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In this article we will discuss about the characteristics of granite and basalts which are the most significant examples of igneous rocks.
Characteristics of Granites:
Granites are the example of the plutonic intrusive igneous rocks which are formed deep within the earth. Since the rate of cooling and solidification of magmas inside the earth is very slow because of very high temperature prevailing underground and hence granites become coarse-grained due to full development of large-sized grains.
Granites are composed essentially of the minerals of quartz, feldspar, and mica but the most abundant mineral is feldspar, mainly orthoclase. Sometimes, the minerals are uniformly distributed and all of them are almost of the same size. Besides, albite, biotite, muscovite and hornblende are also found in granite rocks.
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The granite family includes numerous types of rocks. These granitic rocks are differentiated on the basis of their texture and mineral composition. For example, hornblende granite (when hornblende mineral is most dominant), rhyolite granite, pumice granite, absidian granite, pitchstone granite etc.
From the standpoint of chemical composition granites are acidic rocks wherein silica content ranges between 65 to 85 per cent. Granites are generally light in weight as their density varies from 2.75 to 2.8. Table 8.2 denotes percentage composition of different minerals in granites.
There is also wide range of colour variation in different types of granites. The colour variation is caused mainly because of the number of different minerals present in the rocks and the size of grains. Generally, granites are of light colour but if orthoclase mineral is present in abundance, the granites become pink to yellow of slightly reddish in colour.
If dark coloured hornblende or biotite is a dominant mineral, the granites become of dark black or dark grey colour. Granites are generally resistant to erosion but when the rocks are well jointed, they are easily weathered and a very peculiar landform, ‘tor’ is formed.
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Characteristics of Basalts:
Basalt is a very fine-grained, dark coloured extrusive igneous rock which is formed due to cooling and solidification of molten lavas at the surface of the earth. Sometimes, the cooling of lavas takes place so rapidly that no time is available for the crystallization of basalt and hence no grains are formed, with the result the rock becomes glassy basalt. Basalts having grains, though very small rather minute, are called aphanitic basalts.
Chemically basalts contain 45 to 65 per cent of silica content. Though the rock is heavy in weight but is more susceptible to chemical weathering and fluvial erosion. The dark colour of basalts is because of the abundance of iron. Feldspar is the most dominant mineral (46.2 per cent).
Besides, augite (36.9 per cent), olivine (7.6 per cent), mineral iron (9.5 per cent) etc. (others-2.4 per cent) are other constituent minerals of basalts. Sometimes, polygonal cracks are developed in basalts due to contraction on cooling of lavas. Columnar jointing’s in basalts give birth to peculiar landforms characterized by uneven terrain surfaces.