intrusion(redirected from Intrusive igneous rock)
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a geological term used in two senses:
(1) The process of magma injection into the body of rock that forms the earth’s crust. The solidification of such injected magma leads to the formation of intrusive rock.
(2) An intrusion is a geological body composed of igneous rock and formed in the process of injection and solidification of mag-matic melt in the earth’s crust (intrusion, intrusive body, plutonic intrusion). In relation to the structure of the country rock, intrusions are classified as concordant and discordant. The first group includes sills, laccoliths, and lopoliths, and the second group includes batholiths, stocks, and dikes. Intrusions are classified according to the depths at which the magmatic intrusion occurred as deep-seated intrusions (abyssal) and shallow-depth intrusions (hypabyssal). In this context the conditions of cooling of the magma and its effect on surrounding rock differ sharply. At shallow depths the cooling is rapid and fine crystalline or porphyritic rock is formed, and contact metamorphosis affects a small area of the country rock. At great depths medium-grain and large-grain rock and major changes in the surrounding rock are characteristic.