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Batholith

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Half Dome, a granite monolith in Yosemite National Park and part of the
   Sierra Nevada batholith.
   Enlarge
   Half Dome, a granite monolith in Yosemite National Park and part of the
   Sierra Nevada batholith.

   A batholith (from Greek bathos, depth + lithos, rock) 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, quartz monzonite, or diorite (see also granite dome).

   Although they may appear uniform, batholiths are in fact structures
   with complex histories and compositions. They are composed of multiple
   masses, or plutons, bodies of igneous rock of irregular dimensions
   (typically at least several kilometers) that can be distinguished from
   adjacent igneous rock by some combination of criteria including age,
   composition, texture, or mappable structures. Individual plutons are
   crystallized from magma that traveled toward the surface from a zone of
   partial melting near the base of the Earth's crust.

   Traditionally, these plutons have been considered to form by ascent of
   relatively buoyant magma in large masses called plutonic diapirs.
   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 underworld Pluto). It has also been proposed
   that plutons commonly are formed not by diapiric ascent of large magma
   diapirs, but rather by aggregation of smaller volumes of magma that
   ascended as dikes.

   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 in California. An even larger batholith,
   found predominantly in the Coast 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. 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, exposing 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 (a process 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 located in the world-famous Yosemite
   Valley (see picture).

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