Q 1 – Why do we call the earth a Blue Planet?
Ans. Water is an essential component of all life forms that exist over the surface of the earth. The creatures on the earth are lucky that it is a water planet, otherwise we all would have no existence. Water is a rare commodity in our solar system. There is no water on the sun or anywhere else in the solar system. The earth, fortunately has an abundant supply of water on its surface. Hence, our planet is called the ‘Blue Planet’.
Q 2 – What is a continental margin?
Ans. The continental margin is the extended portion of each continent occupied by relatively shallow seas and gulfs. It is the shallowest part of the ocean showing an average gradient of 1° or even less. The shelf typically ends at a very steep slope, called the shelf break. The width . of the continental shelves vary from one ocean to another. The average width of continental shelves is about 80 km. The shelves are almost absent or very narrow along some of the margins like the coasts of Chile, the west coast of Sumatra, etc.
Q 3 – List out the deepest trenches of various oceans.
Ans. As many as 57 deeps have been explored so far; of which 32 are in the Pacific Ocean; 19 in the Atlantic Ocean and 6 in the Indian Ocean. Some important trenches of the world are as follows:
- Mariana Trench: It is the world’s deepest trench. It lies in Pacific Ocean. It is 11034 km below the ocean.
- Puritonko Trench: It is deepest trench in Atlantic Ocean.
- Sunda Trench: It is deepest trench in Indian Ocean.
Q 4 – What is a thermocline?
Ans. The temperature-depth profile for the ocean water shows how the temperature decreases with the increasing depth. The profile shows a boundary region between the surface waters of the ocean and the deeper layers. The boundary usually begins around 100 – 400 m below the sea surface and extends several hundred of metres downward. This boundary region, from where there is a rapid decrease of temperature, is called the thermocline.
Q 5 – When you move into the ocean what thermal layers would you encounter? Why the temperature varies with depth?
Ans. The temperature structure of oceans over middle and low latitudes can be described as a three-layer system from surface to the bottom.
- The first layer represents the top layer of warm oceanic water and it is about 500m thick with temperatures ranging between 20° C and 25° C. This layer, within the tropical region, is present throughout the year but in mid-latitudes it develops only during summer.
- The second layer called the thermocline layer lies below the first layer and is characterised by rapid decrease in temperature with increasing depth. The thermocline is 500 -1,000 m thick.
- The third layer is very cold and extends upto the deep ocean floor. In the Arctic and Antarctic circles, the surface water temperatures are close to 0° C and so the temperature variation with the depth is very slight.
Q 6 – What is salinity of sea water?
Ans. Salinity is the term used to define the total content of dissolved salts in sea water. It is calculated as the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater. It is usually expressed as parts per thousand (%) or ppt. Salinity is an important property of sea water. Salinity of 24.7% has been considered as the upper limit to demarcate ‘brackish water’. Salinity changes with depth, but the way it changes depends upon the location of the sea. Salinity at the surface increases by the loss of water to ice or evaporation, or decreases by the input of fresh water, such as from the rivers. Salinity at depth is very much fixed, because there is no way that water is ‘lost’, or the salt is ‘added.’
Q 7 – How are various elements of the hydrological cycle interrelated?
Ans. Water is a cyclic resource. It can be used and re-used. Water also undergoes a cycle from atmosphere, land surface and sub surface and the organisms. About 71 per cent of the planetary water is found in the oceans. The remaining is held as freshwater in glaciers and icecaps, groundwater sources, lakes, soil moisture, atmosphere, streams and within life. Nearly 59 per cent of the water that falls on land returns to the atmosphere through evaporation from over the oceans as well as from other places. The remainder runs-off on the surface, infiltrates into the ground or a part of it becomes glacier. The renewable water on the earth is constant while the demand is increasing tremendously. This leads to water crisis in different parts of the world :— spatially and temporally. The pollution of river waters has further aggravated the crisis
Q 8 – Examine the factors that influence the temperature distribution of the oceans.
