Welcome dear students! Today we are going to learn about Hydrosphere from Class 8 Social_Science. After studying this chapter, you will learn to know the different kinds of water bodies, understand the structure of the ocean bed, know the ocean currents, their types and conservation of oceans, and know how to mark oceans, sea, gulf, bay and strait on an Indian map. Let us begin with water, which is the source for marine life and maritime climate. The Earth is popularly called the Blue planet or Watery planet because about 70.78% of the total area of the earth, or 361 million square kilometers, is covered by water. The major water bodies of the world are the Pacific ocean, the Atlantic ocean, the Indian ocean and the Arctic ocean.
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Now let us understand the different types of water bodies. Oceans are deep and large expanse of water found between the continents. For example, the Indian ocean in the south. A sea is a part of the ocean extended or projected far inland. Seas are smaller compared to oceans, for example, the Arabian sea, the Caspian sea, and the Red sea. A gulf is a part of an ocean or sea that penetrated into the land. It is smaller than a sea. The size, shape and depth of a gulf varies from one to another, for example, the Gulf of Mannar, the Persian Gulf, and the Gulf of Mexico. Bays are small semi-circular water bodies partially surrounded by land. The depth, extent and size of bays vary from one to another, for example, the Bay of Bengal, the Bay of Biscay, the Bay of Fundy, and Hudson Bay. A strait is a narrow strip of water body joining two large water bodies, for example, Palk strait, Strait of Gibralter, and Bering strait. An isthmus is a narrow stretch of land mass joining two large land masses. Isthmus is the most important location for constructing ocean canals, for example, the Suez canal between Asia and Africa, and the Panama canal between North America and South America.
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Next, we will explore the relief of the ocean floor. The sea or ocean floor also has various features like the relief features of the earth surface. The knowledge of relief features of the ocean floor is very necessary to know the importance of ocean. On the basis of the characteristic features, the ocean floor is divided into four major parts. They are the continental shelf, the continental slope, the deep sea plain, and the ocean deeps. First, the continental shelf. This is the shallow area along the sea coast. This part is bordered by the sea coast on one side and continental slope on the other side. The average depth of the continental shelf is 100 fathoms, where one fathom is equal to 6 feet. The depth of the sea in this region gradually increases towards the continental slope. The continental shelf is very important for fishing, aquaculture, navigation and extraction of minerals. Second, the continental slope. This is the second part of the ocean floor and it is very steep. It is a link between the continental shelf and the deep sea plain. Sub-marine canyons are a special feature in this region.
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Third, the deep sea plain. This is the vast plain found at the bottom of the sea floor. The deep sea plain is also known as the abyssal plain and it covers the largest area of the ocean floor. In this region, sea mounts and guyots are found. Fourth, the ocean deeps and trenches. The ocean deeps are also called ocean trenches. These are the deepest part of the ocean floor. For example, the Challenger deep of Mariana trench in the Pacific ocean near Philippine islands is around 10,929 meters deep and is the deepest point of the oceans. The other trenches are Tonga trench and Kurile trench in the Pacific ocean. Now let us move on to the temperature and salinity of the ocean water. Sea or ocean water temperature varies according to latitudes and the depth of the ocean floor. The temperature of the ocean water near the equator is more than that of the temperature near the Arctic and the Antarctic circles. With the increasing depth, the temperature of the ocean water decreases. It is because the sunlight can penetrate into the ocean only up to a depth of 200 meters. Within this depth, various marine organisms are found in large numbers.
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Next is salinity. Salinity is the percentage of salts dissolved in the sea or ocean water. The average salinity of the ocean water is 35 PPT, or 35 parts per thousand. The salinity of ocean water is very high near the tropics and it is very low near the poles. At the equatorial region, the salinity is average due to high evaporation and precipitation. You might wonder, why is sea water salty? The water flowing from the land to the sea, which is river water, carries many kinds of salts and deposits them in the sea or ocean. Due to high temperature, water is evaporated continuously and the salts remain in the sea or ocean. This continuous process over a long period of time has resulted in the accumulation of salts in the sea water. Let us look at a facts file on saline water bodies of the world. Lake Van in Turkey has a salinity of 330 parts per thousand. The Dead Sea between Israel and Jordan has a salinity of 300 parts per thousand. Lake Sambar in India has a salinity of 265 parts per thousand. The Red Sea between Asia and Africa has a salinity of 240 parts per thousand.
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Let us now study ocean currents. The ocean water has three types of movements. They are waves, currents and tides. Ocean currents are the regular movement of ocean water from one region to another. Ocean currents are of two types. Warm currents originate and flow from the equatorial regions to sub-polar regions. Cold currents originate in the polar regions and flow towards the equatorial region. Factors that influence ocean currents are rotation of the Earth, temperature, wind, salinity, shape of the landmasses, and others. Let us list the currents of the Pacific Ocean. In the North Pacific ocean, the warm currents are the North equatorial current and the Kuroshio current. The cold currents are the Oyashio current and the California current. In the South Pacific ocean, the warm currents are the South equatorial current and the East Australian current. The cold currents are the Peru or Humboldt current and the West wind drift current. Now, the currents of the Atlantic ocean. In the North Atlantic ocean, the warm currents are the North equatorial current, the Gulf stream, and the North Atlantic drift. The cold currents are the Labrador current, the Norwegean current, and the Canary current.
