Mountain Building - Chapter 17
Mountains are built by a process called orogenesis.
- Typically orogenesis takes place at the edge of continents [such as, the Andes of South America].
- Subduction creates many mountain chains, and may lead to continent-to-continent collisions [as is the case in the Himalayas, where India has collided with the Asian subcontinent].
- Mountains typically will have deep-seated roots that will continue to uplift even after subduction/collision processes end, for instance, the Appalachian Mountains.
- Not all mountains are created by subduction or continental collision. For instance, midocean ridges and hot spots create volcanic mountains. Other mountains can form by block faulting and rifting processes that create great elevation differences. The Sierra Nevada is one example of this [though many of the elevation differences in Yosemite National Park are related to differential erosion].
Types of Mountains
During the subduction process [at a convergent margin], material is compressed along the margin of the nonsubducting plate. This compression, along with igneous intrusion and volcanism, create a complex mountain chain composed of folded and faulted metamorphic rocks and igneous rocks [both intrusive and extrusive].
Under either compression or more typically tension, fault-block mountains can be formed. These are up-faulted or down-dropped blocks that form from major movements on dip-slip faults. The classic example is the Basin and Range of the western United States of America. Nevada is entirely within the Basin and Range.
Upwarped Mountains such as the Black Hills and Adirondacks are created by broad arching of the crust or high angle fault displacements. The rocks in these upfolds were often eroded flat and then were upwarped at a later time. Some folks consider the central Rockies to have formed in this way, but this is debatable; and others consider the Rocky Mountains to be a complex mountain chain.
Isostatic Adjustment
Since rock is compressed, crustal shortening takes place. Any rocks that are squeezed may be folded ductilely and piled up on top of other rocks. This thickens the rock along a continental margin and the portion of the crust that is affected will press into the mantle. The underlying asthenosphere will flow away and this will create a "bump" of overthickened lithosphere. This region will be a root for the mountain chain. As erosion levels the mountains, the roots will rise up and asthenosphere will flow back under the roots. This is called isostatic adjustment. Thus mountain chains will continue to rise long after other orogenic processes have ceased.
Isostatic adjustment can be likened to blocks of wood floating on water. If the part of the wood block that is above water is worn away, the water will flow under the block and continue to lift the block out of the water. Continental lithosphere stands higher on the asthenosphere because it is less dense than oceanic lithosphere. Thus, even the earth's arrangement of ocean basins and continents is related to isostatic processes.
Rock Deformation
As the rock is affected by crustal shortening, individual rock layers will be bent. The rock layers can be folded by plastic deformation and may appear like an accordioned piece of paper. The rock can be folded up and down, like waves, into U-shaped and A-shaped forms. A layer that is bent into a U-shape is called a syncline. A layer that is bent upward into an A-shape [or up- folded arch] is called an anticline. The limb of a syncline will also be the limb of any adjoining anticline.
Domes and basins are created by similar processes to those involved in making anticlines and synclines.
Deformation of rocks on a broad scale has created all of the major complex mountain chains, including the Andes, Alps, Appalachians, Himalayas, Rocky Mountains, and Urals.
Continental Accretion
Sediment from the ocean slab can be accreted onto the continent by obduction.
Small islands and seamounts on the subducting plate can collide with a continent and these foreign terranes can be welded onto the continent [and even the Indian Subcontinent can be considered a terrane welded onto the rest of the Asian continent]. Sometimes these are referred to as suspect terranes.
Accretion builds continents, and accretion of microcontinents and igneous activity combined over billions of years have created the continents that we are familiar with. Most of the rocks of the continental crust were at one time part of an ancient mountain chain. Weathering and erosion have worn away the mountains, but their remnants tell a story of how the modern configuration of the world was created.
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