All rocks on Earth were initially igneous in nature. Igneous rocks form as liquid magma cools, forming crystal structured rocks. There are many different types of igneous rocks.
Igneous rocks that form deep within the Earth’s crust where temperatures are very high might take thousands of years to cool down. This causes the crystals to be much larger, such as in the case of granite. Igneous rocks formed on the surface cool down in just a matter of a few hours. The crystals in these rocks can be microscopically small.
Thus, scientists divide igneous rocks into two different categories: Those which cooled below the surface, and those that cooled on the surface. Those which began their lives below the surface are called intrusive rocks, while those which cooled on the surface are referred to as extrusive rocks.