Ans. The factors which affect the distribution of temperature of ocean water are explained below:
1. Latitude: The temperature of surface water decreases from the equator towards the poles because the amount of insolation decreases poleward. The enclosed seas in the low latitudes record relatively higher temperature than the open seas; whereas the enclosed seas in the high latitudes have lower temperature than the open seas.
2. Unequal distribution of land and water: The oceans in the northern hemisphere receive more heat due to their contact with larger extent of land than the oceans in the southern hemisphere.
3. Prevailing wind: The winds blowing from the land towards the oceans drive warm surface water away from the coast resulting in the upwelling of cold water from below. As a result, there is longitudinal variation in the temperature. On the contrary, the onshore winds pile up warm water near the coast and this raises the temperature.
4. Ocean currents: Warm ocean currents raise the temperature in cold areas while the cold currents decrease the temperature in warm ocean areas. Gulf stream raises the temperature near the eastern coast of North America and the West Coast of Europe while the Labrador current (cold current) lowers the temperature near the north-east coast of North America.
5. Salinity: Saline water absorbs more heat and its temperature rises much higher than fresh water.
All these factors influence the temperature of the ocean currents locally.
Q 9 – What are guyots?
Ans. It is a flat topped seamount. They show evidences of gradual subsidence through stages to become flat topped submerged mountains. It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.
Q 10 – What is a seamount?
Ans. It is a mountain with pointed summits, rising from the seafloor that does not reach the surface of the ocean. Seamounts are volcanic in origin. These can be 3,000 – 4,500 m tall. The Emperor seamount, an extension of the Hawaiian Islands in the Pacific Ocean, is a good example.
Q 11 – What are deep sea plains?
Ans. Deep sea plains are gently sloping areas of the ocean basins. These are the flattest and smoothest regions of the world. The depths vary between 3,000 and 6,000m. These plains are covered with fine¬grained sediments like clay and silt.
Q 12 – What are ocean deeps or trenches?
Ans. These areas are the deepest parts of the oceans. The trenches are relatively steep sided, narrow basins. They are some 3-5 km deeper than the surrounding ocean floor. They occur at the bases of continental slopes and along island arcs and are associated with active volcanoes and strong earthquakes.
Q 13 – Explain about mid-oceanic ridges.
Ans. A mid-oceanic ridge is composed of two chains of mountains separated by a large depression. The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean’s surface. Iceland, a part of the mid- Atlantic Ridge, is an example.
Q 14 – What is atoll?
Ans. These are low islands found in the tropical oceans consisting of coral reefs surrounding a central depression. It may
be a part of the sea (lagoon), or sometimes form enclosing a body of fresh, brackish, or highly saline water
Q 15 – What is shelf break?
Ans. The shelf typically ends at a very steep slope, called the shelf break.
Q 16 – In how many parts can ocean floor be divided?
Ans. The ocean floors can be divided into four major divisions:
- the Continental Shelf;
- the Continental Slope;
- the Deep Sea Plain;
- the Oceanic Deeps.
Q 17 – What minor relief features are found in ocean floors except major divisions?
Ans. Apart from the major relief features of the ocean floor, some minor but significant features predominate in different parts of the oceans. It includes like ridges, hills, seamounts, guyots, trenches, canyons, etc.
Q 18 – What is continental slope?
Ans. The continental slope connects the continental shelf and the ocean basins. It begins where the bottom of the continental shelf sharply drops off into a steep slope. The gradient of the slope region varies between 2-5°. The depth of the slope region varies between 200 and 3,000 m. The slope boundary indicates the end of the continents. Canyons and trenches are observed in this region.
Q 19 – What are submarine canyons? Name the best submarine canyon in the world.
Ans. These are deep valleys, some comparable to the Grand Canyon of the Colorado river. They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers. The Hudson Canyon is the best known submarine canyon in the world.
Q 20 – Name the areas of the world with highest salinity.
Ans. Highest salinity in water bodies are Lake Van in Turkey, Dead Sea, Great Salt Lake.’
Q 21 – In which latitude is salinity highest?
Ans. The highest salinity is recorded between 15° and 20° latitudes.
