Here's why:

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

* Extrusive means the magma or lava erupted onto the Earth's surface.

* Intrusive igneous rocks form when magma cools and solidifies underground.

Examples of extrusive igneous rocks formed from quickly cooled lava include:

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

* Rhyolite: A light-colored, fine-grained rock formed from felsic lava.

* Obsidian: A dark-colored, glassy rock that cools so quickly it doesn't have time to form crystals.

The speed of cooling affects the size and shape of the crystals in the rock. Quick cooling leads to smaller, more numerous crystals, while slower cooling allows for larger crystals to form.