Igneous Rocks By Sarah Barron
Basic Facts About Igneous Rocks • Igneous rocks- crystalline solids which form directly from the cooling of magma. • This exothermic process involves a phase change from a liquid state of matter to asolid state of matter. • The surface of the earth, where it exposed to the coldness of space, is made of igneous rock.
Naming Igneous Rocks • Igneous rocks are given names based upon two things: what they are made of (composition) and how big the crystals are (texture).
Texture • How large are the mineral grains in the final solid rock? • The slower the magma cools, the larger the mineral grains will be. • Classifications by texture name igneous rocks to be either intrusive or extrusive.
Comparisons of Texture Extrusive Coarsely-grained Mineral grains are large enough to see without a magnifying glass Also referred to as phaneritic Finely-grained Mineral grains are only visible with aid of a magnifying glass Also referred to as aphanitic Intrusive
Texture Classifications Aphanitic Granite Gabbro Obsidian Basalt Phaneritic
Composition • The composition of igneous magmas is directly related to where the magma is formed • Magmas are generally mafic if they are associated with crustal spreading • Magmas are felsic if they are associated with crustal compression and subduction
Mafic • Basalt is produced if the magma erupts at the surface • Gabbro is produced if the magma never makes it out of the magma chamber • Basalt and gabbro are compositionally the same however, they are two different rocks based on texture.
Intermediate • Intermediate magmas are associated with crustal compression and subduction like the felsic magmas. • Intermediate magmas go through the same differentiation process as felsic magmas as well. • Intermediate magmas produce diorite and andesite. • Diorite is intrusive whereas andesite is extrusive.
Felsic • Mafic seafloor basalt and continental sediments are subducted back into the crust, where they re-melt, thus continues the differentiation process. • Magma enriched in the lighter elements is the product. • The final result of the differentiation process, the felsic magmas, lead to the formation of granite or rhyolite. • Granite is intrusive whereas rhyolite is extrusive.
References • Google.com/images • jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry13.html