains many.
Rift Valleys • Valleys also form when mountains form. Rift valleys are formed when the block of land between two normal faults slides downward. One example of a rift valley is Death Valley in California. It is a long narrow valley 87 meters below sea level.
Folding • Sometimes when stress is applied to the rocks of the crust, the rocks bend but do not break. A bend in a rock is called a fold.
Anticline and Syncline • A rock can fold wither upward or downward. An upward fold in a rock is called an anticline. A downward fold in a rock is called a syncline. The Appalachian Mountains are made up of many anticlines and synclines.
Fault or Fold • A number of factors determine whether rocks will fault or fold: • Temperature • Pressure • Rock type • How the stress is applied
Temperature • One factor is temperature. If they become extremely hot during compression, they are more likely to fold than to fault.
Pressure • 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 to fault.
Rock Type • 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. Other rocks, such as rock salt, bend easily under stress and are said to be ductile.
How the Stress is Applied • If the stress is applied gradually, the rocks will usually fold. If the stress is applied suddenly, the rocks will usually fault.
Plateaus • A plateau is a large area of flat land that is raised high above sea level. A plateau is wider than it is tall. Although plateaus are often raised up by the same processes that form mountains, the rock layers in a plateau remain flat.
Plateau Formation • Plateaus can be formed by: • Vertical Faulting • Fold • Rivers
Plateau Formation by Vertical Faulting • Plateaus may be formed is through vertical faulting, such as the Colorado Plateau.
Plateau Formation by Fold • One way a plateau may be formed is by a slow, flat-topped fold, such as the Appalachian Plateau.
Plateau Formation by Molten Rock Flows • Plateaus can also be formed by a series of molten rock flows on the surface of the Earth,such as the Columbia Plateau.
Plateau Formation by Rivers • Rivers often carve one large plateau into many smaller plateaus or deep cut valleys. One of the most spectacular is the Grand Canyon.
Domes • Lava often flows onto the surface to form a plateau. Sometimes, magma pushes upward but does not reach the surface. The stress caused by the magma causes the rock layers above it to fold upward, forming an uplifted area. The magma cools and forms hardened rock. The uplifted area formedby rising magma is called a dome.
Dome Mountains • 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.
The Floating Crust • Because the mantle is denser than the crust, the solid rocky crust floats on the mantle. The floating crust exerts a downward force on the mantle. The mantle also exerts a force. A balance exists between the downward force of the crust and the upward force of the mantle. The balancing of these two forces is called isostasy.
Balancing Act • Low-lying regions such as Norway, Sweden and Finland have risen since the ice has melted. Crustal rock can also sink. The Mississippi River has dropped millions of tons of mud and sand particle into the Gulf of Mexico. The addition of materials has caused the crust on the Gulf floor to sink but the depth of water has not changed.
