Types Of Extrusive Rocks
There are many types of igneous rocks, but before we begin to examine those, let’s put things in order first.
There are three different types of rocks on earth: sedimentary rocks, metamorphic rocks, and igneous rocks. Sedimentation is a rather general name that refers to those processes which cause organic particles and minerals to build up and precipitate; precipitation being the process of forming a material within another solid material, or within a solution. Metamorphic rocks, on the other hand, take their name from the process they undergo, namely metamorphism. Metamorphism literally means a “change in the form”, and this is exactly the case with metamorphic rocks: a an already existing rock goes through metamorphism, which means that it becomes subjected to extreme heat and pressure, more specifically temperatures that exceed 150 Celsius degrees and 1500 bars respectively. As for igneous rocks, those are form when magma or lava of volcanoes cools down and becomes solidified. This solidification process can take place either above or below the earth’s surface: when it is takes place below, the producing rock is called intrusive; while when it takes place below the surface, they are called extrusive rocks. More specifically, extrusive rocks take less time to solidify compared to intrusive, and as a result they are fine-grained; that is also why all types of extrusive rocks are more are more smooth in texture and crystalline in their composition. Igneous rocks are classified with respect to their chemical composition, their geometry, their texture, their mineralogy and their mode of concurrence, a classification that reveals a lot about the conditions which lead to the formation of various types of intrusive and various types of extrusive rocks.
Now that you have some background information regarding rocks, let’s go back to our original question: what types of extrusive rocks are there? The most common types of extrusive rocks are: basalt, obsidian, pumice, rhyolite, andesite, dacite, and scoria. As we mentioned already, pretty much all types of extrusive rocks are fine-grained, and it should be taken for granted in the rocks analyzed below. Basalt is basically composed of plagioclase and pyroxene, but although its color is originally dark grey, it soon turns into a brownish/rust-red color due to the fact that it is rich in iron that becomes oxidized. Obsibian is much more dark in color, and it is essentially a type of volcanic glass. Rhyolite, on the other hand, is rather light-colored, and it consists in alkali feldspar, plagioclase, and quartz. Pumice is also a light-colored type of volcanic glass, a glass with a highly vesicular texture. Dacite’s has an aphanite to prophyritic texture and it contains relatively proportionate amounts of quartz and feldspar. Plagioclase, as well mineral likes pyroxene, hornblende, and biotite combined are what make up for the composition of andesite; andesite’s texture also ranges from aphanitic to porphyritic; in fact, in a way andesite is a type of rock that is something between basalt and dacite. Last but not least, scoria can be either brown black, and even purplish red, but in any case it always has a very dark color. Other types of extrusive rocks are: tuff, welded tuff, komatiite, etc.