Introduction to types and classification of rocks

Abstract

Generally rocks can be divided into three major types based on the process of their formation. These are; Igneous rocks, sedimentary rocks and metamorphic rocks. Igneous rocks are formed by the solidification and cooling of magma in volcanic areas, while sedimentary rocks are formed by low temperature accumulation of sediments in tectonic basins and topographical sinks. Metamorphic rocks on the other hand are formed by application of temperature and pressure on pre-existing rocks. Metamorphic rocks therefore form at great depths, but exposed on the surface due to erosion and epirogenic movements. Each of these three rock types can be further classified in terms of; chemistry, how the form and environment of formation. The distributions of these major rock types are critical in regional mapping of natural resources. Igneous is coined from word “ignis” meaning fire and therefore these rocks are good indicators of volcanism and are there associated with geothermal resources and hydrothermal deposits like porphyry copper, and gold etc. sedimentary rocks especially the organic sediments on the other hand are good source rocks and are associated with resources like coal and oil. Sediments are also good proxies for environmental reconstruction, since fossils can be well preserved in these deposits. Metamorphic environments are associated with Gemstones and alteration minerals which form good index minerals as geobarometers and geothermometry. In this paper we look mainly into igneous and metamorphic rocks, their distribution, plate margins and geothermal resources.