The earth’s crust consists of rocks. A rock is any mineral material of the earth.…
THE CLASSIFICATION OF ROCKS
The earth’s crust is made up of various types of rocks, differing from one another in mineral content, texture, structure, colour, permeability, mode of occurrence and degree of resistance to denudation. A knowledge of these rocks is of paramount importance to geologists, who study the composition and physical history of the earth, but the geographer, too, needs a basic knowledge of the most common rocks and their relationship with landforms. Rocks also form the basis for soil formation and determine to some extent the type of natural vegetation and land use, so we must have a fair acquaintance with the rocks around us.
Rocks may be defined as aggregates of minerals, that is, a combination of different minerals. For example, granite which is a common rock in Nigeria is composed essentially of the minerals quartz, feldspar and mica, while limestone contains a bit of quartz and feldspar, and much of calcium carbonate (CaCO3).
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 fined-grained, while sandstone which is coarse-grained 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 rocks, 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 varies 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 ferromagnesian 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.
Rocks may be classified also according to age.
