To determine whether an igneous rock is intrusive or extrusive, you need to examine its texture and mineral composition. Intrusive igneous rocks, such as granite, form from magma that cools slowly beneath the Earth’s surface. This slow cooling allows large crystals to form, giving intrusive rocks a coarse-grained texture. On the other hand, extrusive igneous rocks, like basalt, form when lava cools quickly on the Earth’s surface. The rapid cooling results in fine-grained textures, where crystals are too small to see with the naked eye.
Other characteristics include the presence of vesicles (small gas bubbles) in extrusive rocks, which are formed when gas escapes from the lava as it cools, and the overall color and mineral composition can also give clues. For example, if you see a rock with visible crystals and a rough texture, it’s likely intrusive. If it has a smooth texture or looks glassy, it’s probably extrusive.