Mount Rushmore is not an igneous body. It is a granite formation, which is an igneous rock that has undergone a process called crystallization to form a solid, crystalline structure.

Here's a breakdown:

* Igneous Rock: Formed from the cooling and solidification of magma (underground) or lava (above ground).

* Granite: A type of igneous rock that is usually light-colored and composed of quartz, feldspar, and mica.

* Crystallization: The process where magma or lava cools and minerals within it arrange themselves into a crystalline structure.

Mount Rushmore is essentially a large outcrop of granite, which formed over millions of years from the cooling and crystallization of magma deep within the Earth.