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
Pitchstone Pitchstone Pitchstone is a dull black glassy volcanic rock formed when felsic lava or magma cools quickly. It is similar to obsidian but is defined by the International Union of Geological Sciences as having a higher water content. It is a volcanic glass; however, unlike a glass, pitchstone has an irregular hackly fracture not a conchoidal fracture. That is due to its coarser (than obsidian) crystal structure. Pitchstone has a resinous lustre, or silky in some cases, and a variable composition. Its colour may be mottled, streaked, or uniform brown, red, green, gray, or black. It is an extrusive rock