Rock Deformation Deformation refers to changes in the shape and/or volume of a rock body. Rocks first repond 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.
Rock Deformation • Ductile deformation is a solid state flow that results in a change in size and shape of rocks without fracturing. • It occurs in a high-temperature/high-pressure environment. • In a near-surface environment, when stress is applied rapidly, most rocks deform by brittle failure.
Folds • One of the most basic geologic structures associated with rock deformation is 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.
Folds • Domes (upwarped structures) and basins (downwarped structures) are circular or somewhat elongated folds formed by vertical displacements of strata.
Bell Ringer • Contrast the movements that occur along normal and reverse faults. What type of force is indicated by each fault? Refer to figure 10.9 in your textbook.
Faults • 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 and include both normal and reverse faults. • Low angle reverse faults are called thrust faults. • Normal faults indicate tensional stresses that pull the crust apart.
Faults • 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 the compressional forces are at work.
Faults • Strike-slip faults exhibit mainly horizontal displacement parallel to the fault surface. • Large strike-slip faults are called transform faults and accommodate displacement between plate boundaries.
Joints • Joints are fractures along which no appreciable displacement has occurred. • They generally occur in groups with roughly parallel orientations and are the result of brittle failure of rock units located in the outermost crust.
Bellringer • What happens to a floating object when weight is added? Subtracted? How does this principal apply to changes in the elevation of mountains? What term is applied to the adjustment that causes crustal uplift of this type? Page 301-304
Mountain Building • 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.
Mountain Building at Subduction Zones • Subduction of oceanic lithosphere under continental block gives rise to an Andean-type plate margin that is characterized by a continental volcanic arc and associated igneous plutons. • Sediment derived from the land, as well as material scraped from the subducting plate, becomes plastered against the landward side of the trench, forming an accretionary wedge. • An excellent example of an inactive Andean-type mountain belt is found in the western United States and includes the Sierra Nevada and the Coast Range in California.
Collision Mountain Ranges • Continued subduction of oceanic lithosphere beneath an Andean-type continental margin will eventually close an ocean basin. • The result will be a continental collision and development of compressional mountains that are characterized by shortened and thickened crust as exhibited by the Himalayas. • A common feature of compressional mountains are fold-and-thrust belts. • Many mountain belts have been generated by continental collisions , including Alps, Urals, and Appalachians.
Collision Mountain Ranges • Mountain belts can develop as a result of the collision and merger of an island arc, oceanic plateau, or some other small crustal fragment to a continental block. • Many of the mountain belts of the North American Cordillera, principally those in Alaska and British columbia, were generated in this matter.
Fault Block Mountains • Although most mountains form along convergent plate boundaries, other tectonic processes, such as continental rifting, can produce uplift and the formation of topographic mountains. • The mountains that form in these settings, termed fault-block mountains, are bounded by high-angle normal faults that gradually flatten with depth. • The Basin and Range Province in the western United States consists of hundreds of faulted blocks that give rise to nearly parallel mountain ranges that stand above sediment-laden basins.
The principle of isostasy • 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.
Vertical Movements of the Crust • Most mountainous topography is located where the crust has been shortened and thickened. • Mountains have deep crustal roots that isostatically support them. • As erosion lowers the peaks, isostatic adjustment gradually raises the mountains in response. • The processes of uplifting and erosion will continue until the mountain block reaches “normal” crustal thickness. • Gravity also causes elevated mountainous structures to collapse under their own “weight.”
Erosion and resulting isostatic adjustment of the crust Figure 10.24 AB
Erosion and resulting isostatic adjustment of the crust Figure 10.24 BC