What distinguishes intrusive from extrusive igneous rocks?

The distinction between intrusive and extrusive igneous rocks is based on their formation processes and locations within the Earth's crust. Intrusive igneous rocks, also known as plutonic rocks, originate from magma that cools and solidifies beneath the Earth's surface. This slow cooling process allows for the formation of larger crystals, resulting in a coarse-grained texture.

In contrast, extrusive igneous rocks, or volcanic rocks, form when lava erupts onto the Earth's surface and cools rapidly. This rapid cooling leads to the formation of smaller crystals, and these rocks typically have a fine-grained or glassy texture.

Understanding this difference is fundamental to geology, as it helps in identifying the environment in which a rock formed, which can inform us about the geological history of an area. This knowledge is crucial for various applications, such as resource exploration and environmental studies.