Page 428 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

392 part III The Earth–Atmosphere Interface
◀Figure 13.9 Domes and basins. [(c) Science Source.]
(a) Dome
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(c) The Richat dome, Mauritania; notice that sand dunes are covering up part of the structure on its southeastern side.
A freshly poured concrete sidewalk is smooth and strong. Adding stress to the sidewalk by driving heavy equipment over it may result in strain that causes a fracture. Pieces on either side of the fracture may move up, down, or horizon- tally depending on the direction of stress. Similarly, when rock strata are stressed beyond their ability to remain a solid unit, they express the strain as a fracture. Faulting occurs when rocks on either side of the fracture shift rela- tive to the other side. Fault zones are areas where fractures in the rock demonstrate crustal movement.
Types of Faults The fracture surface along which the two sides of a fault move is the fault plane; the tilt and orientation of this plane is the basis for differentiating the three main types of faults introduced in Figure 13.7: normal, reverse, and strike-slip, caused, respectively, by tensional stress, by compressional stress, and by lateral- shearing stress.
When forces pull rocks apart, the tensional stress causes a normal fault, in which rock on one side moves vertically along an inclined fault plane (Figure 13.11a). The downward-shifting side is the hanging wall; it drops relative to the footwall block. An exposed fault plane sometimes is visible along the base of faulted mountains, where individual ridges are truncated by the movements of the fault and end in triangular facets. A displacement of the ground surface caused by faulting is commonly called a fault scarp, or escarpment.
▼Figure 13.10 Folded mountains in the Zagros crush zone, iran. The Zagros Mountains are a product of the Zagros crush zone, where the Arabian plate pushes northward into the Eurasian plate. [NASA.]
collision more than 400 km wide. In the satellite image of this zone in Figure 13.10, anticlines form the parallel ridges; active weathering and erosion processes are ex- posing the underlying strata.
In addition to the types of folding discussed above, broad warping actions are another cause of bending in continental crust. The bends produced by these actions, however, are far greater in extent than the folds produced by compression. Forces responsible for such large-scale warping include mantle convection, isostatic adjustment (such as that caused by the weight of previous ice loads across northern Canada), and crustal swelling above an underlying hot spot.
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