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Intrusive igneous rocks (also called plutonic rocks, named after Pluto, the Roman god of the underworld) are formed from magma that cools and hardens within the earth. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as a result these rocks are course grained.
Igneous rocks are formed when magma crystallizes and solidifies. After this transition from liquid to solid, igneous rocks are classified as either intrusive or extrusive. In order for an igneous rock to be considered intrusive it must undergo the change from a liquid to a solid state inside of the Earth. Due to the high temperatures inside the Earth, this process can take thousands or even millions of years. This type of igneous rock tends to be much larger and has a coarse texture.