batholith

A very large igneous rock mass, typically granite, intruded while molten into the surrounding country rock, outcropping over at least 100 km²/40 sq mi and extending to unknown depth. It is characteristic of orogenic belts and subduction zones, such as the Andean batholith in South America or the Coast Range of W Canada and Alaska.

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.

Although they may appear uniform, batholiths are in fact structures with complex histories and compositions. 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;

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. For a geographer, a batholith is an exposed area of mostly continuous plutonic rock that covers an area larger than 100 square kilometers. However, the majority of batholiths visible at the surface (via outcroppings) have areas far greater than 100 square kilometers. 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. 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).