How can we tell if a rock is extrusive or intrusive?

Answer 1

To determine if a rock is extrusive or intrusive, examine its texture and grain size. Extrusive rocks, formed from lava that cools quickly on the Earth's surface, typically have a fine-grained or glassy texture with small crystals. In contrast, intrusive rocks cool slowly beneath the surface, resulting in a coarse-grained texture with larger, visible crystals. Additionally, the presence of gas bubbles in extrusive rocks can further indicate their formation from volcanic activity.