Fundamental Of Physical Geography Chapter 5 Minerals And Rocks

A rock is an aggregate of one or more minerals. 

The three major classes of rocks are :

(i)Igneous rocks
(ii)Sedimentary rocks
(iii)Metamorphic rocks

(i)These rocks do not have particles and layers.

(ii)These rocks are compact and massive.

(iii)These rocks are crystalline.

(iv)The igneous rocks are extremely hard and resistant.

(v)These rocks do not possess fossils of plants or animals.

(vi)These rocks are thermal in origin.

Sedimentary rocks are types of rock that are formed by the deposition and subsequent cementation of that material at the Earth's surface and within bodies of water.

(i)The layers of deposits : Loose materials or sediments are deposited in the layers.

Metallic minerals contain metal content and can be sub-divided into three types:
(i) Precious metals : gold, silver, platinum etc.
(ii) Ferrous metals : iron and other metals often mixed with iron to form various kinds of steel.
(iii) Non-ferrous metals : include metals like copper, lead, zinc, tin, aluminium etc

A metamorphic rock is a type of rock which has been changed by extreme heat and pressure.

The types of metamorphic rocks depend upon the original rocks that were subjected to metamorphism. Metamorphic rocks are classified into two major types:
(i)foliated rocks 

(ii)non-foliated rocks

Formation of metamorphic rocks : These rocks are formed due to thermal metamorphism.
It is of two types :

(i)Contact metamorphism: In contact metamorphism the rocks come into the contact with hot magma and lava and the rock material is recrystallized under high pressure and temperature.

The features:
(i)Pyroxene consists of calcium, aluminum, magnesium, iron and silica.
(ii)Pyroxene forms 10 percent of the earth’s crust.
(iii)It is commonly found in meteorites. It is in green or black colour.

(i)Feldspar : Half of the earth’s crust is composed of feldspar. It has a light cream and salmon pink colour. It is used in ceramics and glass making.

(iv)Amphibole : Its major elements are silica, aluminium, calcium and iron. They form the 7% of the earth's crust. It is used in the asbestos industry. It is black or green in colour.

(v)Mica : It comprises potassium, aluminium, magnesium, iron, silica etc. It forms 4% of the earth's crust. It is used in the electrical instruments.

(vi)Olivine : Magnesium, iron and silica are the major elements of olivine. It is used in jewellery. It is usually a greenish crystal often found in basaltic rocks.

(i) Coal and petroleum are major sources of power.

(ii) Bricks from clay, cement from limestone and glass from sand is obtained.

(iii) Limestone is used for buildings purpose.

The igneous rocks are crystalline rocks and hence are very rich in minerals of great economic value. These minerals are : magralite (iron), nickel, copper, lead, zink, chromite, manganese, gold, diamond, platinum etc. These are of great value in metallurgical industry. Ores of lead, tin, copper are also found mixed with quartz and calcite. Igneous rocks abound in silicate minerals containing iron and magnesium. Some mica are also associated with igneous rocks.

Economic significance of igneous rocks :

(i) Granite is used for house-building and idol-making.

(ii) Pumice stone is used as a sharpener.

(iii) Basalt and dolerite are used for road-making.

(iv) Many minerals found in these rocks e.g iron ore, manganese are basic raw materials for many industries.

On the basis of sediments, the sedimentary rocks are of three types.

1. Mechanically formed sedimentary rocks.

2. Organically formed sedimentary rocks.

3. Chemically formed sedimentary rocks.

1. Mechanically formed sedimentary rocks : These are formed due to various weathering processes especially mechanical weathering processes. The rock breaks in various sizes of fragments. They are also called clastic sedimentary rocks.

2. Organically formed sedimentary rocks : These are formed by the active participation of animals and plants through their secretions or formed out of dead remains of organisms. These are non-clastic in nature.

3. Chemically formed sedimentary rocks : Solidified precipitation from chemical solutions forms these rocks. These are also non-clastic rocks like the organically formed ones.

The chief types of rocks are as under:

(i)The igneous rocks - The igneous rocks form out of magma and lava from the interior of the earth. These are formed when magma cools and solidifies. These are hard in nature such as granite.
(ii)The sedimentary rocks - These rocks are formed by the sedimentation in the bottom of sea or lake transported by various exogenous agencies. The sedimentary rocks are soft in nature. Sandstone is the typical example.

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Minerals which have metal content are known as metallic minerals and are sub-divided into three types:
(i) Precious metals : gold, silver, platinum etc.
(ii) Ferrous metals : iron and other metals often mixed with iron to form various kinds of steel.
(iii) Non-ferrous metals : include metals like copper, lead, zinc, tin, aluminium etc.

The main processes of metamorphism are :

(i)Contact Metamorphism 

(ii)Regional Metamorphism

The distinction is mentioned below : 

(i)Contact Metamorphism : It is also known as local metamorphism. It is a process of change affected by heat in the proximity of magma intrusion in the affected rocks. The heat is produced by both-the heat of magma and associated matter such as water vapour. This metamorphism produces chemical replacement, re-crystallization and mechanical changes in the host rock. As the host rock gets metallic substance from the magma, so it becomes a great source of valuable minerals.

(ii)Regional Metamorphism : It is a process in which the rocks are changed mainly by high pressure. It is also termed tectonic or dynamic metamorphism. The deformation of crystal increases the pressure in the affected region. It is associated with mountain chains. Slaty and plastic deformations are the two important results. Re-crystallization is also seen in this process.