Q 22 – Explain the factors affecting salinity of sea water.
Ans. Factors affecting ocean salinity are as follows:
- Evaporation and precipitation: The salinity of water in the surface layer of oceans depend mainly on evaporation and precipitation.
- Fresh water: Surface salinity is greatly influenced in coastal regions by the fresh water flow from rivers, and in polar regions by the processes of freezing and thawing of ice.
- Wind: It also influences salinity of an area by transferring water to other areas.
- Ocean current: The ocean currents contribute to the salinity variations.
- Temperature: Salinity, temperature and density of water are interrelated. Hence, any change in the temperature or density influences the salinity of water in an area.
Q 23 – Explain the relief features of ocean.
Ans. The oceans are confined to the great depressions of the earth’s outer layer. The oceans, unlike the continents, merge so naturally into one another that it is hard to demarcate them. The geographers have divided the oceanic part of the earth into five oceans, namely the Pacific, the Atlantic, the Indian, Southern ocean and the Arctic. The various seas, bays, gulfs and other inlets are parts of these four large oceans. A major portion of the ocean floor is found between 3-6 km below the sea level. The ‘land’ under the waters of the oceans, that is, the ocean floor exhibits complex and varied features as those observed over the land. The floors of the oceans are rugged with the world’s largest mountain ranges, deepest trenches and the largest plains. These features are formed, like those of the continents, by the factors of tectonic, volcanic and depositional processes.
Q 24 – Explain the components and process of hydrological cycle.
Ans. Components of water cycle includes water storage in oceans, atmosphere, ice and snow, groundwater and water bodies. The processes of the water cycle vary in different components. In water stored in oceans evaporation, evapotranspiration and sublimation takes place. In water, in the atmosphere condensation and precipitation takes place. In water stored in ice and snow there is snowmelt runoff to streams. In surface runoff, there is stream flow freshwater storage and infiltration. In groundwater storage, groundwater discharge springs occurs.
Q 25 – Differentiate between continental shelf and continental slope.
Ans. Continental shelf:
- It is an extended margin of each continent occupied by shallow sea, gulfs, etc. It is a submerge part of the land adjoining the coast.
- It is shallow and is about 800m deep with an average gradient 1° or even less.
- Its edges extended upto the denth of 150-200 m. ‘
- It contains the sediments deposited with land its average width is 80km.
- The largest continental shelf is a Siberian shelf in Arctic Ocean which stretches upto 1500 km of width.
- About one-fourth of the petroleum is produced from oil wells on the continental shelfs.
- It is important for coastal transport and trade.
Continental slopes:
- It represents the edge of continental block.
- It connects continental shelf and ocean basin.
- The depth varies from 200-300 m.
- Its average gradient is 2-5°.
- It has a steep slope linking continental shelf and deep sea flow lying at an average depth of 3600 m.
- Canyons are formed here.
Q 26 – Write a short note on variation in salinity
Ans. The salinity for normal open ocean ranges between 33% and 37%. In the land locked Red Sea, it is as high as 41%, while in the estuaries and the Arctic, the salinity fluctuates from 0 – 35 %, seasonally. In hot and dry regions, where evaporation is high, the salinity sometimes reaches to 70%. Salinity decreases from 35% – 31% on the western parts of the northern hemisphere because of the influx of melted water from the Arctic region. In the same way, after 15° – 20° south, it decreases to 33%.
The average salinity of the Atlantic Ocean is around 36%. The highest salinity is recorded between 15° and 20° latitudes. Maximum salinity (37%) is observed between 20° N and 30° N and 20° W—60° W. It gradually decreases towards the north. The North Sea, in spite of its location in higher latitudes, records higher salinity due to more saline water brought by the North Atlantic Drift. Baltic Sea records low salinity due to influx of river water in large quantity. Highest salinity is found in water bodies as Lake Van in Turkey, Dead Sea, Great Salt Lake.
Q 27 – The average temperature of water on oceans floor keeps on falling from equator to poles systematically Explain.
