LITHOSPHERE AND LANDFORMS OF THE EARTH

LITHOSPHERE AND LANDFORMS OF THE EARTH

The earth is a massive spherical body. It is much hotter in the centre than at the surface. The surface of the earth is solid, but its interior is made up of minerals, which are in liquid or molten form because of the great pressure and heat inside the earth. As you move from the surface of the earth to its centre, the temperature keeps increasing with depth.

   THE STRUCTURE OF THE EARTH

The structure of the earth

The earth is made up of three layers, which are in the form of concentric shells.

  

1. The crust

This is the outermost and thinnest layer, constituting the solid part of the earth. The crust of the earth has an average thickness of about 40 km. it is made up of an upper layer of rocks  such as granite, which forms the continents. This rests on a lower layer of basalt, which forms the beds of the oceans and the seas. The crust of the earth is also called the Lithosphere (litho means rock.)

2. The mantle

This is the layer that lies beneath the crust and is made up of very dense, molten minerals. It is also called the mesosphere and is about 2,800 km thick. Iron and manganese are the chief minerals found in the mantle.

3. The core   

v  This is the innermost part of the earth. It has a radius of about 5,500 km. it is also called the bar sphere and is rich in iron and nickel. Evidence shows that the innermost part of the core is solid rock due to the extreme pressure of overlying layers. The temperature increase with depth in the interior of the earth at a rate of 1C for every 32 meters. 


 

 * PLATES OF THE EARTH

Plates of the earth

The earth’s crust consists of eight large and several small plates which carry the continents and the ocean floor. The movements of these plates cause earthquakes and volcanoes. This theory is called plate tectonics. This exciting new idea was put forward by geo-physicists in the 1970s to explain volcanic eruptions and earthquakes. They said that the crust of the earth is divided into several plates or rigid horizontal sections. These are constantly moving and sliding over the earth’s mantle… very slowly. So, we do not feel these movements except during tremors. When two adjoining plates meet, one  moves over the other, volcanic activity takes place at the place where they meet. It is here that new mountains and islands are constantly forming, while other’s mantle. The plate that slides on top. Has the rising fold mountains on its edge.

ROCKS 

The crust of the earth or lithosphere is made up of different types of rocks. A rock is composed of two or more minerals they differ in their properties of colure, texture rough, smooth, etc . porosity i. e,. how much water can pass through them and hardness easily broken or otherwise .the properties depend on the manner in which they are formed and   the minerals that form them rocks can be classified into three main groups on the basis of their origin.

Igneous rocks 

These were the first rocks to be formed and so they are also called primary or basic rocks. These rocks are formed from magma or molten rock found beneath the earth’s crust. The magma may solidify within the crust, in which case, the igneous rocks so formed are called intrusive or plutonic rocks. Sometimes, the magma reaches the surface of the earth through vents or cracks in the crust and then solidifies. Igneous rocks that are formed in this manner are called extrusive or volcanic rocks.

Common example of igneous rocks are granite (which is often plutonic) and basalt igneous rocks is that they are very hard and are thus often quarried for building roads (especially basalt) and monuments (e.g., polished granite).

Sedimentary rocks 

These are stratified or layered (strata means layers) rocks which are formed by the collection of sediment over a long span of time. The weight of the upper layers of accumulated sediment exerts pressure on the lower layers, causing them to form rocks, some rocks are formed by sediments deposited on the sea bed by running water, wind or glaciers, e. g,. sandstone and shale, sometimes, the remains of dead animals in the sea form a large portion of the rocks. Rocks of this type include chalk and limestone. Coal is formed from vegetable matter, I . e., the remains of plants compressed under sediment.

Sedimentary rocks have many uses. Coal and petroleum, found in sedimentary rocks, are major sources of power. Cements is made from limestone and glass is made from sandstone.

Metamorphic rocks 

These rocks are formed when any rock, whether igneous, sedimentary or even metamorphic, is subject to great pressure and great and it changes its original form. This can happen as  a result of volcanic activity, movements in the crust or even under ordinary conditions, when a rock lies deep within the crust. The metamorphic rock is so called because it is completely different from the original rock (metamorphosis means total change). Graphite (used as ‘lead’ in pencils) is actually metamorphosed coal; marble (used in constructing fine buildings or statues) is metamorphosed limestone and quartzite (used for building monuments)is metamorphosed sandstone.

 * USES OF ROCKS

Rocs as well as minerals are useful to us in various ways.

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    Building materials: Rocks like granite, sandstone, slate and limestone are used in building roads and houses. Bricks are made of clay, which is a sedimentary soft rock. Marble is used in buildings, especially for decoration.

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         Fuel: Minerals like coal and oil are found in sedimentary rocks. These are used as fuel.

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   Chemicals: Potash, nitrates and phosphates, found in sedimentary rocks, are used in the manufacture of fertilizers.

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     Glass and pottery: Silica is found in sand and is used in making glass. Pottery is made from clay.

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      Metals: Some minerals contain metals. Metals in this form are called ores. The ore is mined and then the metal is extracted and purified. Iron, aluminum and manganese are some examples. Precious metals like gold, platinum and silver are also mined from rocks.

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    Jewels: Some minerals form regular and beautiful shapes, called crystals. When these are mined, cut and polished, they are used in jewellery. The rarer ones like diamonds, emeralds and rubies are very precious. Diamonds are also used in cutting tools as they are very hard.

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       Fossils: An animal may leave a print in the mud, for example a footprint. The print may get filled   with more mud that may become solid and form a cast. These are known as fossils. Bones of dead animals and parts of plants which get embedded and preserved In rocks are also fossils. Fossils of prehistoric animals and plants have been found in rocks. They are very useful because they tell us about animals and plants that are no longer found on earth.