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Summary
The Earth looks blue from space because about 71 percent of it is covered in water. This water, in all its forms – oceans, rivers, lakes, underground, and in the air – is called the hydrosphere. Water moves around the Earth in a process called the water cycle.
Most of the Earth’s water, about 97 percent, is salty ocean water, which we cannot drink. Only 3 percent is fresh water. Much of this fresh water is frozen in ice caps. A very small amount, less than one percent, is available as fresh water in rivers, lakes, streams, and underground. Water on land comes from rain or snow, or seeps from underground, forming rivers that flow to the sea. Underground water collects beneath the land’s surface after rain or snow. Water also exists as vapor in the air, causing humidity; it evaporates from water bodies, forms clouds, and then falls as rain or snow.
Ocean water moves in three main ways: waves, tides, and currents. Waves are the up-and-down movements of the sea surface, usually caused by wind. Tides are the regular rise and fall of sea level. They are caused by the gravitational pull of the Moon and the Sun. When the water level rises and moves towards the coast, it is high tide. When it falls and moves away, it is low tide. The Earth has two high tides and two low tides about every 24 hours and 50 minutes. The Moon’s distance from Earth changes; it is farthest at apogee and nearest at perigee, which affects tides. When the Sun, Earth, and Moon are in a line (during new moon or full moon), their combined gravity causes very high tides called spring tides. When the Sun and Moon are at a right angle to the Earth, their gravity works against each other, causing weaker tides called neap tides.
Ocean currents are like large rivers of water flowing in the oceans. They can be warm or cold. Warm currents flow from warm parts of the world near the equator towards the colder polar regions. Cold currents flow from polar regions towards the equator. Currents are caused by differences in water temperature and saltiness, the Earth’s rotation, winds, and the shape of landmasses. For example, the Gulf Stream is a warm current in the Atlantic Ocean that helps keep Western Europe warmer than it would otherwise be. The North Atlantic Drift is part of this system. The Labrador Current is a cold current in the Atlantic that brings cold water and icebergs south. The Kuroshio Current, also known as the “Black Stream” because of its dark blue water, is a warm current in the Pacific that warms Japan. The Oyashio Current is a cold Pacific current that meets the Kuroshio, creating rich fishing areas but also fog.
Ocean currents greatly affect climate. Warm currents bring warmer temperatures and more rain to coastal areas. Cold currents bring cooler, drier weather. Where warm and cold currents meet, fog often forms, and these areas are usually rich in fish because plankton, tiny sea organisms that fish eat, grow well there. Currents also affect shipping; ships can travel faster with a current or slower against it. Cold currents can carry dangerous icebergs into shipping lanes.
Textbook solutions (Morning Star/Total)
Multiple-Choice Questions
1. Hydrosphere refers to the combined mass of water
(a) in the oceans
(b) rivers, streams and lakes
(c) in the atmosphere
(d) All of the above.
Answer: (d) All of the above.
2. Ocean: 97% :: Fresh water : ______
(a) 3%
(b) 2.3%
(c) 0.7%
(d) 0.66%
Answer: (a) 3%
3. Underground water is found in:
(a) soil
(b) subsoil
(c) underground streams
(d) All of the above.
Answer: (d) All of the above.
4. Name the oscillatory movements in water, manifested by an alternate rise and fall of the sea surface.
(a) Tides
(b) Waves
(c) Spheres
(d) Oscillations
Answer: (b) Waves
5. Apogee : Earth-moon distance ______ :: Perigee : Earth-moon distance ______
(a) maximum, minimum
(b) minimum, maximum
(c) equal, equal
(d) All of the above.
Answer: (a) maximum, minimum
6. ______ flow from the lower latitudes towards the higher latitudes.
(a) Warm currents
(b) Cold currents
(c) Surface currents
(d) Deep water currents
Answer: (a) Warm currents
7. Which amongst the following is NOT one of the causes of Ocean Currents?
(a) Level of Salinity
(b) Temperature
(c) The Earth’s Revolution
(d) Planetary Winds
Answer: (c) The Earth’s Revolution
8. Name the winds that blow between the Equator and the Tropics.
(a) Westerlies
(b) Trade Winds
(c) Monsoon Winds
(d) Summer Winds
Answer: (b) Trade Winds
9. What is the direction of air circulation over the oceans in the middle latitudes?
(a) Cyclonic
(b) Hemispherical
(c) Spherical
(d) Anticyclonic
Answer: (d) Anticyclonic
10. Name the warm Atlantic Ocean Current which keeps the ports of Great Britain ice free even in winters.
(a) The West Atlantic Drift
(b) The East Atlantic Drift
(c) The North Atlantic Drift
(d) The South Atlantic Drift
Answer: (c) The North Atlantic Drift
11. Which current is also known as the ‘Black Stream?
(a) Atlantic current
(b) Kuroshio current
(c) Oyashio current
(d) Newfoundland current
Answer: (b) Kuroshio current
12. Name the current that keeps the hub of world trade, the Port of Montreal, open all year round.
(a) Gulf Stream
(b) Labrador current
(c) Canaries current
(d) Florida current
Answer: (a) Gulf Stream
13. Which currents have created one of the world’s richest fishing grounds?
(a) Labrador current and Gulf stream
(b) Labrador current and Irminger current
(c) Oyashio current and Kuroshio current
(d) Both (a) and (c).
