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
Magma chamber Magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is under great pressure, and, given enough time, that pressure can gradually fracture the rock around it, creating a way for the magma to move upward. If it finds its way to the surface, then the result will be a volcanic eruption; consequently, many volcanoes are situated over magma chambers. These chambers are hard to detect deep within the Earth, and therefore most of those known are close to the surface, commonly between