Intrusive igneous rocks, also known as plutonic rocks, cool quite slowly compared to their extrusive counterparts. This cooling process occurs beneath the Earth’s surface, where molten magma can take thousands to millions of years to crystallize into solid rock. The primary reason for this slow cooling is the insulating properties of the surrounding rock, which retains heat and prolongs the cooling period.
Since intrusive rocks are formed deep underground, they experience high pressure and the heat from surrounding magma and the Earth’s interior. This slow crystallization allows large mineral crystals to form, giving intrusive igneous rocks their characteristic coarse-grained texture.
For example, granite, a common intrusive igneous rock, often features visible quartz, feldspar, and mica crystals due to the lengthy cooling time. In contrast, extrusive igneous rocks like basalt cool quickly on the Earth’s surface, resulting in much smaller crystals. In summary, the slow cooling of intrusive igneous rocks is a key factor that influences their texture and mineral composition.