Mount Rushmore is carved into intrusive igneous rock. Specifically, it's made of granite, which is a coarse-grained, light-colored igneous rock that forms when magma cools and crystallizes slowly beneath the Earth's surface.

Here's why:

* Intrusive igneous rocks form from magma that cools and solidifies within the Earth's crust. Granite is a classic example.

* Extrusive igneous rocks form from lava that erupts onto the Earth's surface and cools quickly. Examples include basalt and rhyolite.

Since Mount Rushmore is carved into a large granite formation, we know it originated from a slow-cooling magma chamber deep within the Earth's crust.