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Batholith

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File:Yosemite 20 bg 090404.jpg
Half Dome, a quartz monzonite monolith in Yosemite National Park and part of the Sierra Nevada Batholith

A batholith (Template:Etymology) is a large mass of intrusive igneous rock (also called plutonic rock), larger than Template:Cvt in area,<ref name="PetersenEtAl2017">Template:Cite book</ref> 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).

File:Intrusion types.svg
Basic types of igneous intrusions:
1. Laccolith
2. Small dike
3. Batholith
4. Dike
5. Sill
6. Volcanic neck and pipe
7. Lopolith
Note: As a general rule, in contrast to the active volcanic vent in the figure, these names refer to the fully cooled and usually millions-of-years-old rock formations, which are the result of the underground magmatic activity shown.

FormationEdit

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 solidified from magma that traveled toward the surface from a zone of partial melting near the base of the Earth's crust.

File:Vitosha platoto.jpg
The eroded laccolith above the batholith system of Vitosha, the Plana domed mountains, next to Sofia, Bulgaria

Traditionally, these plutons have been considered to form by ascent of relatively buoyant magma in large masses called plutonic diapirs. Because the diapirs are liquified and very hot, they tend to rise through the surrounding native country rock, pushing it aside and partially melting it. Most diapirs do not reach the surface to form volcanoes, but instead they 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). An alternate view is that plutons are formed by aggregation of smaller volumes of magma that ascend as dikes.<ref>Template:Cite book</ref>

A batholith is formed when many plutons converge 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 makes up much of the Sierra Nevada in California. An even larger batholith, the Coast Plutonic Complex, is found predominantly in the Coast Mountains of western Canada; it extends for 1,800 kilometers and reaches into southeastern Alaska.

Surface expression and erosionEdit

A batholith is an exposed area of (mostly) continuous plutonic rock that covers an area larger than 100 square kilometers (40 square miles). Areas smaller than 100 square kilometers are called stocks.<ref>GLENCOE SCIENCE | Earth Science Twelfth Grade High School Textbook (Georgia); pg. 115 paragraph 1, pg. 521 question 9</ref> 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 square kilometers of overlying rock in many areas, exposing the once deeply buried batholiths.

Batholiths exposed at the surface are subjected to huge pressure differences between their former location deep in the earth and their new location at or near the surface. As a result, their crystal structure expands slightly over time. This manifests itself by a form of mass wasting called exfoliation. This form of weathering 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 well-known result of this process is Half Dome in Yosemite Valley.

ExamplesEdit

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AfricaEdit

  • Aswan Granite Batholith
  • Cape Coast Batholith, Ghana
  • Heerenveen Batholith, South Africa<ref name="Clemens_EtAl_2010">Template:Cite journal</ref>
  • Paarl Rock, South Africa
  • Darling Batholith, South Africa
  • Hook granite massif, Zambia
  • Mubende Batholith, Uganda

AntarcticaEdit

  • Antarctic Peninsula Batholith
  • Queen Maud Batholith

AsiaEdit

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  • Gangdese batholith, Himalaya
  • Trans-Himalayan Batholith, Himalaya
  • Kalba-Narym batholith, Kazakhstan
  • Karakorum Batholith, Himalaya
  • Tak batholith, Thailand
  • Tien Shan batholith, Central Asia
  • Ranchi batholith, India

EuropeEdit

  • Revsund Massif
  • Rätan Batholith
  • Småland–Värmland Belt

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North AmericaEdit

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OceaniaEdit

South AmericaEdit

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See alsoEdit

ReferencesEdit

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External linksEdit

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