Here's why:

* Batholiths: These are large, irregularly shaped intrusions of igneous rock that solidify deep underground. They form when magma rises from the Earth's mantle and cools slowly.

* Igneous Rock: This rock type forms from the cooling and solidification of magma or lava.

The process of how batholiths form mountains is complex, but here's a simplified explanation:

1. Magma Intrusion: Magma from the mantle rises and intrudes into the Earth's crust.

2. Cooling and Solidification: The magma cools slowly and solidifies, creating a large body of igneous rock called a batholith.

3. Uplift and Erosion: The immense weight of the batholith can cause the surrounding rock to uplift, forming mountains. Over time, erosion wears down the mountains, exposing the batholith.

So, while mountains themselves are made of a variety of rock types, the core of many mountain ranges, especially those formed by tectonic activity, is composed of igneous rock in the form of batholiths.