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.
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.[2]
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 erosion
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.[3] 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.
^Petersen, James F.; Sack, Dorothy; Gabler, Robert E. (2017). Physical Geography (11th ed.). Boston:
Cengage Learning Inc. p. 614.
ISBN978-1-305-65264-4.
^Hall, Clarence A. Jr. (2007). Introduction to the geology of southern California and its native plants. Berkeley: University of California Press. p. 22.
ISBN9780520249325.
^GLENCOE SCIENCE | Earth Science Twelfth Grade High School Textbook (Georgia); pg. 115 paragraph 1, pg. 521 question 9