Igneous, Sedimentary and Metamorphic Rocks

A rock is a solid cumulative of minerals located in the earth's lithosphere. Rocks are generally classified into igneous, sedimentary and metamorphic rocks. These are classified on the basis of their mineral, chemical and textural composition. Let's find out more about the types of rocks along with the processes that are responsible for the formation of these rocks.

Igneous Rocks

Igneous rocks are created when molten magma cools down and is split into two main types. These are plutonic and volcanic rocks. Plutonic or intrusive rocks come into being when magma cools and crystallizes gradually inside the earth's crust. Granite is an example of plutonic rock. Volcanic or extrusive rocks are formed from magma reaching the surface either as lava or as fragmental ejectiles. Basalt and pumice are two examples of volcanic or extrusive rocks. There is a third, though much rarer category of igneous rocks namely hypabyssal. These are mainly formed between the plutonic and volcanic rocks. They usually form sills, dikes or laccoliths.

The word igneous may be traced back to the Latin term 'Igneous' or fire. There are more than 700 types of igneous rocks. They vary from each other in terms of occurrence, texture, mineralogy, chemical composition and geometry. The two important factors used for the classification of igneous rocks are the particle size, which to a great extent, depends upon the cooling history and the mineral composition of the rock. The upper 10 miles of the earth's crust comprises almost entirely of igneous rocks. Igneous rocks are of tremendous geological significance on account of a variety of factors. They provide an insight into the composition of the mantle besides other crucial information regarding the evolution of the earth itself via radiometric dating.

Sedimentary Rocks

Sedimentary rock formation involves the sedimentation of material in the earth's surface and within water bodies. Though they are extensive in their spread over the earth's crust but in terms of pure volume, they comprise only 5% of the earth's total crust. It is often said that the study of sedimentary rocks provide an insight into the study of life itself. Sedimentary rocks are deposited in layers as in strata, referred to as bedding.

Sedimentary rocks are classified into clastic, chemical precipitate and biochemical. Limestone and sandstone are examples of clastic formations. Iron content usually influences the color of sedimentary rocks. Sedimentary rocks alone can contain fossils. They also are important sources of coal, drinking water and fossil fuels. Civil engineering has courses designed to study sedimentary rocks. This is crucial, especially, during construction-related activity.

Metamorphic Rocks

Metamorphic rocks form when rocks undergo metamorphosis/changes due to heat and pressure. Igneous and sedimentary rocks mainly undergo this change and become metamorphic rocks. The existing rock type which undergoes a change is referred to as the protolith. Hence, the study of metamorphic rocks provides a tremendous understanding of the pressures and temperatures deep within the earth's crust.

Gneiss, marble, slate, schist and quartzite are some of the different types of metamorphic rocks. The major types of metamorphism are contact and regional. The former takes place when magma is injected into the surrounding solid rocks while, the latter is primarily associated with large masses of rock spread over a very wide area. This regional metamorphosis is caused mainly by temperature and pressure.

Though rocks have been classified into igneous rocks, sedimentary rocks and metamorphic rocks, there are no distinct lines between them. Changes in their constituent minerals may result in them changing into the other. Some of the latest technology and tools that we use today may trace their origins to more primitive rock based tools used by our early ancestors.