Igneous rocks refer to those rocks that originate from a high-temperature molten liquid called magma. The term 'igneous' comes from the Greek language, meaning 'born from fire.' Magma from deep underground rises along fractures and erupts onto the Earth's surface. Rocks that remain in a molten state after magma erupts onto the surface are called lava. Lava accumulates layer by layer to form a mountain called a volcano. Volcanic rocks cool quickly enough that large mineral crystals do not have time to form. Typical volcanic rocks include obsidian, basalt, and pumice.
In many cases, magma does not have enough energy to erupt onto the surface. Instead, it begins to cool while still underground. This is a slow process that favors the formation of large mineral crystals. Rocks formed in this way are called 'plutonic rocks' — the name comes from Pluto, the god who rules the underworld in Greek mythology. We can only see these rocks at the surface when the overlying rock layers have disappeared due to erosion. The most common plutonic rocks are granite, diorite, and gabbro.
There are two basic types of igneous rocks: 1) intrusive igneous rocks (solidified below the surface), such as diorite, gabbro, granite, and pegmatite; 2) extrusive igneous rocks (solidified on or near the surface), such as andesite, basalt, obsidian, pumice, rhyolite, and volcanic cinder.
Andesite is a fine-grained extrusive igneous rock, mainly composed of plagioclase along with other minerals such as hornblende, pyroxene, and biotite.
Gabbro is a coarse-grained dark intrusive igneous rock, containing plagioclase and pyroxene, and sometimes olivine.
Granite is a coarse-grained light-colored intrusive igneous rock, mainly composed of quartz and potassium feldspar, and plagioclase minerals.
Pumice is a light-colored, porous, glassy extrusive igneous rock. It is formed by the extremely rapid cooling and solidification of magma. The porous texture of pumice is created by the expansion and escape of volatile gases during the condensation of magma.
