Rocks come in many different colours, shapes, and sizes. Rocks are naturally occurring solid material consisting of various minerals. Geologists classify rocks into three basic groups based on how they were formed in nature. Three major types of rocks are as follow: 1) Igneous Rocks. 2) Sedimentary Rocks. 3) Metamorphic Rocks. 1. IGNEOUS ROCKS: Igneous rocks are those rocks which have solidified from the molten material or the lava ejected from the interior of the Earth. Due to high temperature in the interior of the earth the rocks are in molten state. This molten material in called Magma. |
• Volcanic material that flows on the surface of the earth is called Lava. They were the first rocks to be formed so igneous rocks are also known as primary rocks. These rocks are generally hard, lave no layers and do not contain fossils. Most of them besides being crystalline are impervious i.e. water cannot percolate through them. They are compact and do not get eroded rapidly. They make good building materials. e.g. Granite, Basalt, Gabbro, Feldspar, Pumice and Scoria.
2. SEDIMENTARY ROCKS: They are formed through the process of sedimentation. The rocks formed in this manner are called Sedimentary rocks. They are also called Secondary or derived rocks.
CHARACTERISTICS OF SEDIMENTARY ROCKS: 1) Sedimentary rocks are found in layers, so they are known as stratified rocks (strata- layer). 2) Most of these rocks contain fossils. 3) Sedimentary rocks have pores in which water can easily enter. 4) They do not have any type of crystals. 5) These rocks are prone to weathering because they are soft. 6) These hold reserves of coal, oil and natural gas. E.g. Sandstone, Clay, Shale, Lime stone etc.
3. METAMORPHIC ROCKS: Metamorphism means alteration or transformation. They are formed through the process of metamorphism. Metamorphism of rocks takes place when they are subjected to high temperature or pressure. CHARACTERISTICS OF METAMORPHIC ROCKS: 1) These rocks are very hard and have close banded structure. 2) These rocks are formed when igneous or sedimentary rocks are transferred by the action of intense heat and pressure. 3) The formation of metamorphic rocks place over a long period of time. e.g. Quartzite, Marble, Slate, Gneiss, Schist, Graphite.
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