The rate at which magma cools determines whether an igneous rock is intrusive or extrusive.

Extrusive rocks are formed when the molten lava present inside the Earth’s interior comes out during volcanic eruption. As the lava reaches the earth, also called magma, it cools down and solidifies. This solidified magma forms extrusive rocks.

When the molten magma cools down deep inside the earth’s crust then the solid rocks so formed are called intrusive rocks.