Millions and even billions of years ago molten rock was cooling and thus hardening into igneous rocks deep under the surface of the Earth. These rocks are now visible because mountain building has thrust them upward and erosion has removed the softer rocks exposing the much harder igneous rocks. These are called intrusive igneous rocks because the magma has intruded into pre-exiting rock layers. Types of intrusive igneous rocks are granite and basalt. The diagram above shows you a large intrusive igneous body called a batholith. A batholith is the largest of the intrusive bodies. They are larger than 100 square kilometers and usually form granite cores. Click on the "Next" button in the diagram to learn about more intrusive igneous bodies!

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