Mountain Building begins with a brief examination of the processes of crustal uplifting, including the concepts of isostasy and isostatic adjustment and rock deformation. The various types of folds (anticlines, synclines, domes, and basins) and faults (both dip-slip and strike-slip) are investigated. Following an overview of the structural characteristics of the four main mountain types, the chapter concludes with an extensive presentation of mountain building and its association with plate boundaries.
Learning Objectives
After reading, studying, and discussing this chapter, you should be able to:
•Describe the process of isostasy and the role of isostatic adjustment during crustal uplifting.
•Explain the difference between elastic and plastic rock deformation.
•List the major types of folds and faults and describe how they form.
•Describe the four main mountain types and give examples of each.
•Describe the relation between mountain building and plate tectonics.
Chapter Summary
•The name for the processes that collectively produce a mountain system is orogenesis. Earths less dense crust is believed to float on top of the denser rocks in the mantle, much like wooden blocks floating in water. This concept of a floating crust in gravitational balance is called isostasy. As erosion lowers the peaks of mountains, isostatic adjustment gradually raises the mountains in response.
•Rocks that are subjected to stresses greater than their own strength begin to deform, usually by folding or fracturing. During elastic deformation, a rock behaves like a rubber band and will return to nearly its original size and shape when the stress is removed. Once the elastic limit is surpassed, rocks either deform plastically or fracture. Plastic deformation results in permanent changes in the size and shape of a rock through folding and flowing.
•The most basic geologic structures associated with rock deformation are folds (flat—lying sedimentary and volcanic rocks bent into a series of wavelike undulations) and faults. The two most common types of folds are anticlines, formed by the upfolding, or arching, of rock layers, and synclines, which are downfolds, or troughs. Most folds are the result of horizontal compressional stresses. Folds may be symmetrical, asymmetrical, or, if one limb has been tilted beyond the vertical, overturned. Domes (upwarped structures) and basins (downwarped structures) are circular or somewhat elongated folds formed from vertical displacements of rocks.
•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 also called thrust faults. In strike-slip faults, horizontal movement causes displacement along the trend, or strike, of the fault. Normal faults indicate tensional stresses that pull the crust apart. Along the spreading centers of plates, 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. A joint is a fracture along which no appreciable displacement has occurred.
•Mountains can be classified according to their structural characteristics. The four main mountain types are 1) fault block mountains (e.g., Basin and Range Province and Sierra Nevada of California), associated with tensional stresses and at least on one side by normal faults, 2) folded mountains (e.g., Alps, Urals, Himalayas, Appalachians), the most complex which form most of the major mountain belts, 3) upwarped mountains (e.g., Black Hills, Adirondack Mountains), caused by broad arching, and 4) volcanic mountains. Some regions, such as plateaus deeply dissected by erosion, exhibit mountainous topography without appreciable crustal deformation.
•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 volcanic arc. Sediment from the land as well as that scraped from the subducting plate becomes plastered against the landward side of the trench, forming what is called an accretionary wedge. The best example of an Andean-type mountain belt is found in the western United States, including the Sierra Nevada and the Coast Ranges in California. Continental collisions, in which both plates may he carrying continental crust, have resulted in the formation of the Himalaya Mountains and Tibetan Highlands. Recent investigations indicate that accretion, a third mechanism of orogenesis, takes place where smaller crustal fragments collide and merge with continental margins along some plate boundaries. Many of the mountainous regions rimming the Pacific have formed in this manner. The accreted crustal blocks are referred to as terranes.