Rocks that form when magma cools within the Earth are called intrusive igneous rocks.

Here's why:

* Intrusive: This means the magma cooled and solidified *inside* the Earth's crust.

* Igneous: This means the rock formed from the cooling and solidification of molten rock (magma or lava).

Here are some examples of intrusive igneous rocks:

* Granite: A very common and durable rock often used in construction.

* Gabbro: A dark-colored rock found in oceanic crust.

* Diorite: A rock with a mix of light and dark minerals, found in both continental and oceanic crust.

* Peridotite: A very dense rock found deep within the Earth's mantle.

The slow cooling process of intrusive magma allows for larger crystals to form, giving these rocks a characteristic coarse-grained texture.