What is Rocks?
Rocks are the materials that form the essential part of the Earth’s solid crust. “Rocks
are hard mass of mineral matter comprising one or more rock forming minerals”. Rocks are
formed from the molten material known as magma. The study of rocks is called Petrology
(in Greek, petra means rock, logos means science). Petrology deals with the description of
rocks; petrogenesis is the study of the origin of rocks.
Formation of rocks
1. Cooling and consolidation of molten magma within or on the surface of earth =
Igneous or Primary rocks
2. Transportation and cementation of primary rocks = Sedimentary or Secondary
3. Alteration of the existing primary and secondary rocks = Metamorphic rocks
1. Igneous rocks (primary or massive rocks)
These are first formed in the earth crust due to the solidification of molten magma.
Based on the mode of formation, they are further classified as extrusive and intrusive rocks
Extrusive rocks or volcanic rocks
These rocks are formed due to the consolidation of magma on the surface of the
earth. The magma, when it flows on the Earth surface is called LAVA. E.g. Basalt.
Intrusive rocks or plutonic rocks
These rocks are produced due to solidification of magma below the surface of the earth.
Plutonic – intrusive rocks solidifies at greater depth and Hypabassal rocks solidifies at
shallow depth from the surface. E.g. Granite, syenite, diorite, Gabbro etc. Rocks formed in
vertical cracks are called dykes and in horizontal cracks are called sills.
Vesicular rocks: Molten magma cools on the surface. Steam of water is entrapped into
rocks and forms vesicles.
Based on the silica content, rocks are also classified as
1. Acid rocks : >65% SiO2 (Granite, Rhyolite)
2. Intermediate : 56 to 65% SiO2
(Sub acid rocks 60 to 65% SiO2 (Syenite and Trachyte))
(Sub basic rocks 56 to 60 % SiO2 (Diorite and Andesite))
3. Basic rocks : 40 to 55% (Gabbro, basalt)
2. Sedimentary rocks
These rocks are formed from the consolidation of sediments accumulated through
wind or water action at the surface of the earth. Many are deposited in layer or formed
through chemical reactions as precipitates from aqueous solutions. Sediments may contain
various size particles cemented together by substances like SiO2, Fe2O3 or lime. These rocks are also called as clastic rocks.
Based on the origin, the sedimentary rocks are classified as
1. Residual : Laterite
a. Deposited as solids in suspension : Sandstone, shale
b. Deposited by chemical precipitation : Limestone, ironstone
c. Deposited through agency of organic matter: Peat, Phosphatic deposits
Based on the grain size, sedimentary rocks are classified as
1. Rocks with boulder pebbles sized minerals (Rudaceous) : Conglomerate
2. Rocks with sand size particles (Arenaceous) : Sandstone
3. Rocks with silt size particles (silt rocks) : Siltstone
4. Rocks with clay size particles (Argillaceous) : Shale
3. Metamorphic rocks
These are formed from igneous and sedimentary rocks under the influence of heat,
pressure, chemically active liquids and gases. Change may occur in mineral composition or
texture or both. The changes due to water is called hydro metamorphosis and due to
pressure is called dynamo metamorphosis
Sand stone : Quartizite
Shale : Slate/mica, schist
Lime stone : Marble
Granite : granite gneiss
Dolerite : Hornblende gneiss