Here's why:

* Igneous rocks are formed from the cooling and solidification of magma (molten rock) or lava.

* Extrusive rocks form when lava erupts onto the Earth's surface and cools quickly. The rapid cooling results in smaller crystals and a finer-grained texture.

Examples of extrusive igneous rocks include:

* Basalt: A dark-colored, fine-grained rock common in volcanic flows.

* Rhyolite: A light-colored, fine-grained rock rich in silica.

* Obsidian: A glassy, black rock that cools so quickly that crystals don't form.

Let me know if you'd like to know more about igneous rocks!