A fault is a fracture in the rock along which rocks have been relatively displaced. The amount of displacement, which can be horizontal or vertical, may be very small and hardly distinguishable from a joint, or it may be many tens of kilometers. It is possible to describe faults by several geometric or genetic criteria. For our purposes, three types may be distinguished. In a normal fault, usually the result of tensional forces, the inclination of the fault plane and the direction of downthrow are either both to the-left or both to the right. In a reverse fault, resulting from compressional forces, the beds on one side of the fault plane are thrust over the other. A tear or lateral fault is one in which rocks are displaced horizontally along a line of fracture, with little or no vertical movement. The amount of vertical displacement, if any, on a fault is referred to as the throw of the fault the amount of lateral displacement is known as the heave.
The surface expression of a fault depends on the hardness of the rocks involved and the size of the displacement. Normal faulting in competent rocks produces a fault scarp, sharply defined cliff-like feature. With the passage of time, the original fault scarp may become subdued, but the fault plane may have a control on the landscape for long afterwards, especially if rocks of different hardness have been brought together on either side of the fault. Such features are called fault-line scraps. Where a softer rock was originally uplifted in respect of a harder rock, in time, as the weaker rock is more rapidly eroded away, the resistant rock may come to form the higher relief on the fault-line scarp.
Faults frequently tend to occur in zones, where the affected strata may form a dislocated belt perhaps several hundred meters wide, as on parts of the San Andrea's fault. These zones may be characterized by crushed or shattered rock known as fault breccia, and present situations, which favor linear erosion. The fjords of western Norway largely coincide with faulted and crushed zones, indicating that flu-vial and subsequent glacial erosion were largely structurally controlled. In the block-faulted country described earlier, faults may stand out boldly, but equally, a large number of faults exist without any trace in the landform.
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