Final answer:
Igneous rocks can be classified as extrusive, forming on Earth's surface with fine grains, or intrusive, forming underground with coarse grains. Extrusive rocks are generally volcanic, made from rapidly cooled lava, while intrusive rocks, often plutonic, have larger crystal grains formed from slowly cooled magma.
Step-by-step explanation:
Igneous rocks may be either extrusive if they form on or near Earth's surface, or intrusive if they solidify underground. When magma erupts onto the surface and solidifies, it forms extrusive rocks, which tend to have a fine-grained texture due to rapid cooling. This rapid cooling doesn't allow large crystals to form, resulting in rocks like basalt. On the other hand, when magma cools slowly beneath the surface, it forms intrusive rocks with coarse-grained textures, such as granite. The slow cooling allows large crystals to grow over time.
Moreover, the environment of formation influences mineral composition and grain size. Extrusive rocks, often volcanic and shiny due to their small crystal sizes, are produced by lava that has cooled quickly. Intrusive rocks are typically found deeper within Earth's crust, forming large, easily visible crystals, giving them a sugary appearance as light reflects off the crystal faces. Examples include the basalt rocks of Hawaii and the granite found in the Catalina Mountains. Understanding these characteristics helps in identifying and classifying the various igneous rocks based on whether they are volcanic or plutonic.