Faults

The last type of fault is known as a thrust fault. A thrust fault is similar to a reverse fault, in that it is the result of compressional forces on the rock, and one side appears to move upward relative to the other. In most faults, the fault face, the area where the faulting takes place, is at a steep angle, in a thrust fault though, the fault face is at a very shallow angle. The shallow angle of these faults allows huge slabs of rock to move over other rocks. This generally results in the formation of extensive mountain ranges. The mountains of western Wyoming, Montana and Idaho are part of the Cordilleran Thrust Belt, where huge slabs of rock have been thrusted onto and over other rocks.

Faults provide us with a couple more landscape features. Most faults occur in a series or group, not singly. In these fault groups, we see areas where a block has moved vertically downward relative to the two blocks on either side. This dropped block is known as a graben. When a block is left higher than the surrounding blocks that have moved downward, it is called a horst.

Faults are the result of compressional or extensional forces in the earth, occurring along brittle regions in the rock. Several types of faults can occur depending on whether the rock underwent compression or extension. Faults not only show us the physical evidence of where the earth has moved, but help to create much of the terrain we see on the earth. The next time you hear about, or experience an earthquake, remember that at the heart of the earthquake is a fault along which the earth has moved along a weakened plane of rock.

Glossary

Dip-slip Fault: Description of fault movement that has only a vertical component, no side-to-side movement.
Fault: Physical evidence of vertical and/or horizontal movement in the earths surface.
Fault Face: The surface along which a fault moves.
Graben: The topographic feature where a block has moved vertically downward relative to the two blocks on either side.
Horst: The topographic feature where a block is left higher than the surrounding blocks that have moved downward.
Normal Fault: Fault that occurs when tensional forces are applied to rocks where one side appears to drop relative to the other side.
Oblique-slip Fault: Description of fault movement that has both vertical and horizontal movement.
Reverse Fault: Fault that occurs when compressional forces

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