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Fault Earth's crust, the surface layer of the planet, is not solid and unbroken. The forces that rage inside the planet have fractured this brittle layer. Some of these fractures, called faults, lie beneath the surface of the crust. Other faults, however, have ruptured the surface, cracking the crust into various-sized blocks of rock. These blocks dip and rise along faults in response to pressure underground. One block may move up while the other moves down. Sometimes the movement is enough to form valleys or mountains. Other times that movement is not vertical but horizontal, as one block slips along the fault relative to the block on the other side. Movement of crustal blocks along faults may be regular and slow or sporadic and sudden. When two blocks are forced to move against each other but are locked into position, stress builds up. When that stress becomes greater than the forces holding the blocks together, the blocks are forced to move suddenly and violently. The ground vibrations accompanying that release of energy are better known as an earthquake. There are more than one million earthquakes a year on Earth, though more than 60 percent of those are too faint to be felt. Crustal movements along faults are occurring continuously across most of the planet's surface. A fault is defined as a crack or fracture in Earth's crust along which rock on one side has moved relative to rock on the other. (When no movement has occurred, the fracture is known as a joint). When a fault breaks the planet's surface, it may range in length from a few inches to thousands of miles. The line on Earth's surface defining the fault is known as the fault line or fault trace. Forces and changes: Construction and destruction Aerial view of the San Andreas Fault slicing through the Carrizo Plain just east of San Luis Obispo, Any rock subjected to intense stress or pressure over time will deform. At higher temperatures and pressures, rock will soften and bend.. At lower temperatures and pressures, however, rock will break or fracture instead of bending. This type of deformation happens to rock in the upper part of Earth's crust. Faults are a clear example of brittle deformation. The stress that is continually acting on and deforming Earth's surface may be in different forms: tensional stress, which stretches or pulls rock; compressional stress, which squeezes and squashes rock; and shear stress, which changes the shape of rock by causing adjacent parts to slide past one another. All of these stresses are directly related to events occurring deep within the planet. Earth's internal processes, from the core to the crust, have put the surface of the planet in motion, constantly changing its shape