Intrusive igneous rocks form when magma cools and solidifies beneath the Earth’s surface. Here are three primary methods of their formation:
- Batholiths: These are large masses of igneous rock that form deep within the Earth’s crust. They are created when a massive amount of magma cools slowly over thousands to millions of years. Batholiths are often exposed at the surface due to erosion of the overlying rock.
- Dikes: Dikes are vertical or near-vertical intrusions of magma that cut across existing rock layers. They form when magma is forced into fractures or cracks in the surrounding rock. Dikes can vary in thickness from a few centimeters to several meters.
- Sills: Sills are horizontal or near-horizontal intrusions of magma that form between layers of existing rock. They are created when magma is injected along bedding planes or other flat surfaces. Sills can extend over large areas and are typically parallel to the rock layers they intrude.
These methods of formation result in coarse-grained textures due to the slow cooling process, which allows large crystals to form.