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# Types of Faults

Introduction: The Earth's lithosphere is divided into large, moving plates. This plate motion is the foundation of plate tectonic theory. As the plates move, stress builds within rocks that can result in fault formation. Faults are fractures, or breaks, in rock masses where movement has occurred on at least one side of the break. The sudden movement of rocks along a fault can result in an earthquake.

The plane along which either or both rock masses slide is the fault plane. The fault line is where the fault plane intersects with the ground or rock surface. Often there are multiple fractures in one location that have experienced rock movement. These regions are called fault zones.

Geologists classify faults by the type of motion. This includes both the direction and angle of motion. The three major fault types are described below.
 A strike-slip fault occurs when rocks move horizontally, or slide past each other. These faults form under shearing, or sliding, forces. Strike-slip faults are identified as right-lateral or left-lateral depending on the direction of movement of the rock blocks. The diagram shows a left-lateral fault. If you placed one foot on each side of the fault line while the rock block moved, your left foot would move toward you. The famous San-Andreas fault is a right lateral strike-slip fault. It is also a transform fault, or a strike-slip fault that forms along plate boundaries. A normal fault forms when tensional stress pulls rocks apart. When this happens, the rock block on the upper side of the fault will drop down compared to the rock block on the lower side of the fault plane. The rock block that drops is called the hanging wall. The rock block on the other side of the fault plane is the footwall. The diagram shows the footwall on the left of the fault plane and the hanging wall on the right. A reverse fault forms under compressional forces. Think of it as the opposite, or reverse, of a normal fault. The rocks are squeezed inward. The hanging wall moves upward along the fault plane. The diagram shows the hanging wall on the right of the fault plane and the footwall on the left. A thrust fault is a normal fault with a low angle fault plane.
Directions for This Lesson: Try the practice questions to see what you know about the types of faults. Then, apply what you learned in the activity and practice sections.

Practice:
Practice what you have learned by completing the post-lesson worksheet.

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