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. , 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. 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==