Ans. The average temperature of surface water of the- oceans is about 27°C and it gradually decreases from the equator towards the poles. The rate of decrease of temperature with increasing latitude is generally 0.5°C per latitude. The average temperature is around 22°C at 20° latitudes, 14° C at 40° latitudes and 0° C near poles.
The oceans in the northern hemisphere record relatively higher temperature than in the southern hemisphere. The highest temperature is not recorded at the equator but slightly towards north of it.
The average annual temperatures for the northern and southern hemisphere are around 19° C and 16° C respectively. This variation is due to the unequal distribution of land and water in the northern and southern hemispheres.
Q 28 – Explain about horizontal distribution of salinity
Ans. Horizontal distribution of salinity:
- The salinity for normal Open Ocean ranges between 33% and 37%. In the land locked Red Sea records higher salinity due to high evaporation.
- Salinity is, however, very low in Black Sea due to enormous fresh water influx by rivers.
- The average salinity of the Indian Ocean is 35 %.
- The low salinity trend is observed in the Bay of Bengal due to influx of river water.
- On the contrary, the Arabian Sea shows higher salinity due to high evaporation and low influx of fresh water.
Q 29 – Explain about vertical distribution of salinity.
Ans. Vertical distribution of salinity
- ‘Salinity changes with depth, but the way it changes depends upon the location of the sea. Salinity at the surface increases by the loss of water to ice or evaporation, or decreased by the input of fresh waters, such as from the rivers.
- Salinity at depth is very much fixed, because there is no way that water is ‘lost’, or the salt is ‘added.’
- There is a marked difference in the salinity between the surface zones and the deep zones of the oceans. The lower salinity water rests above the higher salinity dense water.
- Salinity, generally, increases with depth and there is a distinct zone called the halocline, where salinity increases sharply.
- Other factors being constant, increasing salinity of seawater causes its density to increase. High salinity seawater, generally, sinks below the lower salinity water. This leads to stratification by salinity.
Q 30 – Explain about vertical distribution of temperature.
Ans. The temperature structure of oceans over middle and low latitudes can be described as a three-layer system from surface to the bottom.
- The first layer represents the top layer of warm oceanic water and it is about 500 m thick with temperatures ranging between 20° and 25° C. This layer, within the tropical region, is present throughout the year but in mid-latitudes it develops only during summer.
- The second layer called the thermocline layer lies below the first layer and is characterised by rapid decrease in temperature with increasing depth. The thermocline is 500 – 1,000 m thick.
- The third layer is very cold and extends upto the deep ocean floor. In the Arctic and Antarctic circles, the surface water temperatures are close to 0 C and so the temperature change with the depth is very slight.
Q 31 – Explain the features of the deep sea plains, submarine ridges, ocean deeps and seamount.
Ans. Abyssal plains (the deep sea plains):
- These are the world’s flattest and smoothest regions.
- About 40% of the oceanic floor is flat.
- It is formed accumulation of sediments on the sea floor.
- Its depth varies from 3000-6000 m.
Submarine ridges:
- Oceanic floors have submarine narrow and elongated ridges.
- They resemble mountain ridges on the earth surface.
- Peak of these ridges may rise above the sea level to form islands.
- Eg-Philippines Ice land is a mid-Atlantic ridge.
Oceanic deeps of submarine trenches:
- Deep narrow steep sided depression is found along the abyssal plain.
- The depth of these trenches may vary from 6,000 to 11,000 m. Example, Marina trenches is the deepest trench in Pacific Ocean.
- Trenches are formed as a result of tectonic forces and normally occur along the marines of Pacific Ocean.
- They occur at the base of the continental slopes and along the island area. Normally associated with volcanoes and strong earthquake.
- There are 32 trenches in Pacific Ocean, 19 in Atlantic Ocean and just 6 in Indian Ocean.
Seamount:
- It is the mount with a pointed submit rising from the sea floor but do not reach the surface of ocean. For example
- Emperor seamount, an extension of Hawaiian islands in Pacific Ocean
- The tallest seamount between Soman and New Zealand.
- Seamounts are volcanic in origin and can be 3000 to 4500 m tall.
