Geography Notes For W.B.C.S-Earth’s Crust-Elements, Minerals and Rocks.

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Elements in the Earth’s Crust

• The earth is composed of various kinds of elements.Continue Reading Geography Notes For W.B.C.S-Earth’s Crust-Elements, Minerals and Rocks.
• About 98% of the total crust is made up of eight elements as oxygen, silicon, aluminium, iron, calcium, sodium, potassium, and magnesium.
• The rest is constituted by elements like titanium, hydrogen, phosphorous, manganese, sulphur, carbon, nickel and others.
• The elements in the earth’s crust are rarely found exclusive but are usually combined with other elements to make various substances.
• These substances are recognised as minerals.
• The table below shows the percentage wise share of each element in both the earth’s crust and the whole earth.

Minerals in the Earth’s Crust

• A mineral is a naturally occurring organic or inorganic substance, having an orderly atomic structure and a definite chemical composition and physical properties.
• A mineral is composed of two or more elements. But, sometimes single element minerals like sulphur, copper, silver, gold, graphite, etc are also found.
• The basic source of all minerals is the hot magma in the interior of the earth.
• When magma cools, crystals of the minerals appear and a systematic series of minerals are formed in sequence to solidify so as to form rocks.
• The minerals which contain metals are called as metallic minerals (eg: Haematite) and the metallic minerals which are profitably mined are called as the ores.
• The crust of the earth is made up of more than 2000 minerals, but out of these, only six are the most abundant and contribute the maximum.
• These six most abundant minerals are feldspar, quartz, pyroxenes, amphiboles, mica and olivine.

Characteristics of some of the major minerals

1. Feldspar:

• Silicon and oxygen are major elements of all types of feldspar.
• Sodium, potassium, calcium, aluminium, etc are found in specific feldspar varieties.
• Half of the earth’s crust is composed of feldspar (plagioclase (39%) and alkali feldspar (12%)).
• It has light cream to salmon pink colour.
• It is commonly used in ceramics and glass making.

2. Quartz:

• It is one of the most important components of sand and granite.
• It consists of silica and it is a hard mineral virtually insoluble in water.
• It is usually white or colourless.
• They are used in the manufacturing of radio, radar, etc.

3. Pyroxene:

• The common elements in pyroxene are Calcium, aluminium, magnesium, iron and silicon.
• About 10% of the earth’s crust is made up of pyroxene.
• It is commonly found in meteorites.
• Its colour is usually green or black.

4. Amphibole:

• Aluminium, calcium, silicon, iron and magnesium are the major elements of amphiboles.
• They form 7% of the earth’s crust.
• It is green or black in colour and is used in asbestos industries commonly.
• Hornblende is another form of amphiboles.

5. Mica:

• It is made up of elements like potassium, aluminium, magnesium, iron, silicon, etc.
• It forms 4% of the earth’s crust.
• It is commonly found in igneous and metamorphic rocks.
• Mica is widely used in electronic instruments.

6. Olivine:

• Magnesium, iron and silica are the major elements of olivine.
• It is commonly found in basaltic rocks with a greenish colour.
• Olivine is used commonly in jewellery.

Rocks in the earth’s Crust

• A rock is nothing but a composition of minerals.
• They are aggregates or a physical mixture of one or more minerals.
• Rocks may be hard or soft and in varied colours.
• Feldspar and quartz are the most common minerals found in all type of rocks.
• The science dealing with the study of rocks is called as Petrology.

Igneous Rocks

• Igneous rocks are formed by the cooling of highly heated molten fluid material called as Magma.
• Asthenosphere, which is just below the upper mantle, a region beneath Lithosphere is the main source of magma.
• They might be formed directly by cooling of magma from the interior of the earth itself or by cooling of lava from the surface of the earth.
• As they comprise the earth’s first crust and all other rocks are derived from them, they are also called as the parents of all rocks or the Primary Rocks.
• They are the most abundant rocks in the earth’s crust.
• On the basis of their mode of occurrence, igneous rocks can be classified as Intrusive and Extrusive Igneous Rocks.

1. Intrusive Igneous Rocks

• They are formed when magma solidifies below the earth’s surface.
• The rate of cooling below the earth’s surface is very slow which gives rise to the formation of large crystals in the rocks.
• That is, the mineral grains of intrusive igneous rocks are very large.
• Deep-seated intrusive igneous rocks are called as Plutonic rocks and shallow depth intrusive igneous rocks are called as Hypabyssal Rocks.
• Eg: Granite, dolerite, etc.

2. Extrusive Igneous Rocks

• They are formed by the cooling of the lava on the earth’s surface.
• As lava cools very rapidly on the surface, the mineral crystals forming extrusive igneous rocks are very fine.
• These rocks are also called as Volcanic Rocks.
• Eg: Gabbro, Basalt, etc.
• On the basis of chemical properties, igneous rocks can be classified as Acid and Basic Igneous rocks.
• They are formed as a result of solidification of acidic (high viscous) or basic lava (low viscous).
• Acidic igneous rocks are composed of 65% or more of silica. They are coloured, hard and very strong (Eg: Granite).
• Basic igneous rocks contain less than 55% of silica and have more iron and magnesium. They are dark in colour, weak enough for weathering (Eg: Basalt, Gabbro).

Sedimentary Rocks

• These rocks are formed by successive deposition of sediments.
• These sediments may be the debris eroded from any previous existing rock which may be igneous, metamorphic or old sedimentary rocks.
• The process of successive deposition and formation of sedimentary rocks is called as Lithification.
• Due to successive depositions, they have a layered or stratified structure and hence are also called as Stratified Rocks.
• Depending upon the mode of formation, sedimentary rocks can be classified as:

1. Mechanically formed/ Clastic Sedimentary Rocks

• They are formed by the consolidation of sediments under excessive pressure and cementation.
• Eg: Conglomerate, Breccia, Sandstone, Shale, etc.

2. Organically/ Biologically formed Sedimentary Rocks

• The consolidation of organic matters derived from plants and animals form this type of rocks.
• Eg: Coal, limestone, chalk, chert, etc.

3. Chemically formed Sedimentary Rocks

• They are formed by various chemical reactions.
• Eg: Gypsum, rock salt, limestone, etc.

Metamorphic Rocks

• The word metamorphic means ‘change of form’.
• Hence, these rocks form under the action of temperature, pressure and volume changes on original rocks.
• Metamorphic rocks are formed under the influence of heat or pressure on original rocks which cause to change their colour, hardness, structure and composition.
• The process of recrystallization and reorganisation of materials within the original rock is called as metamorphism.
• When the metamorphism happens without any appreciable chemical change, it is called as Dynamic Metamorphism.
• If metamorphism happened due to the influence of heat, it is called as Thermal Metamorphism. It has two types: Contact Metamorphism and Regional Metamorphism.
• When the reorganisation occurs due to direct contact with the hot magma, it is called as Contact Metamorphism.
• If the rocks undergo reorganisation due to tremendous heat/ pressure formed as a result of tectonic shearing, it is called as Regional Metamorphism.
• Metamorphic Rocks can be classified into Foliated (Slate, Schist, Gneiss) and Non-Foliated (Quartzite, Marble) Metamorphic Rocks on the basis of the presence or absence of bands of mineral grains.

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