Igneous rocks that cool quickly below the Earth's surface are called intrusive igneous rocks.

Here's why:

* Intrusive refers to the formation of igneous rocks when magma cools and solidifies *within* the Earth's crust.

* Extrusive igneous rocks form when lava cools and solidifies *on* the Earth's surface.

Key characteristics of intrusive igneous rocks:

* Slow cooling: Magma cools much more slowly than lava because it's insulated by surrounding rock.

* Large crystals: The slow cooling allows for larger mineral crystals to form.

* Examples: Granite, gabbro, diorite

Key characteristics of extrusive igneous rocks:

* Fast cooling: Lava cools rapidly in the open air.

* Small crystals: The rapid cooling doesn't allow time for large crystals to form, resulting in a fine-grained texture.

* Examples: Basalt, rhyolite, obsidian

Let me know if you have any other questions!