Q 32 – Ocean seems to be water body but it has many types of landforms within it”. Justify the statements by giving some examples.
Ans. The statement is absolutely right. Like the surface of the earth ocean flow is neither level or flat, it is undulating and varying. It comprises of different types of landforms below it. Some of these are:
- Continental shelf: It is an extended margin of each continent occupied by shallow sea, gulfs, etc. It is a submerge part of the land adjoining the coast.
- Continental slopes: It represents the edge of continental block. It connects continental shelf and ocean basin. The depth varies from 200-300 m.
- Abyssal Plains (The Deep Sea Plains): These are the world’s flattest and smoothest regions. About 40% of the oceanic floor is flat. It is formed accumulation of sediments on the sea floor. Its depth varies from 3000-6000m.
- Sub Marine Ridges: Oceanic floors have sub-marine narrow and elongated ridges. They resemble mountain ridges on the earth surface. Peak of these ridges may rise above the sea level to form islands. Eg- Philippines Ice land is a mid-Atlantic ridge.
- Oceanic deeps of sub-marine trenches: Deep narrow steep sided depression found along the abyssal plain. The depth of these trenches may vary from 6,000 to 11,000 m. Example, Marina trenches is the deepest trench in Pacific Ocean.
Q 33 – Explain hydrological cycle.
Ans. The hydrological cycle is a conceptual model that describes the storage and movement of water between the biosphere, atmosphere, lithosphere, and the hydrosphere. Water on our planet can be stored in any one of the following major reservoirs: atmosphere, oceans, lakes, rivers, soils, glaciers, snowfields, and groundwater. Water moves from one reservoir to another by way of processes like evaporation, condensation, precipitation, deposi-tion, runoff, infiltration, subli¬mation, transpiration, melting, and groundwrater flow. The oceans supply most of the evaporated water found in the atmosphere. Of this evaporated water, only 91% of it is returned to the ocean basins by way of precipitation. The remaining 9% is transported to areas over landmasses where climatologically factors induce the formation of precipitation. The resulting imbalance between rates of evaporation and precipitation over land and ocean is corrected by runoff and groundwater flow to the oceans.
Q 34 – What are brines?
Ans. Brines are salt solutions commonly containing a higher portion of dissolved salt than that occurring in seawater.
Q 35 – How would you classify the large water bodies?
Ans. The large water bodies can be classified into four groups, viz., oceans, intercontinental seas, enclosed seas and fringing seas.
Q 36 – Name four major oceans of the earth.
Ans. The four major oceans of the earth are the Pacific Ocean, Atlantic Ocean, Indian Ocean and the Arctic Ocean.
Q 37 – Give three examples of intercontinental seas.
Ans. They are Malays Sea, Central American Sea and the Mediterranean Sea.
Q 38 – Give a few examples of enclosed seas.
Ans. Baltic Sea, Red Sea, Hudson Bay and the Persian Gulf are some examples of enclosed seas.
Q 39 – Identify some fringing seas.
Ans. Baring Sea, Japan Sea, East China Sea, Andaman, California, North Sea, Laurentian Sea, Bass Sea, English Channel and Irish Seas.
Q 40 – What are the factors on which the amount of rainfall entering the soil depends?
Ans. The amount of rainfall entering the soil depends upon the rate of rainfall and the infiltration rate of the soil.
Q 41 – What are the first order relief features of the earth?
Ans. Continents and ocean basins are the first order relief features of the earth.
Q 42 – The continental margins consists of two major submarine features.” Name them.
Ans. They are the continental shelf and the continental slope.
Q 43 – Mention three types of features of the ocean basin floor.
Ans. The ocean basin floor contains three types of features, viz.,
- abyssal plains and hills,
- oceanic rise and
- seamounts.
Q 44 – Name the second layer of oceans identified on the basis of their temperature structure.
Ans. It is Theraseoline.
Q 45 – What is the major salt presents in the seawater?
Ans. The major salt present in seawater is sodium chloride.
Q 46 – Name the deepest part of the ocean.
