Rock Types, Formation, Characteristics, and Use

The term rock is used to define the agglomeration of mineral particles that exist on or beneath the earth’s surface naturally (Byerlee, 1978). Rocks contain materials which are closely grouped together. However, their structure and hardness vary depending on their chemical composition. Rocks are grouped into three major categories namely sedimentary rocks, metamorphic rocks and igneous rocks (Irvin & Bargar, 1971). This classification is made mainly on the basis of their particle size, mineral composition and chemical composition. This paper explores the various types of rocks in detail, including their ways of formation, characteristics and their uses.

Sedimentary rocks

Just as the name suggests, these rocks are formed when the sediments accumulate either on the earth’s surface or it could be in water bodies (Byerlee, 1978). Sedimentary rocks are made up of three basic types: Clastic sedimentary rocks which are formed as a result of mechanical weathering of big particle such as debris. Clastic sedimentary rocks consist of sandstone, shale, precise, and conglomerate (Pearce & Can, 1973). Tectonic setting of basic volcanic rocks

The clastic sedimentary rocks have different distinguishable features (Maitre, Streckeisen & Zanettin, 1986). Breccia is composed of fragments which are large with angular shape. Brace has a diameter of over two millimetres and the spaces which exist between large fragments can be filled with smaller particles which bind the rock together. Conglomerates contain large particles which are round, they are greater than two millimetres. The space between the fragments are also filled with small particles hence binds the rock together. Sandstone is made up of weathered debris which are in the size of the sand. Sandstone is majorly found in environments such as deserts, deltas, beaches and flood plains. Shale is made up of weathering debris and they are in the size of clay. The second type of sedimentary rocks is the chemical sedimentary rocks. Byerlee (1978) assert that chemical sedimentary rocks are formed when precipitation of dissolved materials from a solution occurs. Sedimentary rocks are formed mainly in water bodies such as rivers, lakes, oceans and even seas. As the rivers flow over rocks with minerals, the minerals dissolve in the water and are carried away. They are deposited together with the water in the river mouth such as lake, ocean or seas. With the high temperatures, water evaporate and leave behind the rock (Pearce & Can, 1973). Examples include rock salt and some limestone.

Limestone can be formed by two processes. It can form organically where coral and shell accumulate (Byerlee, 1978). It can also be formed through chemical process as a result of precipitation of calcium carbonate from the ocean or lake water. Limestone is made up of majorly calcium carbonate. Rock salt on the other hand is a chemical sedimentary rock that forms when the ocean or lake water evaporates. Rock salt is found mainly in areas where the climate is very arid (Pearce & Can, 1973). The last type of sedimentary rocks is the organic sedimentary rocks. The sedimentary rocks are formed as a result of accumulation of plant and animal remains (Byerlee, 1978). The remains of dead plants and animals accumulate for several years leading to the formation of the rocks. The example of sedimentary rocks includes coal and some limestone. Coal is formed when plants’ remains accumulate especially in a swampy environment. Coal is usually mined to be used as a fuel since it is combustible. Limestone can be formed chemically or organically. Organically it is formed when coral, shell, algal fecal particles accumulate. (Pearce & Can, 1973).

Metamorphic rocks

Byerlee (1978) assert that metamorphic rocks are formed when the sedimentary rocks and igneous rocks are subjected to heat, pressure or both. The igneous and sedimentary rocks are reduced into smaller sizes as a result of deformation and re-crystallization caused by the forces forming metamorphic rocks. The type of metamorphic rock formed will therefore depend on the amount of heat, pressure and time the rock was subjected to. Metamorphic rocks usually form deep in the earth where the factors such as heat and pressure are present. (Pearce & Can, 1973) They usually form at 15 kilometres below the earth’s surface. The heat used in the process of formation comes from the magma and pressure is derived from layers of rocks piled on each other. Pressure increases with increase in the thickness of the layers. There are two types of metamorphism which includes contact metamorphism and regional metamorphism. Contact metamorphism takes place when magma forces its way into the existing rocks (Byerlee, 1978). The rocks are subjected to heat of the magma changing their form. However, the changes are relatively small. Marble is a good example formed due to contact metamorphism. Regional metamorphism takes place over a wide area and the changes are very high. Regional metamorphism is associated with formation of high mountains (Maitre, Streckeisen & Zanettin, 1986).

Metamorphic rocks are classified into two major groups namely foliated and non foliated metamorphic rocks. This classification is based mainly on their texture. Foliated rocks appear in layers and are formed due to the difference in metamorphism (Maitre, Streckeisen & Zanettin, 1986). Foliated metamorphic rocks include, slate, phyllite, schist and gneiss. The rocks have different distinguishable features. Slate has a smooth dull surface due to its very fine grain size. The parent rocks of slate or shale, mudstone and siltstone. Phyllite has a fine grain size and the parent material is slate(Pearce & Can, 1973). Schist which has a grain size that is medium to coarse originate from phyllite. Gneiss also has a medium to coarse grain size and the parent rocks are schist, granite and volcanic rocks.

Non foliated metamorphic rock has a uniform texture and they contain one Mineral (Byerlee, 1978). Non foliated sedimentary rocks include, marble, quartzite and anthracite. Anthracite has a fine grain size and the parent material is bituminous coal. Both marble and quartzite have medium to fine grain size. The parent material for marble is limestone and for quartzite is quartz sandstone.

Igneous rocks

Igneous rocks are formed when molten magma solidify. Solidification can take place either on the earth’s surface or beneath. Igneous rocks are classified into two categories namely, intrusive and extrusive igneous rocks.

Intrusive Igneous Rocks

These are formed when magma cools before reaching the earth’s surface. The magma cools very slowly over a long period of time and this enables the individual mineral grains to grow to large sizes (Byerlee, 1978). These rocks have coarse grain texture. Examples of intrusive igneous rocks are, gabbro, diorite, granite and pegmatite.

Extrusive igneous rocks

Maitre, Streckeisen & Zanettin, 1986) assert when magma flows very fast reaching the earth’s surface before cooling it becomes lava. Lava cools immediately it reaches the earth’s surface due to the atmospheric temperature which is very cold.

Intrusive igneous rocks include; andesite, pumice, obsidian, basalt and scoria.

Rocks have several uses. Some of the uses of rocks include the following;

Coal is used in production of electricity in power plants (Byerlee, 1978). Limestone is used in the manufacture of cement, insecticides, glass, paper and petrochemicals. Granite is used for architectural construction and monuments. Gneiss rocks are used as building stones and other structural purposes. Pumice is used as an abrasive material in hand soap and emery boards (Pearce & Can, 1973).

In conclusion, it is evident that one rock can become another type of rock by erosion and deposition forming sedimentary rock, melting and cooling forming igneous rocks or metamorphic rocks formed through subjection of the igneous rocks and sedimentary rocks to heat, pressure or both.


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Byerlee, J. (1978). Friction of rocks. Pure and applied Geophysics, 116(4), 615-626.

Irvine, T., & Baragar, W. (1971). A guide to the chemical classification of the common volcanic rocks. Canadian journal of earth sciences, 8(5), 523-548.

Pearce, J. A., & Cann, J. R. (1973). Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth and planetary science letters, 19(2), 290-300.