<P> The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite . When exposed by erosion, these cores (called batholiths) may occupy huge areas of the Earth's surface . </P> <P> Intrusive igneous rocks that form at depth within the crust are termed plutonic (or abyssal) rocks and are usually coarse - grained . Intrusive igneous rocks that form near the surface are termed subvolcanic or hypabyssal rocks and they are usually medium - grained . Hypabyssal rocks are less common than plutonic or volcanic rocks and often form dikes, sills, laccoliths, lopoliths, or phacoliths . </P> <P> Extrusive igneous rocks, also known as volcanic rocks, are formed at the crust's surface as a result of the partial melting of rocks within the mantle and crust . Extrusive igneous rocks cool and solidify quicker than intrusive igneous rocks . They are formed by the cooling of molten magma on the earth's surface . The magma, which is brought to the surface through fissures or volcanic eruptions, solidifies at a faster rate . Hence such rocks are smooth, crystalline and fine - grained . Basalt is a common extrusive igneous rock and forms lava flows, lava sheets and lava plateaus . Some kinds of basalt solidify to form long polygonal columns . The Giant's Causeway in Antrim, Northern Ireland is an example . </P> <P> The molten rock, with or without suspended crystals and gas bubbles, is called magma . It rises because it is less dense than the rock from which it was created . When magma reaches the surface from beneath water or air, it is called lava . Eruptions of volcanoes into air are termed subaerial, whereas those occurring underneath the ocean are termed submarine . Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity . </P>

What igneous rock crystallizes at the earth's surface