The Igneous rocks, also referred as Plutonic rocks are one of the main rock types apart from metamorphic and sedimentary. These rocks can be formed either by intrusive or extrusive process depending on the location on the earth’s surface. The cooling and crystallization of magma (melted rock) inside the ground or above the ground creates the igneous rocks. Eruption of volcanoes also causes the formation of Igneous Rocks when the magma intensifies above the surface of the earth. The magma that rises above the surface is referred as Lava and the igneous rocks form when the lava solidifies. When the igneous rocks are formed inside the surface of the earth, the magma cools and solidifies in the subsurface of the earth and is visible in the form of crystals, for example Granite, Gabbro; however, the igneous rocks formed outside the surface do not contain any visible crystals because of the temperature, for example, Obsidian, Basalt.
Many igneous rocks contain foreign material in the form of xenoliths, torn from conduit walls during magma ascent, or present in a disaggregated state as individual xenocrysts (Gill 17). These rocks can be identified by the texture, chemical composition, color, the minerals and the geometry of the rocks. The igneous rocks can be divided into Felsic Rocks and Mafic Rocks. The Felsic rocks are light in color, contain low temperature minerals, and high amount of silica, for example Quartz and Potassium Feldspar, whereas the Mafic rocks are dark in color, contain high temperature minerals and high amount of iron and magnesium, for example Olivine, Biotite Mica and Hornblende. These rocks can be further classified as Intermediate Rocks and Ultramsfic rocks.
The other igneous rocks are pumice, andesite, diorite, porphyry, pegmatite, and rhyolite. The Pumice and Obsidian are generally volcanic glasses as the texture is glassy. Pumice is light weight and hovers on water. The common color of Pumice is gray and Obsidian comes in white to gray color. The aphanitic textured igneous rocks are rhyolite, andesite and basalt. All these are fine grained rocks. The minerals in rhyolite cannot be seen though the content is similar to that of granite, it comes in the shades of gray, pale yellow, pink and light brown. Andesite’s are either gray or light brown and they are quite visible crystals. Basalt contains large iron content that provides brown or black color to it. It may even turn red when oxidized.
The granite, gabbro, and diorite are coarse grained or phaneritic textured igneous rocks. Granite is made up of orthoclase feldspar, quartz, white mica, or black mica, and depending on the mineral the color of granite can be pink or red. Diorite contains white feldspar mineral and it can be seen in light white to dark colors. The gabbro is very dark compared to other igneous rocks and the contents are similar to basalt. The porphyry and pegmatite igneous rocks contain large and small grains. The mineral content pegmatite is similar to that of granite and they are found in the form of large crystals.
Works Cited
Gill, Robin. Igneous Rocks and Processes: A Practical Guide. (Illustrated). John Wiley & Sons. 2010. Print.
