Types of Igneous Rocks

Igneous rock can be thought of as the mother of all rocks. Earth was molten rock as it formed and over time, cooled to form igneous rock. Over millions of years, this igneous rock was put under heat and pressure to one day become metamorphic rock and likewise, was subject to surface processes of weathering, erosion, deposition, and lithification to become sedimentary rock.

Igneous rock forms in two environments – in the subsurface (underground) or at Earth’s surface. When igneous rock forms in the subsurface, it is known as plutonic (intrusive) igneous rock. Plutonic igneous rock forms when magma slowly cools over millions of years. This slow cooling process is the reason you see large mineral grains in plutonic igneous rock (think granite).

Igneous rock that forms at Earth’s surface is known as volcanic (extrusive) igneous rock. Volcanic igneous rock forms as a result of volcanic activity. This can be volcanic eruptions, lava fields associate with fissure eruptions, or lava cooling at mid-ocean ridges. This type of igneous rocks cools much more quickly and therefore the rock does not tend to have large visible mineral grains.