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Forces in Earth’s crust. After this lesson you will be able to . Vocabulary and key questions. Stress Tension Compression Shearing Normal fault Reverse fault Strike-slip fault Plateau How does stress change Earth’s crust? How do faults form?
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Forces in Earth’s crust After this lesson you will be able to
Vocabulary and key questions • Stress • Tension • Compression • Shearing • Normal fault • Reverse fault • Strike-slip fault • Plateau • How does stress change Earth’s crust? • How do faults form? • How does plate movement create new landforms?
How does stress change Earth’s crust? • Rocks are hard and stiff. But the movement of Earth’s plates can create strong forces that slowly bend or fold many rocks like a caramel candy bar. Like the candy bar, some rocks may only bend and stretch when a strong force is first applied to them. But beyond a certain limit, all rocks in Earth’s brittle upper crust will break. • Forces created by plate movement are examples of stress. Stress is a force that acts on rock to change its shape or volume. Geologists often express stress as force per unit area. Because stress increases as force increases, stress adds energy to the rock. The energy is stored in the rock until the rock changes shape or breaks.
How does stress change Earth’s crust? • Three kinds of stress can occur in the crust-tension, compression, and shearing. Tension, compression, and shearing work over millions of years to change the shape and volume of rock. • Most changes in the crust occur only very slowly, so that you cannot directly observe the crust bending, stretching or breaking.
Tension • Rock in the crust can be stretched so that it becomes thinner in the middle. This process can make rock seem to act like a piece of warm bubble gum. • The stress force that pulls on the crust and thins rock in the middle is called tension. Tension occurs where two plates pull apart.
Compression • One plate pushing against another plate can squeeze rock like a giant trash compactor. • The stress force that squeezes rock until it folds or breaks is called compression. Compression occurs where two plates come together.
Shearing • Stress that pushes a mass of rock in two opposite directions is called shearing. • Shearing can cause rock to break and slip apart or to change its shape. • Shearing occurs where two plates slip past each other.
How do faults form? • Recall that a fault is a break in the rock of the crust where rock surfaces slip past each other. Most faults occur along plate boundaries where the forces of plate motion push or pull the crust so much that the crust breaks. • When enough stress builds up in the rock, the rock breaks, creating a fault. There are three main types of faults: normal faults, reverse faults, and strike-slip faults.
Normal faults • The Rio Grande River flows through a wide valley in New Mexico. Here tension has pulled apart two pieces of Earth’s crust, forming the valley. Where rock is pulled apart by tension in Earth’s crust, normal faults form. • In a normal fault, the fault cuts through rock at an angle, so one block of rock sits over the fault, while the other block lies under the fault. The block of rock that sits over the fault is called the hanging wall. The rock that lies under the fault is called the footwall. • When movement occurs along a normal fault, the hanging wall slips downward. Normal faults occur where two plates diverge, or pull apart.
Reverse faults • The northern Rocky Mountains rise high above the western United States and Canada. These mountains were gradually lifted up over time by movement along reverse faults. • A reverse fault has the same structure as a normal fault, but the blocks move in the reverse direction. That is, the hanging wall moves up and the footwall moves down. • Reverse faults form where compression pushes the rock of the crust together.
Strike-slip faults • The hilly plains in southern California are split by the San Andreas fault. Here, shearing has produced a strike-slip fault. • In a strike-slip fault, the rocks on either side of the fault slip past each other sideways, with little up and down motion. • A strike-slip fault that forms the boundary between two plates is called a transform boundary. The San Andreas fault is an example of a transform boundary.
How does plate movement create new landforms? • Most changes in the crust occur so slowly that they cannot be observed directly. But what if you could speed up time so that a billion years passed by in minutes? Then, you could watch the movement of Earth’s plates fold, stretch, and uplift the crust over wide areas. • Over millions of years, the forces of plate movement can change a flat plain into features such as anticlines and synclines, folded mountains, fault-block mountains, and plateaus.
Folding Earth’s crust • Have you ever skidded on a rug that wrinkled up as your feet pushed it across the floor? Sometimes plate movements can cause Earth’s crust to fold much like the rug. Then, rocks stressed by compression may bend without breaking. • Folds are bends in rock that form when compression shortens and thickens Earth’s crust. A fold can be a few cm across or hundreds of km wide.
How anticlines and synclines form • Geologists use the terms anticline and syncline to describe upward and downward folds in rock. A fold in rock that bends upward into an arch is an anticline. • A fold in rock that bends downward to form a V shape is a syncline. Anticlines and synclines are found in many places where compression forces have folded the crust. • The central Appalachian Mountains in Pennsylvania are folded mountains made up of anticlines and synclines.
How folded mountains form • The collision of two plates can cause compression and folding of the crust over a wide area. Folding produced some of the world’s largest mountain ranges. • The Himalayas in Asia and the Alps in Europe formed when pieces of the crust folded during the collision of two plates. These mountains formed over million of years.
Stretching Earth’s crust • If you traveled by car from Salt Lake City to Los Angeles, you would cross the Great Basin. This region contains many mountains separated by broad valleys or basins. The mountains form from tension in Earth’s crust that causes faulting. Such mountains are called fault-block mountains. • How do fault-block mountains form? Where two plates move away from each other, tension forces create many normal faults. Suppose two normal faults cause valleys to drop down on either sides of a block of rock. This process is shown in the diagram that accompanies the photograph in figure 5 page 108. • As the hanging wall of each normal fault slips downward, the block in between now stands above the surrounding valleys, forming a fault-block mountain.
Uplifting Earth’s crust • The forces that raise mountains can also uplift, or raise, plateaus. A plateau is a large area of flat land elevated high above sea level. • Some plateaus form when forces in Earth’s crust push up a large, flat block of rock. Like a fancy sandwich, a plateau consists of many different flat layers, and is wider than it is tall. • Forces deforming the crust uplifted the Colorado Plateau in the “Four Corners” region of Arizona, Utah, Colorado, and New Mexico.