Ans. The deepest part of the ocean is the Mariana Trench in the Pacific Ocean. It is from 11,034 m. deep below the sea level.
Q 47 – Where is the tallest sea mount situated?
Ans. The tallest seamount is situated between Samoa and New Zealand which is 8690 m. high from the surrounding ocean floor.
Q 48 – What are the factors determining the salinity of seawater?
Ans. The salinity of seawater depends upon the extent of evaporation, river discharge and precipitation.
Q 49 – What is residence time?
Ans. The average time that an element remains dissolved in the ocean before removal is known as residence time.
Q 50 – How do you measure the salinity of seawater?
Ans. It is measured in gm. of salt per thousand gm of seawater.
Q 51 – Why is the earth called Blue planet?
Ans. Because of the abundance of water on its surface, the earth is often called a Blue planet.
Q 52 – What are oceanic deeps?
Ans. Trough like depressions or trenches in the seafloor is called oceanic deeps. Deeps occur mostly at the convergence of plates in subduction zones.
Q 53 – How is the hydrological cycle expressed mathematically?
Ans. The hydrological cycle is expressed mathematically as:’
RF = RO + ET
RF includes all types of precipitations, RO is round off and ET is evapotranspiration.
Q 54 – Which is the largest ocean in the world?
Ans. The Pacific Ocean.
Q 55 – Which sea has the maximum salinity?
Ans. Dead Sea – 238 per thousand.
Q 56 – How much part of the earth is covered by the Pacific Ocean?
Ans. 1/3 part.
Q 57 – How much percentage of the earth is covered with water?
Ans. 71 % of the surface of the earth is covered with water.
Q 58 – What is the average salinity of ocean water?
Ans. 35 per thousand.
Q 59 – Name the two main banks of the world.
Ans.
- The Duggar bank,
- The Grand bank,
Q 60 – What percentage of the ocean floor is covered by the continental shelf?
Ans. 7.6%.
Q 61 – Why is earth called the ‘Blue Planet’?
Ans. The earth is called a Blue planet because of the abundance of water found on the surface of the earth. Water covers 71 % of the earth surface.
If we take into account only the water surface of the earth, then 43% lies in the northern hemisphere and 57% in the southern hemisphere.
Q 62 – What are oceanic deeps?
Ans. The oceanic deeps are deeper than the abyssal plains. They occupy 2% of the oceanic area. Although deeps are found in the Pacific ocean. Mariana deep is the deepest deep in the Pacific Ocean.
Q 63 – Name the most common features found on the ocean floor.
Ans. The most common features found on the ocean floor are:
- Continental shelf
- Continental slope
- Submarine canyon
- Abyssal plains
- Submarine ridge
- Ocean deeps
- Troughs
Q 64 – Define the continental shelf.
Ans. From the coasts towards the open sea, there often exists a stretch of the shallow sea. The bed of this shallow sea bordering the continental platform is known as the continental shelf. It is a continuation of the continental block submerged below the sea. It slopes gently towards the sea, the angle of slope usually being less than one degree. The depth of the shallow seas rushing over a continental shelf varies from low tide to about 100 fathoms. Along the eastern coast of India, there exists a fairly wide strip of the continental shelf.
Q 65 – What are oceanic deeps?
Ans. Trough like depressions or trenches in the seafloor is called oceanic deeps. Deeps occur mostly at the convergence of plates in subduction zones.
Q 66 – What are seamounts?
Ans. Seamounts are topographical features rising from the ocean floor. A seamount is an isolated peak, usually a volcano, with a pointed summit, usually lies below the ocean surface, sometimes 3000 m. below.
Q 67 – What is an abyssal plain?
Ans. Abyssal plain is a deep-sea plain. It is a very large and relatively level area of the ocean floor, covered with a thin layer of sediment. Thus abyssal plain is an area of the deep ocean floor having a flat bottom with a very’ faint slope. Characteristically situated at the foot of the continental rise, the abyssal plain is present in all ocean basins. It is formed by long-continued deposition of very’ fine sediments and, therefore, has a nearly perfect flatness.
