Geology And Geophysics

How Igneous Rocks are Formed



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The formation of igneous rock is a process, that in its basic form, is taught in most schools. The scientific process though is of course more complex dealing with pressure and chemical reactions, but the formation of igneous rocks has shaped this planet.

The term "igneous" is itself an indication of the formation process of the rock type, deriving from the Latin word "ignis", meaning fire. Early observers saw how new rocks were formed out of volcanic lava, and a name that linked the fire of the volcano to the rocks seemed to be apt.

The basic process behind the formation of igneous rocks starts below the surface of the earth, between the crust and the mantle where molten rock flows. The heat and pressure deep within the earth ensures that rock remain in a liquid state; as this molten rock  flows towards the surface of the earth though the magma starts to cool down, and solidifies into rock, igneous rock.

Today there are hundreds of different igneous rocks classified by geologists, although geologists do split them into two main types, intrusive and extrusive igneous rocks. Intrusive rocks are formed from magma, and so are formed beneath the surface of the earth; extrusive rocks on the other hand are formed on the surface normally as a result of lava flows.

The vast majority of igneous rocks are intrusive, and are generally to be found through mining, although erosion does help to leave outcrops of rocks visible. Intrusive igneous rocks are formed through the slow solidification of the magma. The amount of time that this solidification period takes, depends upon a number of factors, although most prevalent of these is the closeness of the molten magma to the earth’s surface. The temperatures involved though are still extreme, and as such the solidification process could take millions of years.

The slowness of the solidification also gives the inclusive igneous rocks their distinctive look. Slow solidification leads to slow crystallisation, mean in turn means large crystals and the texture of the rocks. The cooling process also allows seems of precious metals to form in between the solidifying rock formation.

Intrusive igneous rocks include the likes of granite, gabbro (or black granite) and peridotite.

The other main type of igneous rock is the extrusive type. These rocks are formed when magma becomes lava upon its escape to the earth’s surface. The cooling time for these rocks is not millions of years, and in the case of most lava flows is a few weeks at most.

The speed that extrusive igneous rocks are solidifies ensures that the crystals that make up the rock are much smaller, especially compared to their intrusive counterparts. The most well known rock of this type is probably basalt.

In super fast solidification, normally where lava comes into contact with a water source, the crystallisation process is so rapid that the crystals cannot be seen with the naked eye. This results in an amorphous appearance, and the rocks look like coloured glass. The most famous of such amorphous igneous rocks is obsidian.

Water, and in particular steam can have another impact upon the creation of extrusive igneous rock, and can form pumice, a rock full of bubbles which enables it to float.

Igneous rock is also classified by its content, and of course different rocks have different compositions. The primary method of categorising based on content is the quartz content of the igneous rock, although classification can also be made upon the presence of iron or other elements.

As previously mentioned all other types of rock do come from igneous rock initially. Sedimentary rocks are primarily formed by the breakdown of igneous rocks through erosion or weathering. Metamorphic rocks are formed as igneous or sedimentary rocks are altered through pressure or heat. At the same time though igneous rock can also help to create more igneous rock. This is primarily observed underground, and as molten magma travels past rock formations, the heat is sufficient to turn the rock once again into liquid form, until it solidifies into new rock forms.

Igneous rocks are in essence formed from molten rock, be it magma or lava, this though allows for a great deal of variation in the individual types.

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