The Natural Realms of the Earth Lithosphere-NOTES

 

The Natural Realms of the Earth Lithosphere-NOTES

People who study the Earth – the Earth Scientists – usually talk of four natural realms on the Earth. Realms are areas which have some common features. These are 1. Lithosphere, 2. Hydrosphere, 3. Atmosphere and 4. Biosphere. You have read much about many of these in the earlier classes but in this chapter, we will see some broad features of these spheres or realms, and how they are interrelated and also how human beings interact with them. 

1. Lithosphere: It is the solid crust or the hard top part of the Earth. It is made up of rocks and minerals and covered with a thick layer of soil. (In Greek ‘Litho’ means stone or rock and ‘sphaira’ means sphere or ball.) It is not a smooth surface as you see on the globe, but has high mountains, plateaus or high lands, low plains, deep valleys and very deep basins which are filled with water (oceans). Many of these features are shaped by wind and water. Portions of this crust, in the form of dust etc., are mingled with the air too. When the lithosphere heats up due to sunrays or cools down, it influences the air and water too. We and most other living beings live on this realm. We use the rocks and soils and other things found on this hard crust, in many ways. 

2. Hydrosphere: The realm of water is called Hydrosphere. (It comes from the Greek word ‘hudor’ meaning water.) Some part of the water is found deep down under the earth among rocks (ground water or mineral water). It comprises of the various sources of water and different types of water bodies like rivers, lakes, seas, oceans etc.

 3. Atmosphere: The thin layer of air that surrounds the earth is Atmosphere (The Greek word ‘atmos’ means vapour). It consists of a large number of gases including oxygen, nitrogen, carbon dioxide, water vapour etc and dust particles.

The surface of the continents is not even – they have plain low lands, plateaus and high mountains. These are also, in a way, a result of the internal processes of the earth . These landforms like mountains, plains and plateaus are called ‘Second Order landforms.

The Jigsaw Puzzle and the Moving Plates! In the previous chapter, we saw how many continents look like pieces of jigsaw puzzle (Fig. 1.4); how scientists thought that, in the beginning, probably all continents were held together and how they broke up and gradually drifted and came to their present places. After years of careful study, geologists have concluded that all the continents and even the oceans are actually situated on massive base of rocks called ‘plates’.

There are about seven major plates on the Earth and several minor ones. (The major plates are African, North American, South American, Indo Australian, Antarctic, Eurasian and Pacific Plates. Among the minor plates are the Nazca and Arabian plates). What is special about these ‘plates’? These plates actually ‘float’ on the mantle. They are constantly being pushed and therefore keep moving slowly. They move so slowly that we can’t feel the movement.

As a result of this movement, one plate pushes another neighbouring plate. The region where the two plates meet and push each other, a lot of pressure is exerted by each of them on the other. One plate is pushed under the mantle while the other plate is pushed up to form a chain of mountains. This movement of plates is called ‘plate tectonics’. This process causes earthquakes etc. Now, why are these plates being ‘pushed’? Who pushes them?

Sea floor spreading: Geologists studying the crust under the sea have discovered that under some oceans like the Pacific Ocean, there are mid ocean ridges or ranges. They are formed by the lava rising up from the mantle. The eruptions on the ridge create new ocean floor made of basalt rocks, which then spreads laterally from the ridge. Thus, the mid-ocean ridges contain the newest crust formed on the planet. This fresh crust is being slowly pulled away from the ridge widening the ocean basin. This leads to what is called ‘sea-floor spreading’.

Drama at the margins: The margins of the plates or the boundaries where the plates meet are the sites of highest geologic activity. We saw how new crust is formed leading to sea-floor spreading along the mid ocean ridges. Similarly, in other margins of the plates where one plate meets another, often the incoming plate dips under the stable plate. In fact, the incoming plate actually goes into the mantle of the earth and becomes molten due to the heat of the mantle. The plate thus going into the mantle actually pulls the rest of the plate with it.

This, in turn, pulls the newly formed sea floor near the ocean ridges. For example, the Indian plate (on which the Deccan plateau of south India ‘rides’) pushes the Eurasian plate and goes under it just where the Himalaya mountains are. Just imagine – one day in very distant future the land you are standing upon will go under the Himalayas and join the molten mantle! In fact, the Himalaya mountains were formed by this process of the Indian plate pushing into the Eurasian plate (just as if you spread a sheet of cloth on a table and push it from one side it will fold and form mountain like formations). Recently earthquakes are occured in Nepal and Pakistan as the Indian plate subduct the Eurasian plate. Many of the plate boundaries are also charecterised by volcanic eruptions and earthquakes. They are the most earthquake prone and volcano prone zones. Slow Movements and Sudden Movements In the above section, we saw two kinds of changes in the Lithosphere - first, the very slow movements leading to the formation of the crust, movement of the continental plates and their eventual return to the Mantle. Second, the sudden and dramatic eruption of volcanoes and earthquakes. The sudden movements can be destructive and cause much damage.

 At the same time, they also lead to changes in landforms. Volcanoes: See the figure of a volcano. Volcanoes are places on the earth’s surface where molten material from the mantle erupts on the Earth’s surface. This molten material is also accompanied by steam, smoke and various forms of gases from the depths of the earth. The smoke, ash and dust spreads out in the atmosphere while the molten materials cool and form hard rocks called ‘Igneous rocks’. Some part of the lava may not reach the surface and may cool under the surface and become rocks. These are called ‘intrusive landforms’. They are usually covered with older rocks and are exposed sometimes due to erosion of the covering rocks. A part of the lava which pours on the surface of the earth forms the ‘extrusive landforms’. Not all of them come from volcanoes – some of them are poured out of fissures on the earth’s surface and spread all around them. Such flows of lava, for example, occurred many times on the Deccan leading to the formation of the extensive lava plateau.