Ecology And Environment

Igneous Rocks Basalt

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Among the igneous (volcanic) rock that covers the earth, ‘basalt’ could be considered the most common, because most of the ocean floor is made up of it. As with any other igneous rocks, basalt is also derived from cooling of magma and in most instances, it is considered an extrusive type of igneous rock as the formation of the rock follows the outpouring of magma through volcanic eruptions. Given its widespread nature, this article will briefly look at the formation, composition, consistency and the spread of basalt in comparison to other members of the igneous rock family.

Formation of Basalt

As described earlier, basalt is formed through the cooling of magma as it flows on the surface of the earth following its exit from the earth’s crust through volcanic eruptions. Most of the time, the volcanic eruptions that give rise to basalt take place under the sea and therefore the cooling of magma that take place when forming basalt would be much faster than cooling of other types of extrusive igneous rocks. This can also explain why basalt covers most of the ocean floor, while the predominant igneous rock covering the continents is granite.

As the cooling of magma when forming basalt is fast, it may give rise to a significant porosity in the basalt structure and therefore is given the name ‘vesicular’ basalt. The reason for such porosity is the presence of bubbles and gaseous constituents within magma at the time of rapid cooling when flowing on the earth’s surface. However, such pores can also take place due to chemical reactions and physical weathering on the already formed basalt, which gives rise to cracks and widening the existing cracks. As magma cools rapidly, it does not allow its minerals to grow and therefore the mineral constituents of basalt would be fine grained as compared to some other igneous rocks such as granite, which consists of coarse grains.

Composition of basalt

As with most other igneous rocks, basalt is also comprised of minerals from the silicate group and the darker and denser appearance of basalt is the result of having heavy minerals, which bear magnesium and iron. In other forms of igneous rocks, the mineral composition will mostly bear silicon and aluminum. Thus, oxidized basalt may appear red in color as the iron acquires ‘rust’ through its reactions with the oxygen.

Consistency of basalt

As described earlier, many types of basalt are fine-grained and may appear quite glassy. However, basalt may contain a porphyritic structure with larger crystals embedded in a finely crystalline groundmass. In a porphyritic structure, the commonest minerals to be seen are olivine and augite, although plagioclase feldspar could also be found. At the same time, the cavities of basalt lavas could be secondarily filled with minerals such as calcite, chlorite and zeolites.

Spread of basalt igneous rocks

Geologists classify the basalt igneous rocks into two types based on their site and these are the ocean island basalts and large igneous provinces. Thus, the ocean island basalts are commonly seen in places such as Hawaii and Iceland, while the best places to see large igneous provinces made out of basalts are Columbian plateau of the northwestern United States, the Deccan region of western India and the Karoo of South Africa. However, different types of basalt can be found in relation to many volcanic islands, as well as throughout the world’s great mountains.

More about this author: Dr Pandula Siribaddana

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