Published by: BhumiRaj Timalsina
Published date: 02 Feb 2022
The earth is our common habitat. However, it is considered spherical in shape but it is not like a ball at all. Its equatorial region is bulged and the polar part is slightly flattened. The diameter of the equatorial part of the earth is about 12800 km where as that of polar part is about 12751 km. The earth is only one planet where the existence of life is possible. Water, sunlight, suitable temperature and presence of oxygen in the atmosphere available on the earth are the basis of survival of life on it. It is looked blue in colour from the space. It is because of water present on it.
The earth is divided into three parts as lithosphere, hydrosphere and atmosphere. The land part of the earth is called lithosphere and its water part is called hydrosphere. The earth is surrounded by a layer of air called atmosphere.
Structure of the earth is also divided into two parts as External part and Internal part.
The outer part of the earth is found to be hard and about 50km in thickness. One fourth of the earth’s surface is land where as about three fourth part is covered by water. The maximum water part is occupied by seas. The highest peak of the land is Mt. Everest present in our country which is 8848 m high from the sea level. The water body of the earth consists of rivers, lakes, seas, oceans etc. The geological disturbances and changes cause the formation of mountains, valleys, plains etc. The outer part of the earth can be separated into water part and land part.
Generally the land mass about 300 m or higher than it from the see level is called mountain. Some of them are higher in altitude where as others are lower. The mountain with lower altitude and steeped land surface are called hills. On the basis of the (height) altitude their height is lower than 300 m. The height and the steepness of mountain are different. Some of them are too steep where as others are less steeped. So many mountains are there in the northern belt of our country. There is snowfall on the mountains at the height of more than 1829 metre (6000 feet) throughout the year. Such mountains are called Himals. The row of Himals is called Himalayas. Including smaller and higher, there are so many Himalayas in our country. Out of ten top peaks in the world, seven top peaks are there in our country.
The inner part of the earth is generally made up of hard rocks. Few feet of outer surface of the mountain is covered by soil and gravel. Some mountains are formed by hard rocks where the plants do not grow.
The plane land mass upto 200m in altitude from the sea level is called plain. It is formed by the deposition of soil and humus of animals, plants etc carried by the streams and rivers from the himals and mountains. Therefore, the plain is fertile. The soil of plain is suitable for cultivation. The Terai region of Nepal is one of its examples.
The plain surrounded by the mountains is called valley. The shape of valley is like a bowl. Since it lies in the low altitude, the fertile soil and biomass are carried by the river and rivulets lets of surrounding mountains in the plain. As a result, such valleys become more fertile. The capital city Kathmandu of our country is the same type of valley. Pokhara, Dang, Surkhet, Arun etc are also the valleys. Valleys can be formed by the deposited soil of river, dried or burried lakes etc.
The high land mass with a plane surface at the top is called plateau. Plateaus are also the Himalayan region but with plane surface instead of steepness at the top. Tibet is a type of plateau.
The big mass of water which flows continuously is called a river. Rivers are originated from any lakes, sources of water or Himalaya. While flowing down rivers mix with each other and end at the sea or ocean.The flowing rivers carry soil, leaves, rocks etc. from the surrounding and its speed decrease in the plain surface. As a result such things get deposited at the bottom. The river is a base for human life. The ancient civilizations of the world are found to be developed on the banks of the rivers. In Nepal rivers are used for fishing, irrigation, hydropower production etc. The main rivers of Nepal are Koshi, Gandaki and Karnali.
The big mass of water surrounded by the land is called lake. It is the main need for the aquatic, wet land animals and plants. Big lakes are regarded very attractive from the tourism point of view. Fish farming, rowing (boating), swimming etc can be done in the lakes. Many lakes like Rara lake, Tilicho lake, Phewa lake, Ghoda Ghodi lake etc are in Nepal.
The huge mass of water on the surface of the earth is called sea. The maximum part of the earth is covered by seas. The huge ocean like Pacific Ocean, Atlantic Ocean, Indian Ocean, etc are the seas. Shipping, fishing, cultivation of marine plants and animals can be done in the seas. Due to easy and chief transportation, it is also known as the base of development. Ranging from unicellular organism to huge multicellular organism like whale are found in the seas. Thus the sea is the huge habitat of the aquatic life.
The earth was like a red hot molten sphere when it was formed. Since it is cooling slowly, although its outer part is seen to be hard and cold, the inner part is still red hot and in the molten state. Even the outer part contains, mountains, plains, oceans and solid rocks, the inner part contains molten rocks and metals. On the basis of state of rocks and structure, the earth is divided into four layers, they are
(a) Crust
(b) Mantle
(c) Outer core
(d) Inner core
The outer most layer of the earth is crust. The thickness of the crust is not uniform. It lies upto 50 km below the earth’s surface. It is made up of soil and rocks. Activities like housing, cultivation, urbanization, road construction, etc. are done on this layer. Oceans, rivers, forests, mountains etc. all are situated on the crust. Mainly the silicate mines are in this layer.
Mantle lies below the crust. Its thickness is about 2900 km. Although its temperature is comparatively higher than that of crust, substances (objects) are still found to be in the solid and flexible state. It is made up of hard silicate and the minerals like iron and magnesium. The density of the rocks in this layer is more than two to three times than that of outer layer. The temperature of this layer ranges from 750°C to 2500°C.
The thickness of this layer is about 2100 km. Heavier metals like iron, cobalt and nickel are there in this layer. Since the temperature of this layer ranges from 2500°C to 3000°C, these rocks and metals are in molten state.
Inner core of the earth is its central core. The thickness of this core is about 1300 km. Due to the tremendous high pressure in this core, the temperature ranges from 3000°C to 5000°C. Result of the high pressure on it, the molten matters remain in the dense and tight solid state. Rocks in this layer are about five times heavier than those in the outer layer.