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In the South Atlantic ocean, the warm currents are the South equatorial current and the Brazilian current. The cold currents are the Falkland current and the Benguela current. The current of the Indian ocean is unique. The north Indian ocean currents change their direction seasonally following the monsoons. During the south west monsoon, the currents flow in a south west to north east direction, and during the north east monsoon, they flow in a north east to south west direction. The south Indian ocean currents are the Mozambique, Madagascar, and Agulhas as warm currents, and the West Australian as a cold current. Here is another important facts file about major fishing grounds. The Honshu Hokkaido fishing grounds are the meeting place of the Kuroshio warm current and the Oyashio cold currents in Japan. The Grand bank is the meeting place of the Gulf stream warm current and the Labrador cold currents in New Foundland, United States of America. The Dogger bank is located in the North sea region of Europe.
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Now let us learn about tides, their types, causes and uses. Tides are the periodic rise and fall in the sea or ocean level. They occur due to gravitational attraction by the moon, gravitational force by the sun, rotation of the earth, and centrifugal force of the earth. Tides are of two types. High tide, also known as flood tide, and low tide, also known as ebb tide. On the water bodies of the earth, there are alternate high tides and low tides. Between one high tide and a low tide, the time is 6 hours and 13 minutes. Between one high tide and another, the time gap is 12 hours and 26 minutes. In 24 hours and 52 minutes, there are two high tides and two low tides. There are two types of tides on the basis of the positions of the moon and the sun. Spring tides take place when the earth, the moon and the sun are in the same straight line, for example, during new moon and full moon. When the spring tides take place, high tides are the highest and low tides are the lowest.
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Neap tides occur on the first quarter and last quarter days in the phase of the moon. During neap tides, the sun and the earth are in the same straight line and the moon is in right angle to the earth. In this type of tides, the high tides are not very high and low tides are not very low. The uses of tides are directly and indirectly useful to mankind in the following ways. First, it helps navigation. Second, it helps in the location of ports and harbours. Third, tides help to keep the harbours clean. Fourth, it helps in salt preparation. Fifth, they help fishing. Sixth, they also help to generate energy in the form of tidal energy. Conservation of oceans is very important. Oceans help us in many ways and they are the habitat of millions of marine organisms. Considering the rate at which the oceans are being contaminated, they may not remain what they should be in the future. The following are the important methods of conservation of oceans. First, oil transportation should be done through pipelines. Second, nuclear waste should not be dumped into the sea or ocean. Third, petro-chemical industries in the coastal regions must avoid dumping effluents into the sea or ocean. Fourth, dumping of any waste near the ports and harbours must be controlled. Fifth, ore deposition and mineral exploitation along the coast must be controlled. Sixth, exploitation and destruction of beaches must be controlled.
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Now, let us solve the exercises to prepare for your exams. Exercise one asks you to fill in the blanks with suitable words. The answers are as follows. Number one, the average depth of the continental shelf is 100 fathoms. Number two, one fathom is equal to 6 feet. Number three, the deepest point in the ocean is the Challenger deep of Mariana trench. Number four, the average salinity of the ocean water is 35 parts per thousand. Number five, during new moon and full moon, spring tides occur. Exercise two asks you to answer briefly. Question six, what is hydrosphere? The hydrosphere refers to all the water covering about 70.78% of the earth surface, which is 361 million square kilometers. Question seven, mention the four major parts of the ocean floor. They are the continental shelf, the continental slope, the deep sea plain, and the ocean deeps or trenches. Question eight, state the difference between ocean currents and tides. Ocean currents are the regular movement of ocean water from one region to another, while tides are the periodic rise and fall in the sea or ocean level. Question nine, distinguish between spring tide and neap tide. Spring tides occur when the earth, moon, and sun are in a straight line, causing the highest high tides and lowest low tides. Neap tides occur during the first and last quarter of the moon when the sun and earth are aligned and the moon is at a right angle, resulting in moderate tides. Question ten, how can we conserve the oceans? We can conserve oceans by transporting oil through pipelines, not dumping nuclear waste, preventing coastal industries from dumping effluents, controlling waste dumping near ports, controlling ore deposition and mineral exploitation along the coast, and controlling the exploitation and destruction of beaches.
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Exercise three asks you to match the following. The correct matches are: one, fathom matches with measuring the depth of ocean. Two, oyashio matches with cold current. Three, gulf stream matches with eastern coast of United States of America. Four, sea mounts matches with deep sea plain. Five, agulhas current matches with Indian ocean current. Exercise four asks you to define the following terms. Number eleven, continental shelf is the shallow area along the sea coast, bordered by the sea coast on one side and continental slope on the other side, with an average depth of 100 fathoms. Number twelve, salinity is the percentage of salts dissolved in the sea or ocean water. Number thirteen, warm currents originate and flow from the equatorial regions to sub-polar regions, while cold currents originate in the polar regions and flow towards the equatorial region. Number fourteen, high tide, also known as flood tide, and low tide, also known as ebb tide, are the periodic rise and fall in the sea or ocean level. Number fifteen, the Benguela current is a cold current in the South Atlantic ocean. Number sixteen, tides are the periodic rise and fall in the sea or ocean level caused by gravitational forces of the moon and sun, earth rotation, and centrifugal force.
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Exercise five lists terms to remember: gulf, salinity, warm currents, kuroshio current, high tide and low tide, and tidal energy. Exercise six is an activity. List out the cold and warm currents in the Atlantic ocean. The warm currents in the North Atlantic are the North equatorial current, the Gulf stream, and the North Atlantic drift. The cold currents in the North Atlantic are the Labrador current, the Norwegean current, and the Canary current. In the South Atlantic, the warm currents are the South equatorial current and the Brazilian current. The cold currents are the Falkland current and the Benguela current. We have now covered every important aspect of the hydrosphere, from the distribution of water bodies and the structure of the ocean floor, to ocean currents, tides, and the crucial steps for ocean conservation. Remember to practice marking these features on your map and revising the definitions regularly. Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]