Igneous rocks are formed from magma (molten rock) that has cooled and solidified. The cooling process determines the texture and mineral composition of the resulting igneous rock. There are three main types of igneous rocks: intrusive (plutonic), extrusive (volcanic), and hypabyssal.
Intrusive Igneous Rocks
Intrusive igneous rocks are formed when magma cools slowly below the surface of the Earth. As the magma cools over thousands to millions of years, mineral crystals have time to grow. This results in large mineral crystals that are visible to the naked eye, giving intrusive igneous rocks a phaneritic texture. Granite, gabbro, and diorite are examples of intrusive igneous rocks.
Granite is an intrusive felsic igneous rock, meaning it has high silica content. It contains abundant quartz, feldspar, and mica minerals. The interlocking network of feldspar and quartz crystals gives granite a speckled appearance. Granite often contains distinctive mineral deposits and pegmatites with very large crystals.
Gabbro is an intrusive mafic igneous rock. It has relatively low silica content and contains more dark-colored minerals like pyroxene, amphibole, and olivine. The ophitic texture of gabbro consists of pyroxene and plagioclase feldspar crystals in a typical arrangement that resembles a snake swallowing an egg.
Diorite is an intrusive intermediate igneous rock. Its mineral composition falls between felsic granite and mafic gabbro. Diorite consists mainly of plagioclase feldspar with a scattering of dark amphibole and pyroxene minerals. The salt-and-pepper speckled texture of diorite is due to the mix of light and dark mineral crystals.
Extrusive Igneous Rocks
Extrusive igneous rocks form when magma reaches the surface as lava or volcanic ash, cools quickly and solidifies. The fast cooling only allows small mineral crystals to form, resulting in fine-grained rocks. Basalt, andesite, obsidian, pumice, and scoria are extrusive igneous rocks.
Basalt is an extrusive mafic volcanic rock low in silica. It is typically dark colored with calcic plagioclase, pyroxene, and olivine minerals. Columnar jointing is a common feature of basalt flows.
Andesite is an extrusive intermediate volcanic rock, transitional between basalt and rhyolite. It contains visible crystals of plagioclase feldspar, amphibole, and pyroxene minerals in a fine-grained matrix. Andesite lava commonly forms thick, blocky flows.
Obsidian is an extrusive felsic volcanic glass that forms when rhyolitic lava cools extremely rapidly. It lacks crystal structure and breaks with a distinctive conchoidal fracture. Obsidian was used by ancient people to make tools and weapons.
Pumice and scoria are porous extrusive igneous rocks ejected from volcanoes. Pumice has silica-rich composition and forms frothy fragments from gas-rich lava. Scoria has mafic composition and forms dark-colored cinder-like clinkery fragments.
Hypabyssal Igneous Rocks
Hypabyssal rocks form from magma intruded at moderately shallow depths in the crust. They cool slower than volcanic rocks but faster than plutonic rocks. This results in a texture between the two extremes: medium-sized mineral crystals visible without magnification. Diabase (dolerite) is a common example of a hypabyssal igneous rock.
