Intrusive igneous rocks form from magma that cools and solidifies below the Earth's surface. The rate at which the magma cools and solidifies determines the grain size of the resulting rock.
Intrusive igneous rocks generally have a coarse grain size because they cool slowly at depths below the Earth's surface. This slow cooling process allows the minerals in the magma to crystallize over a longer period of time, resulting in larger mineral grains in the rock.
Examples of intrusive igneous rocks with a coarse grain size include granite, gabbro, and diorite. These rocks are characterized by their large mineral grains and can be easily distinguished from other types of rocks by their texture.
On the other hand, extrusive igneous rocks form from lava that cools and solidifies above the Earth's surface. These rocks have a fine grain size because they cool rapidly due to their exposure to the cooler air and water. Examples of extrusive igneous rocks with a fine grain size include basalt, andesite, and rhyolite.