Q 68 – Write a short note on the continental rise.
Ans. Continental rise is a submarine feature of the continental margins. Along its seaward margin, the continental shelf gives way to the continental slope. The slope is abruptly replaced by the continental rise. Continental rise is a surface of much gentler slope decreasing in steepness toward the ocean basin floor. It generally has a moderate to low relief.
Q 69 – What is run off? How does it occur?
Ans. Run off is the water flowing from a drainage area. It is that part of precipitation which run off the land surface into streams. Runoff occurs when precipitation, that does not have an opportunity to infiltrate into the soil, flows across the land surface. However, most of it enters the stream channel ultimately, which carries it to the oceans. A part of precipitation that infiltrates the soil percolates downward to the water table through springs. Broadly speaking, runoff is composed of water from both surface flow and seepage flow. It is an extremely important-segment of hydrological cycles.
Q 70 – Discuss the factors affecting the distribution of temperature of the ocean water.
Ans. The salinity of seawater varies from place to place. At the equator, the salinity is somewhat lower due to abundant rains, greater cloudiness and low rate of evaporation. In latitudes about 20° in both the hemispheres, where evaporation is more intense and precipitation is lower, salinity is greater and in the temperate latitudes, where evaporation is less and rains. are more abundant, salinity is lower.
In Red sea, where no river flows, the salinity is 40 to 41 per thousand. In the Black sea, in which numerous rivers discharge, the salinity is 17 to 18 per thousand. In the White sea. it is between 25 and 26 per thousand, while in the Baltic sea it is 3 to 4 per thousand on an average.
Q 72 – Why is the seawater saline?
Ans. The seawater is saline because a number of salts are dissolved in it. The total weight of salts dissolved in the sea is 4800 million tonnes. Of these, sodium chloride, or common salt, accounts for 3800 million tonnes, sulphate for 3000 million tonnes, magnesium for 1600 million tonnes, potassium for 480 million tonnes and bromide for 83 million tonnes. The seawater may be less saline or more saline depending upon the extent of evaporation, river discharge and precipitation, but salt composition remains invariable.
Q 73 – What are the direct and indirect uses of oceans to man?
Ans. Oceans are directly or indirectly useful to man in many ways:
Direct uses of the oceans:
- Oceans are the storehouse of fishes.
- They are a storehouse of minerals.
- They provide the cheapest route for transportation.
Indirect uses: Indirectly-oceans control the climate.
Q 74 – How is the depth of seafloor measured?
Ans. The depth of seafloor is measured with sound waves. A depth recorder is used for this purpose. The echo of the sound returning after striking the sea bottom forms the basis of it. It helps to measure the velocity and depth.
Q 75 – What do you understand by hydrosphere?
Ans. The hydrosphere is the part of the earth submerged by oceans and seas. The vast oceans cover a total area of about 71% of the total earth’s surface. Earth is called a blue planet. Approximately 61% of the northern hemisphere and 81 % of the southern hemisphere is known as water hemisphere. The distribution of land and water is antipolar on earth. The Arctic Ocean surrounds the north pole and the Antarctic continent encircles the south pole.
Q 76 – Prepare a map of the Atlantic Ocean floor, mentioning ocean basins and mid-Atlantic ridges.
Ans. The Atlantic Ocean covers about one-sixth of the earth’s area. Resembling ‘S’ in shape, the ocean is bounded on the west by North America and South America, and On the east by Europe and Africa. To the south, it extends up to Antarctica and in the north, it is bounded by Greenland and Iceland.
The mid-Atlantic ridge, running from north to south in ‘S’ form is about 14,450 km. long and about 4000 m. high. The ridge has a broad fracture in the middle and slopes on both sides gently. The mid-oceanic ridge in the North Atlantic is known as Dolphin Ridge, and that in the South Atlant ic as the Challenger Ridge. A number of islands are located on the ridge.
The mid-Atlantic ridge divides the ocean into two major basins, namely, the East and West Atlantic basins. Some other basins in the Atlantic are the North American basin, The Labrador basin, The Brazil basin. The Argentina, Agulhas, Angola, and the West.
