Intrusive rock

Intrusive rock is formed when magma penetrates existing rock, crystallizes and solidifies underground to form intrusions, for example plutons, batholiths, dikes, sills, laccoliths, and volcanic necks. Some geologists use the term plutonic rock synomymously with intrusive rock but other geologists subdivide intrusive rock, by crystal size, into coarse-grained plutonic rock (typically formed deeper in the Earth’s crust in batholiths and other plutons) and medium-grained subvolcanic or hypabyssal rock (typically formed higher in the crust in dikes and sills).

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