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.
Although they may appear uniform, batholiths are in fact structures with complex histories and compositions. They are composed of multiple masses, or
plutons, of magma that traveled toward the surface from a zone of partial melting at the base of the Earth's crust. While moving, these plutons of relatively
buoyant magma are 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).
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).
*
Laccolith*
Dike*
Sill*
Volcanic Neck*
Pluton* Plummer, McGeary, Carlson,
Physical Geology, Eight Edition (McGraw-Hill: Boston, 1999) pages 61-63 ISBN 0-697-37404-1
* Glazner, Bartley, Coleman, Gray, Taylor,
Are plutons assembled over millions of years by amalgamation from small magma chambers?, GSA Today: Vol. 14, No. 4, pp. 4–11
*
Idaho Batholith*
The Cornubian Batholith
