On the basis of their origin, characteristics, and information, the rocks are classified into following three types:
- Igneous Rocks
- Sedimentary (Aqeous) Rocks
- Metamorphic Rocks
(A). IGNEOUS ROCKS:
The term igneous is based on the Latin word ignis meaning fire. Thus it denotes those rocks which originated through fire. Technically speaking, these rocks were formed as a result of cooling of molten magma (Lava) either inside the crust or on the surface. According to H.J.de Blij,
“Magma is not only a complex melt of many minerals, it also means gases including water vapor. It is a surging, swelling mass that pushes outward and upward. Sometimes facing itself into and though existing layers of rocks in the crust. If its upward thrust ceases before it reaches the surface, the resulting rock formed from the cooled magma are called intrusive igneous rocks. If it penetrates all the way to the surface and spills out as lava or ash, the rocks formed from these materials are called extrusive igneous rocks.”
(B). SEDIMENTARY ROCKS
Sedimentary rocks are also called secondary rocks as these formed from the igneous rocks. These are also termed as stratified and aqueous rocks. According to Goth Cheng Leong,
“Sedimentary rocks are formed from the sediments accumulated over long periods of usually underwater. They are distinguished from other rocks by their characteristic layer formation and are termed stratified rocks. The strata may vary in thickness from a few centimeters to many meters. The rocks may be coarse or fine grained, soft or hard. The materials that form sedimentary rocks may be brought by streams, glaciers, winds and animals. They are non-crystalline and often contain fossils of animals, plants and other organism.”
The word sedimentary is derived from the Latin word sedimentum meaning “Settling”. Thus sedimentary rocks are those which result from the deposition and compaction of rock and mineral grains derived from other rocks. According to H.J.de Blij,
“Many sedimentary rocks begin their existence as loose deposit of sand or gravel at the bottom of a sea or lake, on a beach or in a desert. Later the sediment is lithified-compressed into a rock. As successive layer of sediment accumulate the weight of the sediments expels most of water between grains. Pressure caused by the weight of overlying materials will compact and consolidate lower strata. The grains are squeezed tightly together, especially in fine grained sediments such as clays and silts. This is process of compaction.
Characteristics of Sedimentary Rocks:
The sedimentary rocks are the following features:
- The Sedimentary rocks are different from other rocks due to their layer formation. These layer are called stratas.
- The rock may be coarse or fine-grained, soft or hard.
- The materials that form sedimentary rock are transported by streams, glaciers or winds.
- These rocks possess mud cracks or marks of ripples of water waves because they are formed under water.
- The particle forming sedimentary rocks are the different sizes varying in thickness from few centimeter to many meters.
- With the passage of time, these rocks may be folded, faulted or deformed.
(C). METAMORPHIC ROCKS:
The terms metamorphic is derived from a Greek word meaning change. Thus the metamorphic rocks are those that have been changed by heat and pressure. Thus actually these are those igneous or sedimentary rocks which have changed through metamorphism. Metamorphism is the process which transforms the existing rocks to metamorphism rocks through heat and pressure. According to F.J. Monkhouse,
“ Igneous and sedimentary rocks may undergo changes, both physical and chemical, which produce either new minerals or new structure within the rocks. These changing may be brought about by Earth’s movements subjecting the rocks to both heat (thermal) and pressure (dynamic).” Similarly, Goh Cheng Leong explains the formation of metamorphic rocks in these words,
“ All rocks whether igneous or sedimentary may become metamorphic or changing rocks under great heat and pressure. Their original character and appearance may be greatly altered by such forces, particularly during Earth movements. In this manner, clay may be a metamorphosed into slate, limestone into marble, sandstone into quartzite, granite into gneiss, shale into schist and coal into graphite,”
Metamorphism may be defined as the process of transformation of igneous or sedimentary rocks to metamorphic rocks ( changed rocks ) through the Earth’s movements ( producing heat and pressure). The metamorphism can be thermal when the intrusion of hot mass of igneous rock raises temperature of surrounding rocks. Similarly these can be a contact metamorphism when flowing when action by magma effects rocks in the zone near the batholiths or dike.
Metamorphism produces changes in the minerals. Mostly the change reshapes the rocks particles along parallel planes which are different from the bedding. It is called flow cleavage and its best example is slate. Moreover, through the act of metamorphism fine-grained sediments such as shale are turned into slates and schists, coarse grained or crystalline rocks form quartzites, gneisses and granulites and most of the rocks show foliation.
Types of Metamorphism:
On the basis of the agents which bring about the transformation in the rocks, the metamorphism can be divided into two types:
- Contact Metamorphism
- Regional Metamorphism
- Contact Metamorphism:
This types of metamorphism takes place due to the movement of hot molten magma (Lava). When the hot lava intrudes into the cavities, hollows or cleavage planes of rocks, it exerts pressure and hears up the surrounding rocks. High temperature brings about many changes. For example, it bakes limestone into marble and coal is turned into graphite.
- Regional Metamorphism:
It takes place due to the internal mountains building movements which exert great pressure on the rocks. Thus the entire shape of the rocks is changed. For example, shale becomes slate which then changes into schist. Due to mountain building movements, the submerged rocks are totally changed and become crystalline. Similarly, these submerged also bring back to the surface of Earth the deep rocks. Rocks such as graphite and quartzite are formed in this manner. Similarly, shale or muddy clay is turned into slates which have fine grains, Sandstones are changed into quartzite which are white-colored but very hard in the texture.