Q 77 – What type of relief features exists in the ocean basin floor?
Ans. The ocean basin is an extensive region of the basin floor, generally lying at a depth ranging between 2,500 and 6,000 m. It covers about 76.2% of the ocean’s area.
The ocean basin floor contains three types of relief features:
- abyssal plains and hills,
- oceanic rise and
- seamounts.
An abyssal plain is an area of the deep ocean floor having a flat bottom with a very faint slope. Characteristically situated at the foot of the continental rise, the abyssal plain is present in all ocean basins. Abyssal plains are surfaces formed by long-continued deposition of very fine sediments and, therefore, have a nearly perfect flatness. Abyssal hills are small hills rising to heights of a few tens of metres to a few hundred metres above the sea basin floor.
The ocean rise is an area hundreds of km. in the breadth over which the surface rises several hundred metres above the surrounding abyssal plain. Within the rise, the relief may range from subdued to very rugged.
Seamounts are the isolated peaks that rise 1000 ms. or more above the seafloor. Many of the seamounts are conspicuously flat-topped and extremely steep riched, named guyot.
Q 78 – Identify various layers of the ocean based on the level of salinity.
Ans. The salinity of the water is the ratio of the total weight of dissolved
solids to the weight of water. It is variable in quantity, differentiating in value from place to place over the oceans and at various depths. The average salinity is 35 per thousand. The relationship of salinity to depth is analogous with the three-layer temperature systems.
- First and the topmost layer is the shallow surface layer of high salinity (35.0 to 36.5 per thousand)
- Below this layer is a zone of the rapid decrease in salinity, called a halocline. It corresponds with the thermocline.
- Below the halocline, differences in salinity’ are very small and salinity lies in the range of 34.6 to 34.9 per thousand for most of the ocean body. Thus, the salinity decreases with the increasing depth.
Q 79 – Discuss the longitudinal variation in the salinity of ocean water.
Ans. The salinity of seawater varies from place to place. At the equator, the salinity is somewhat lower due to abundant rains, greater cloudiness and low rate of evaporation. In latitudes about 20° in both the hemispheres, where evaporation is more intense and precipitation is lower, salinity is greater and in the temperate latitudes, where evaporation is less and rains. are more abundant, salinity is lower.
In Red sea, where no river flows, the salinity is 40 to 41 per thousand. In the Black sea, in which numerous rivers discharge, the salinity is 17 to 18 per thousand. In the White sea. it is between 25 and 26 per thousand, while in the Baltic sea it is 3 to 4 per thousand on an average.
Q 80 – Describe the hydrological cycle in brief.
Ans. The hydrological cycle is the continuous circulation of water from the earth’s surface to the atmosphere. Water from the oceans is evaporated and lifted into the atmosphere. It is eventually condensed and is returned back to the earth’s surface in the form of rain, hail, snow or sleet.
Some of the precipitation, after wetting the foliage and ground, runs off over the surface to the streams. It is the water that sometimes causes erosion and is the main contribution to floods. Of the precipitation that soaks into the ground, some are available for growing plants and for evaporation. Some reach the deeper zones and percolates through springs and seeps to maintain streams during the dry period. The streams, in turn, eventually return the water back to the oceans where it originated. It is because of this never-ending circulation that the process has come to be known as the hydrological cycle.
The hydrological cycle is mathematically expressed as:
RF = RO + ET, where RF-includes all types of precipitation, RO is runoff and ET is evapotranspiration.
Run off occurs when precipitation, that does not have an opportunity to infiltrate into soil, flows across the land surface. However, most of it enters the stream channel ultimately, which carries it to the oceans. A part of precipitation that infiltrates the soil percolates downward to the water table through springs. Broadly speaking, runoff is composed of water from both surface flow and seepage flow. It is an extremely important segment of the hydrological cycle. Rainwater that reaches the soil surface is wholly or partly absorbed by the soil in the process of infiltration. The amount of rainfall entering the soil depends upon the s* rate of rainfall and infiltration rate of the soil.
