Basic types ofigneous intrusions: 1.Laccolith 2. Smalldike 3.Batholith 4.Dike 5.Sill 6.Volcanic neck andpipe 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.
Although they may appear uniform, batholiths are in fact structures with complex histories and compositions. They are composed of multiple masses, orplutons, 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 ofpartial melting near the base of the Earth's crust.
The eroded laccolith above the batholith system ofVitosha, thePlana domed mountains, next toSofia,Bulgaria
Traditionally, these plutons have been considered to form by ascent of relatively buoyant magma in large masses calledplutonicdiapirs. Because the diapirs are liquified and very hot, they tend to rise through the surrounding nativecountry rock, pushing it aside and partially melting it. Most diapirs do not reach the surface to formvolcanoes, but instead they slow down, cool, and usually solidify 5 to 30 kilometers underground as plutons (hence the use of the wordpluton; in reference to theRoman god of the underworldPluto). An alternate view is that plutons are formed by aggregation of smaller volumes of magma that ascend asdikes.[2]
A batholith is formed when many plutons converge to form a huge expanse of granitic rock. Some batholiths are mammoth, paralleling past and presentsubduction zones and other heat sources for hundreds of kilometers incontinental crust. One such batholith is theSierra Nevada Batholith, which is a continuous granitic formation that makes up much of theSierra Nevada in California. An even larger batholith, theCoast Plutonic Complex, is found predominantly in theCoast Mountains of western Canada; it extends for 1,800 kilometers and reaches into southeastern Alaska.
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 calledstocks.[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 oferosion accelerated bycontinental 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, theircrystal structure expands slightly over time. This manifests itself by a form ofmass wasting calledexfoliation. This form of weathering causes convex and relatively thin sheets of rock to slough off the exposed surfaces of batholiths (a process accelerated byfrost wedging). The result is fairly clean and rounded rock faces. A well-known result of this process isHalf Dome inYosemite 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
