The Relationship Between Folding and Faulting Because different rocks behave differently under stress, we expect that some rocks when subjected to the same stress will fracture or fault, while others will fold. When such contrasting rocks occur in the same area, such as ductile rocks overlying brittle rocks, the brittle rocks may fault and the ductile rocks may bend or fold over the fault. Also since even ductile rocks can eventually fracture under high stress, rocks may fold up to a certain point then fracture to form a fault. Since different rocks have different resistance to erosion and weathering, erosion of folded areas can lead to a topography that reflects the folding. Mountain Ranges - The Result of Deformation of the Crust One of the most spectacular results of deformation acting within the crust of the Earth is the formation of mountain ranges.
Mountains originate by three processes, two of which are directly related to deformation.
As discussed previously, both normal and reverse faults can cause the uplift of blocks of crustal rocks. The Sierra Nevada mountains of California, and the mountains in the Basin and Range province of the western U.S., as discussed previously, were formed by faulting processes and are thus fault block mountains. When this occurs the rocks between the two continental blocks become folded and faulted under compressional stresses and are pushed upward to form fold and thrust mountains. Similarly the Appalachian Mountains of North America and the Alps of Europe were formed by such processes. Volcanic Mountains - The third type of mountains, volcanic mountains, are not formed by deformational processes, but instead by the outpouring of magma onto the surface of the Earth.