Answer: (d) Both (a) and (c).
14. Currents influence which of the following?
(a) temperature
(b) rainfall
(c) fish population
(d) all of the above.
Answer: (d) all of the above.
Short Answer Questions
1. Name any two ways in which movement of ocean water takes place.
Answer: Two ways in which the movement of ocean water takes place are through waves and currents. (Alternatively: waves and tides, or currents and tides).
2. What are tides? Name one factor that causes tides.
Answer: Tides are the rise and fall of sea water due to gravitational forces. One factor that causes tides is the gravitational force of the moon.
3. What is the time interval between tides? Name the factors responsible for this time interval.
Answer: On average, a place experiences tides twice a day, with each tide being delayed by 26 minutes compared to the previous day. A particular tide centre takes 24 hours and 52 minutes to come under the moon again, with another tide on the opposite side occurring after 12 hours and 26 minutes. The factors responsible for this time interval are the Earth’s rotation and the moon’s revolution around the Earth.
4. What are Spring and Neap tides?
Answer: Spring Tides are very high tides that occur when the sun, the moon, and the Earth are almost in the same line (during full moon and new moon). Neap Tides are tides with a lower range than normal, occurring when the sun, the Earth, and the moon form a right angle (during the seventh or eighth day of each fortnight).
5. Name two types of ocean currents based on their temperature. Give an example of each.
Answer: Based on their temperature, two types of ocean currents are Warm Currents and Cold Currents. An example of a Warm Current is the Gulf Stream, and an example of a Cold Current is the Labrador Current.
6. For what is the Gulf Stream famous?
Answer: The Gulf Stream is famous for being a powerful, warm, and swift Atlantic Ocean current that originates in the Gulf of Mexico. It influences the climate of the east coast of North America and Western Europe, making them warmer, and is also a potential source of renewable power.
7. What happens when warm and cold currents meet?
Answer: When warm and cold currents meet, dense fog often occurs. This meeting point also frequently creates rich fishing grounds due to the mixing of waters and concentration of nutrients. Violent storms can also follow the line where these currents meet.
8. Mona had gone to visit her friend’s boat on the Visakhapatnam jetty. She was surprised to see that the boats seemed to have sunk because the water level in the jetty was low and the gangways to the boats were almost vertical. As a student of geography name and explain the phenomena that caused this.
Answer: The phenomenon Mona observed is an ebb tide, which results in low tide water. Tides are the periodic rise and fall of sea levels. When the sea water falls and moves towards the sea, it is called an ebb, and the resultant low water level is known as low tide water. This makes the boats appear lower and the gangways steeper.
Structured Questions
1. (a) How are tides formed?
Answer: Tides are formed primarily due to the gravitational forces exerted by the moon and, to a lesser extent, the sun on the Earth’s waters. The part of the Earth facing the moon experiences a strong gravitational pull, causing the water to bulge outwards, creating a high tide. A similar high tide occurs on the opposite side of the Earth due to the reactionary (centrifugal) force related to the Earth-moon system’s rotation.
(b) Differentiate between High Tides and Low Tides.
Answer: High tide refers to the rise of sea water and its movement towards the coast, leading to the highest water level reached during a tidal cycle. Low tide (or ebb tide) refers to the fall of sea water and its movement away from the coast towards the sea, leading to the lowest water level reached during a tidal cycle.
(c) Give a reason for each of the following:
(i) There are two high and two low tides in a day.
Answer: Most coastal locations experience two high tides and two low tides in approximately 24 hours and 50 minutes. This is because as the Earth rotates on its axis, a point on its surface passes through two tidal bulges (causing high tides) and two tidal troughs (causing low tides) created by the moon’s gravitational pull and the associated centrifugal force.
(ii) Each day a tide is delayed by 26 minutes.
Answer: Each successive high or low tide at a particular location is typically delayed by about 26 minutes (or the entire cycle by about 52 minutes daily) because the moon revolves around the Earth in the same direction that the Earth rotates. So, by the time the Earth completes one rotation, the moon has moved slightly in its orbit, and the Earth has to rotate a little longer for the same point to be directly under or opposite the moon again.
(iii) The tidal range differs from one waterbody to the other.
