Here in this article, we’d help give you almost all you need to know about igneous rocks.
Igneous Rocks Definition
Igneous rocks are rocks formed from the cooling of molten magma. If one visits the island of Hawaii, one might observe red hot lava flowing over the land which cools and solidify into a fine-grained black rock called basalt.
Magma is a molten rock which may contain suspended solid crystals usually silicate minerals and gas. Magma in the Earth’s interior contains many volatile substances like hydrogen fluoride, carbon-monoxide, halogenic elements, e.t.c.
Classifications of Igneous Rocks
In this article igneous rocks have been classified based on mode of occurrence, chemical composition, and texture. Samples of each rock have been provided below under there various classifications. It should be noted that one rock type can occur under multiple classifications. Here are the classifications of igneous rocks:
- Classification Based on Mode of Occurrence
- Classification Based on Chemical Composition
- Classification Based on Texture.
Classification Based on Mode of Occurrence
Intrusive Rocks: These are igneous rocks formed when molten magma solidifies within the Earth’s interior. Examples of such rocks are diorite, gabbro, granite, peridotite and pegmatite.
Extrusive Rocks: These are igneous rocks formed from the solidification of molten magma on the surface of the earth. Examples of such rocks are andesite, basalt, dacite, obsidian, pumice and tuff.
Hypabyssal Rocks: These are “shallow-seated” Intrusive rocks, that is igneous rocks formed a few kilometres beneath the subsurface. Examples of such rocks are dolerite, microgranite, microdiorite, diabase.
Classification Based on Chemical Composition
Felsic or Silicic Rocks: These are igneous rocks with very high silica content (usually 65% or more SiO2). Due to the high silica content, such rocks are light coloured. Chemical analysis show that rocks in this category have small amounts of oxides of calcium, magnesium and iron. The remaining 25% to 35% is mostly aluminum oxide, sodium oxide or potassium oxide. Examples of felsic or silicic rocks are granite and rhyolite.
Mafic Rocks: These rocks are considered silica-poor, they contain about 50% silica and other minerals like pyroxene, amphibole, olivine and mica. These minerals have high magnesium and iron oxides, there presence gives mafic rocks its dark colour. Examples of mafic rocks are basalt, diabase, gabbro.
Ultramafic Rocks: These rocks are also known as ultrabasic rocks, they have very low silica content (usually less than 45%). Ultramafic rocks are composed entirely or almost entirely of ferromagnesian minerals. Examples are Peridotite, kimberlite, lamprophyre, lamproite and dunite.
Intermediate Volcanic Rocks: These are rocks with a chemical content between that of felsic and mafic. Andesite is the most common intermediate volcanic rock. Other examples are diorite, dacite and granodiorite.
Classification Based on Texture
Phaneritic Textured Rocks: These are deep-seated igneous rocks. They form when molten magma cools slowly within the earth’s interior and as a result forms coarse-grained igneous rocks with large crystalline grains. Examples of such rocks are gabbro, diorite and granite.
Aphantic Textured Rocks: These rocks are fine-grained, they form when molten magma cools rapidly at the Earth’s surface as such crystals have little time to form. Examples are basalt, andesite and rhyolite.
Porphyritic Textured Rocks: These are rocks that began crystallizing slowly underground followed by the Eruption and rapid solidification at the Earth’s surface. Porphyritic rocks may be fine-grained or coarse-grained, if they are coarse grained intrusive names are used, for instance porphyritic granite. If they are fine-grained extrusive names are used, for instance Porphyritic andesite. Other examples of rocks with porphyritic textures are porphyritic basalt and porphyritic rhyolite.