Here's why:
* Extrusive means the magma erupted onto the surface.
* Intrusive means the magma cooled and solidified underground.
The rapid cooling of extrusive rocks leads to:
* Fine-grained textures: The minerals have less time to grow large crystals.
* Volcanic glass (obsidian): Cooling can be so fast that minerals don't form at all.
* Porphyritic texture: Sometimes larger crystals form first in the magma chamber, and then the remaining magma cools rapidly, resulting in a mix of large and small crystals.
Examples of extrusive igneous rocks include:
* Basalt: A dark-colored, fine-grained rock often found in volcanic flows.
* Rhyolite: A light-colored, fine-grained rock, often with a glassy texture.
* Andesite: An intermediate-colored, fine-grained rock, often with a porphyritic texture.
