When a magma chamber cools slowly and hardens underground, it forms intrusive igneous rocks.

Here's why:

* Intrusive means the rock formed beneath the Earth's surface.

* Igneous means the rock formed from cooled and solidified magma or lava.

Examples of intrusive igneous rocks include:

* Granite: A very common, light-colored rock with large crystals.

* Gabbro: A dark-colored rock with coarse grains.

* Diorite: A medium-colored rock with a mix of light and dark minerals.

Slow cooling allows for the formation of larger crystals, as the minerals have more time to grow and arrange themselves. This is a defining characteristic of intrusive igneous rocks.