Intrusive rock Intrusive rock 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. 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
Lava of lava dome eruptions include the Novarupta dome, and successive lava domes of Mount St Helens. Lava tubes are formed when a flow of relatively fluid lava cools on the upper surface sufficiently to form a crust. Beneath this crust, which being made of rock is an excellent insulator, the lava can continue to flow as a liquid. When this flow occurs over a prolonged period of time the lava conduit can form a tunnel-like aperture or "lava tube", which can conduct molten rock many kilometres from the vent without cooling appreciably. Often these lava tubes drain out once the