Intrusion (geology)

Intrusive rock (also called plutonic rock) is formed when magma crystallizes and solidifies underground to form intrusions, for example plutons, batholiths, dikes, sills, laccoliths, and volcanic necks.

Intrusive rock forms within Earth's crust from the crystallization of magma. Magma slowly pushes up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the magma slowly cools into a solid, the different parts of the magma crystallize into rocks. Many mountain ranges, such as the Sierra Nevada in California, are formed mostly from large granite (or related rock) intrusions; see Sierra Nevada Batholith.

Intrusions are one of the two ways igneous rock can form; the other is extrusive rock, that is, a volcanic eruption or similar event. Technically speaking, an intrusion is any formation of intrusive igneous rock; rock formed from magma that cools and solidifies within the crust of the planet. In contrast, an extrusion consists of extrusive rock; rock formed above the surface of the crust.

Large bodies of magma that solidify underground before they reach the surface of the crust are called plutons. Plutonic rocks form 7% of the Earth's current land surface.

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