Igneous rocks may be classified into extrusive igneous rocks and intrusive igneous rocks.
Extrusive igneous rocks are formed by cooling molten magma on the surface of the Earth. The molten magma comes onto the Earth’s surface through cracks, fissures, and volcanic eruptions.
They are also known as volcanic rocks. Because these rocks solidify at a faster pace, they are smooth, crystalline, and fine-grained.
Basalt is a common example of extrusive igneous rock.
Also, Read Intrusive Igneous Rocks