Answer: The tidal range, which is the difference in height between high tide and low tide, varies significantly from one waterbody to another. This variation is due to factors such as the shape and depth of the ocean basin, the configuration of the coastline, the presence of estuaries, and the relative positions of the sun and moon, which affect the strength of the gravitational forces.
(d) Draw a well labelled diagram showing the formation of spring and neap tides.
Answer: See Fig. 11.4 and Fig. 11.5 of Morning Star/Total Geography book or click here.
2. (a) Describing the two types of ocean currents based on their temperature.
Answer: Ocean currents are broadly divided into two types based on their temperature:
Warm Currents: These currents originate in low latitudes (tropical zones) and flow towards higher latitudes (temperate and sub-polar zones). They carry warm water into colder regions. An example is the Gulf Stream.
Cold Currents: These currents originate in high latitudes (polar regions) or are driven by upwelling of deep cold water, and flow towards lower latitudes (warm equator region). They carry cold water into warmer regions. An example is the Labrador Current.
(b) State any two factors responsible for causing the currents.
Answer: Two factors responsible for causing ocean currents are:
- Differences in Temperature: Warmer water is less dense and tends to rise and flow over colder, denser water. This temperature difference creates movement.
- Planetary Winds: Prevailing winds like the Trade Winds and Westerlies exert a force on the surface of the ocean, dragging the water and causing it to move in the direction of the wind, thus forming currents.
(c) Give a reason for each of the following:
(i) Warm currents produce a milder climate.
Answer: Warm currents produce a milder climate in the coastal regions they flow past because they transport large amounts of warm water from tropical areas to cooler, higher latitudes. The air above these warm currents also gets heated and carries warmth and moisture, moderating the temperatures of adjacent landmasses, especially during winter.
(ii) The eastern coasts of USA are comparatively cold.
Answer: The north-eastern coasts of the USA are comparatively cold, particularly in winter, due to the influence of the cold Labrador Current. This current brings frigid water southward from the Arctic Ocean, significantly cooling the adjacent coastal areas.
(iii) The coasts of Norway are not frozen in winter whereas its adjoining coasts are frozen for most parts of the year.
Answer: The coasts of Norway remain largely ice-free in winter, despite their high latitude, due to the warming influence of the North Atlantic Drift. This warm current, an extension of the Gulf Stream, brings relatively warm waters to the region, preventing the sea from freezing. Adjoining areas not reached by this warm current can be ice-bound.
(d) Describe one effect of each of the following three ocean currents.
(i) Labrador Current of the Atlantic Ocean.
Answer: One significant effect of the Labrador Current is its transportation of icebergs from the glaciers of Greenland southward into the trans-Atlantic shipping lanes, especially in spring and early summer, posing a hazard to navigation.
(ii) The Kuroshio current
Answer: One effect of the warm Kuroshio Current is that it sustains the coral reefs of Japan, enabling them to exist as the northernmost coral reefs in the world.
(iii) Oyashio Current of the Pacific Ocean.
Answer: One effect of the Oyashio Current is that its cold, nutrient-rich waters create some of the world’s richest fishing grounds due to the high concentration of nutrients and favorable conditions for marine life.
3. (a) State the origin and flow of the Gulf Stream.
Answer: The Gulf Stream originates in the Gulf of Mexico. It flows out through the Straits of Florida, then moves northeastward along the eastern coast of the United States. Near Newfoundland, it continues across the Atlantic Ocean, where it broadens and becomes known as the North Atlantic Drift as it moves towards Europe.
(b) What is the effect of Gulf Stream on the coasts of North America and Western Europe?
Answer: The Gulf Stream has a significant warming effect on the climate of the east coast of North America, from Florida to Newfoundland. Its extension, the North Atlantic Drift, makes the climate of Western Europe (including the British Isles and Scandinavia) much milder than it would otherwise be at such high latitudes, keeping ports ice-free in winter and supporting agriculture.
(c) Give a reason for each of the following:
(i) The waters of the Oyashio Current form the richest fishing grounds in the world.
Answer: The waters of the Oyashio Current form exceptionally rich fishing grounds primarily because of the extremely high nutrient content of its cold water. Additionally, very high tides in some areas (up to ten metres) further enhance the availability of these nutrients, supporting a vast amount of marine life, which in turn supports large fish populations.
(ii) There is heavy rainfall in Queensland but the Atacama desert is arid.
Answer: Queensland, on the eastern coast of Australia, receives heavy rainfall partly due to the warm East Australian Current (driven by South-East Trade Winds), which brings warm, moist air to the coast, leading to precipitation. Conversely, the Atacama Desert on the western coast of South America is extremely arid partly due to the influence of the cold Peru Current. This cold current cools the air above it, reducing its capacity to hold moisture and leading to very little rainfall.
(iii) Rich fishing grounds are located on the Pacific coast of North America.
