The Classification Of Rocks

Rock Texture, Structure and Colour

Rock texture refers to the sizes of the mineral grains, that is, whether the rock is fine grained or coarse grained, dense glass or frothy glass. The texture of an igneous rock varies according to the environment in which solidification took place. Very rapid cooling of lava at the earth's surface results in a glassy texture. Moderately rapid cooling produces fine texture, while coarse textures are as a result of slow or gradual cooling of molten rock deep down in the crust. Textures of sedimentary rocks are recognizable on the basis of the size of the constituent grains and the minerals contained. Shale is a sedimentary rock formed from mud or clay and it is made up of particles of sand size 0.02mm - 2.00mm.
The colours of rocks are essentially a consequence of the minerals which make up the roc
ks, that is, the relative proportions of the minerals determine the colour of the rocks. Granite is a coarse-grained rock formed as a result of slow cooling in the crust, and it is light-coloured. Granite is composed mainly of quartz, feldspar which vary from white to pinkish, and mica which consists of black (biotite) and white (muscovite) types. On the other hand, basalt is dark-coloured and very fine-grained in texture. The dark colour is due to the presence of dark-coloured ferro-magnesian minerals such as olivine, hornblende, biotite mica, and so on.
Rock structures refer to features such as folds, faults, joints and bedding planes, as well as rock massiveness and physical hardness. Rock structures directly influence the resultant landforms. Very simple rock structures are the sedimentary strata, intrusions of sill type and basic lava extrusions. complex structures include folds which abound over large parts of the earth's surface.
Generally speaking, all rocks may be classified into three major groups - igneous, sedimentary and metamorphic, according to their origin and appearance.