Igneous rocks are some of the oldest known rocks and one of the three main rock types, the others being sedimentary and metamorphic. These rocks are always formed from the cooling of lava or magma and the exact type of rock formed will depend on several factors. Igneous rock types can be separated into two distinct categories and their are about six main kinds of rock in each category.
Considering that igneous rocks are all formed by lava/magma, it would seem that there would only be a few types but this not true. The conditions in which the rocks cool from the lava/magma will determine exactly which of the twelve basic types of igneous rock will form. The cooling process of the lava/magma will result in a phase change from liquid to form a solid and crystallization will occur. This change could take place relatively quickly or over a long period of time. Depending on the source material, as there is more than one type of lava/magma, the igneous rock can have slight differences in composition but are commonly formed from the following minerals: quartz, olivine, biotite, amphibole, plagioclase, orthoclase, and pyroxene.
The cooling will take place outside or inside of the Earth’s crust. This is actually the basis of the separation of igneous rock types into two categories, because the differences in cooling produce distinctive difference in texture and grain size. Magma that cools inside the Earth’s crust forms intrusive igneous rock while lava that cools on the surface of the Earth forms extrusive igneous rock.
Intrusive Igneous rocks
Magma that cools inside the Earth’s crust is deemed intrusive or plutonic igneous rock. The cooling is very slow and can take thousands of years to even millions of years before the cooling has finished. As a result of the slow cooling and the pressure inside the Earth, intrusive rocks have courser grains and what is known as a paneritic texture. This essentially means that the individual crystals on the intrusive rocks are large and easy to see. The rocks look like a mixture of different base components. Common intrusive igneous rocks include: granite, gabbro, pegmatite, diorite, and peridotite.
Many of these igneous rocks are known for their strength and durability which is why they are often used as building materials. The best example of this is granite because the chemical composition of granite makes it ideal for many different applications.
Extrusive Igneous rocks
Lava is just magma that has come to the surface and the rapid cooling that it experiences in the presence of the comparitively cooler atmosphere forms extrusive igneous rocks. The cooling process is extremely fast compared to intrusive rocks and only takes days or weeks. Some rocks are even rapidly cooled when they are ejected into the air during an eruption and can partially or completely solidify before reaching the ground.
Common intrusive igneous rocks include: andesite, tuff, obsidian, rhyolite, pumice, scroia, and basalt. These rocks are mostly fine-grained and deemed as having an aphanitic texture which means that any crystals are small and difficult to see, usually beyond what the human eye is able to distinguish. However, there are three other different textures that may result in extrusive igneous rock.
The lava can cool to form a very dark volcanic glass or obsidian if quenching occurs. Some extrusive rocks may also have a unique feature called a vesicular texture. This texture results from gases trapped in the magma being released during the faster cooling. Two vesicular extrusive rocks are pumice and scoria. The texture of these igneous rocks is similar to what happens to dough when it is heated to form bread. The yeast in the dough produces carbon dioxide that creates many small pores and pockets inside the rising bread. The same texture is present in igneous rocks which are formed in this manner.
The final texture is porphyritic and is characterized by the presence of both the small crystals of fine-grained rock and some large crystals of course-grained rock. This occurs when some cooling happens when the rock is still magma and just inside the Earth’s crust. It soon exits the crust during an eruption but because of the time it sat and cooled, some larger crystals will have formed. This texture can happen with several of the extrusive rocks.