Answer: Rich fishing grounds can be located on the Pacific coast of North America due to factors influenced by ocean currents. Cold currents, like the California Current, can bring nutrient-rich waters from deeper ocean layers towards the surface (a process often associated with upwelling), which supports large populations of plankton, the base of the marine food web. Areas where different currents meet can also concentrate these food sources, attracting fish.
(d) Describe three major effects of currents.
Answer: Three major effects of ocean currents are:
- Influence on Climate: Currents significantly affect the temperature and rainfall of coastal regions. Warm currents bring milder, wetter conditions to higher latitudes, while cold currents can lead to cooler, drier conditions, sometimes contributing to desert formation.
- Impact on Marine Organisms: Currents transport nutrients and plankton, which are essential for marine life. The meeting points of warm and cold currents are often rich fishing grounds because they concentrate these food sources.
- Effect on Commerce and Navigation: Warm currents can keep ports in high-latitude regions ice-free throughout the year, facilitating trade. Historically, currents have also aided or hindered sailing ships depending on their routes and the direction of the currents. Cold currents can transport icebergs, posing a hazard to shipping.
Thinking Skills
1. You are watching the sea waves from the balcony of a seaside resort in Mumbai on a New Moon Day. Suddenly you noticed rise of sea water and its movement towards the coast. What was this wave and after how much time will it be repeated?
Answer: The phenomenon I observed is a high tide. Since it is a New Moon Day, it is likely a spring tide, which is a particularly high tide. This high tide will generally be followed by a low tide, and then another high tide will occur approximately 12 hours and 26 minutes later.
2. Imagine that the Ocean Currents stop flowing. What would be its consequences for the human beings?
Answer: If ocean currents stopped flowing, the consequences for human beings would be severe and far-reaching:
Climatic Extremes: Temperatures in equatorial regions would become significantly hotter, and temperate and polar regions would become much colder, as the currents would no longer redistribute solar heat around the planet. This would drastically alter weather patterns, leading to more extreme droughts, floods, and storms.
Collapse of Marine Ecosystems: The transport of nutrients that supports plankton growth would cease. Since plankton forms the base of the marine food web, fish populations would decline drastically, leading to widespread food shortages for communities dependent on seafood and a collapse of the fishing industry.
Agricultural Disruption: Changes in temperature and rainfall patterns would make many current agricultural regions unproductive, threatening global food security.
Impact on Coastal Communities: Sea levels might change unpredictably in different regions, and coastal climates would be severely affected, impacting human settlements.
Practical/Map Work
On an outline map of the world name and show the direction of one warm and one cold current each of the Pacific Ocean and the Atlantic Ocean.
Answer: Do it yourself. Here are the instructions:
- First, take a blank outline world map (you can print it or draw a simple one with the continents and oceans labelled).
- Use coloured pencils or pens—one colour for warm currents and another for cold currents.
For the Pacific Ocean:
- Warm Current – Kuroshio Current (Japan Current):
- Locate Japan on the map.
- Draw an arrow starting near the east coast of Taiwan going northeast past Japan.
- Label it “Kuroshio Current” and mark it as warm.
- Cold Current – Oyashio Current:
- Locate the eastern part of Russia (near Kamchatka Peninsula).
- Draw an arrow starting from the Bering Sea down southwards past the eastern coast of Japan.
- Label it “Oyashio Current” and mark it as cold.
For the Atlantic Ocean:
- Warm Current – Gulf Stream:
- Locate the Gulf of Mexico.
- Draw an arrow from the Gulf of Mexico through the Florida Strait, up the east coast of the USA, and then curving towards Western Europe.
- Label it “Gulf Stream” and mark it as warm.
- Cold Current – Labrador Current:
- Locate the area between Greenland and Canada (Labrador region).
- Draw an arrow moving southwards along the east coast of Canada towards Newfoundland.
- Label it “Labrador Current” and mark it as cold.
Tips:
- Use a key or legend to show which colour represents warm and cold currents.
- Make sure arrows clearly show the direction of flow.
- Keep your labelling neat and clear.
Extras
Additional MCQs
1. What percentage of Earth’s surface is covered in water?
A. 61%
B. 71%
C. 81%
D. 91%
Answer: B. 71%
80. The high point of a sea wave is called the:
A. Crest
B. Trough
C. Ridge
D. Bulge
Answer: A. Crest
Additional Assertion and Reason
1. Assertion (A): The Earth appears blue when viewed from outer space.
Reason (R): Approximately 71 per cent of the earth is covered in water.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R does not explain A.
(c) A is true but R is false
(d) A is false but R is true
Answer: (a) Both A and R are true and R is the correct explanation of A.
15. Assertion (A): The Oyashio Current forms very rich fishing grounds.
Reason (R): The cold water of the Oyashio Current has extremely high nutrient content.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R does not explain A.
(c) A is true but R is false
(d) A is false but R is true
Answer: (a) Both A and R are true and R is the correct explanation of A.
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