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Batholith

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Half Dome - larger image
A batholith is a large emplacement of igneous intrusive[?] (also called plutonic) rock that forms from cooled magma deep in the earth's crust. Batholiths are almost always made mostly of felsic or intermediate[?] rock-types such as granite, diorite[?] or lighter colored forms of andesite[?].

In spite of their apparent uniformity, batholiths are in fact structures with complex histories and compositions. They are composed of multiple blobs, or plutons, of magma that traveled toward the surface from a zone of partial melting at the base the earth's crust. While moving, these blobs of relatively buoyant magma are called plutonic diapirs (not to be confused with diaper). Because the diapirs are liquefied and very hot, they tend to rise through the surrounding country rock pushing it aside and partially melting it. Most diapirs do not reach the surface to form volcanoes, but instead slow down, cool and usually solidify 5 to 30 kilometers underground as plutons (hence the use of the word pluton; in reference to the Roman god of the underword Pluto).

A batholith is formed when many plutons converge together to form a huge expanse of granitic rock. Some batholiths are mammoth, paralleling past and present subduction zones and other heat sources for hundreds of kilometers in continental crust. One such batholith is the Sierra Nevada Batholith[?] which is a continuous granitic formation that forms much of the Sierra Nevada mountains in California. An even larger batholith, found predominantly in the mountains of western Canada, extends for 1,800 kilometers and reaches into southeastern Alaska.

There is also an important geographic usage of the term batholith to consider. For a geographer, a batholith is an exposed area of mostly continuous plutonic rock[?] that covers an area larger than 100 square kilometers. Areas smaller than 100 kilometers are called stocks. However, the majority of batholiths visible at the surface (via outcroppings) have areas far greater than 100 square kilometers. These areas are exposed to the surface through the process of erosion accelerated by continental uplift[?] acting over many tens of millions to hundreds of millions of years. This process has removed several tens of kilometers of overlying rock in many areas which exposes the once deeply buried batholiths.

Batholiths exposed at the surface are also subjected to huge pressure differences between their former homes deep in the earth and their new homes at or near the surface. As a result, their crystal structure expands slightly and over time this manifests itself by a form of mass wasting[?] called exfoliation[?]. This form of erosion causes convex and relatively thin sheets of rock to slough off the exposed surfaces of batholiths (this process is accelerated by frost wedging[?]). The result is fairly clean and rounded rock faces. A famous example of the result of this process is Half Dome which is visible from the world famous Yosemite Valley (see picture).

Reference

  • Plummer, McGeary, Carlson, Physical Geology, Eight Edition (McGraw-Hill: Boston, 1999) pages 61-63 ISBN 0-697-37404-1

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