Rocks

• Are the materials that form the essential part of the earth’s crust.
• Rocks are formed from the molten material known as magma.
• The study of rock is called Petrology (in Greek, “petra” means rock, “logos” mean study).
• 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 rocks
3. Alteration of the existing primary and secondary rocks = Metamorphic rocks

Types of 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

a. 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

b. 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.

2. Sedimentary 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.

Types of sedimentary rock

A. Based on the origin

1. Residual: Laterite
2. Transported

• Deposited as solids in suspension: Sandstone, shale
• Deposited by chemical precipitation: Limestone, ironstone
• Deposited through agency of organic matter: Peat, Phosphatic deposits

B. Based on the grain size

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

• 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.

Note:

Sand stone       : Quartizite

Shale                 : Slate/mica, schist

Lime stone       : Marble

Granite              : granite gneiss

Dolerite             : Hornblende gneiss