sentence: Intrusions are one of the two ways igneous rock can form.
sentences_after: The other is in a volcanic eruption or similar event. Technically speaking, an intrusion is a formation of igneous rock formed from magma that cools and solidifies within the crust of a planet. In contrast, an 'extrusion' consists of rock formed above the surface of the crust.
concept_name: intrusion
headings: Intrusion
categories: Geology Igneous rocks