• Deformation refers to changes in the shape and/or volume of a rock body. Rocks deform differently depending on the environment (temperature and confining pressure), the composition and of the rock, and the length of time stress is maintained. Rocks first respond by deforming elastically, and will return to their original shape when the stress is removed. Once their elastic limit (strength) is surpassed, rocks either deform by ductile flow or they fracture. Ductile deformation is a solid state flow that results in a change in size and shape of rocks without fracturing. Ductile deformation occurs in a high temperature/high pressure environment. In a near-surface environment, when stress is applied rapidly, most rocks deform by brittle failure.

• One of the most basic geologic structures associated with rock deformation are folds (flat-lying sedimentary and volcanic rocks bent into a series of wavelike undulations). The two most common types of folds are anticlines, formed by the upfolding, or arching, of rock layers, and synclines, which are downfolds. Most folds are the result of horizontal compressional stresses. Domes (upwarped structures) and basins (downwarped structures) are circular or somewhat elongated folds formed by vertical displacements of strata.

  

• Faults are fractures in the crust along which appreciable displacement has occurred. Faults in which the movement is primarily vertical are called dip-slip faults. Dip-slip faults include both normal and reverse faults. Low-angle reverse faults are called thrust faults. Normal faults indicate tensional stresses that pull the crust apart. Along spreading centers, divergence can cause a central block called a graben, bounded by normal faults, to drop as the plates separate.

  

• Reverse and thrust faulting indicate that compressional forces are at work. Large thrust faults are found along subduction zones and other convergent boundaries where plates are colliding.

• Strike-slip faults exhibit mainly horizontal displacement parallel to the fault surface. Large strike-slip faults, called transform faults, accommodate displacement between plate boundaries. Most transform faults cut the oceanic lithosphere and link spreading centers. The San Andreas fault cuts the continental lithosphere and accommodates the northward displacement of southwestern California.

  

• Joints are fractures along which no appreciable displacement has occurred. Joints generally occur in groups with roughly parallel orientations and are the result of brittle failure of rock units located in the outermost crust.

• The name for the processes that collectively produce a mountain system is orogenesis. Most mountains consist of roughly parallel ridges of folded and faulted sedimentary and volcanic rocks, portions of which have been strongly metamorphosed and intruded by younger igneous bodies.

• Major mountain systems form along convergent plate boundaries. Andean-type mountain building along continental margins involves the convergence of an oceanic plate and a plate whose leading edge contains continental crust. At some point in the formation of Andean-type mountains a subduction zone forms along with a continental volcanic arc.

• Continental collisions, in which both plates are carrying continental crust, have resulted in the formation of the Himalaya Mountains and the Tibetan Plateau. Continental collisions also formed many other mountain belts, including the Alps, Urals, and Appalachians.

• Recent investigations indicate that accretion, a third mechanism of orogenesis, takes place where small crustal fragments collide and accrete to continental margins along plate boundaries. The accreted crustal blocks are referred to as terranes. The mountainous topography of Alaska and the westernmost regions of Canada, the United States, and Mexico formed as the result of the accretion of terranes to North America.

• Earth's less dense crust floats on top of the denser and deformable rocks of the mantle, much like wooden blocks floating in water. The concept of a floating crust in gravitational balance is called isostasy. Most mountainous topography is located where the crust has been shortened and thickened. Therefore, mountains have deep, buoyant crustal roots that isostatically support them. As erosion lowers the peaks, isostatic adjustment gradually raises the mountains in response. The process of uplifting and erosion will continue until the mountain block reaches "normal" crustal thickness.

Copyright © 1995-2010, Pearson Education, Inc., publishing as Pearson Prentice Hall Legal and Privacy Terms