Samacheer Kalvi 9th Social Science Hydrosphere Exercise Solutions | KalviGavani

Hydrosphere — Exercise Solutions Guide

Solve every exercise on your own first, then cross-check your answers here. Understanding why an answer is correct matters more than memorising it.

I. Multiple Choice Questions (MCQs)

**Q1. Approximately what percentage of the Earth's surface is covered by water?

(a) 51% (b) 61% (c) 71% (d) 81%

**Q2. The deepest point in the world's oceans is

(a) Puerto Rico Trench (b) Mariana Trench (c) Java Trench (d) Tonga Trench

**Q3. The warm ocean current that flows along the eastern coast of North America is the

(a) Canary Current (b) Labrador Current (c) Gulf Stream (d) Benguela Current

**Q4. The average salinity of ocean water is approximately

(a) 25 parts per thousand (b) 35 parts per thousand (c) 45 parts per thousand (d) 55 parts per thousand

**Q5. Which ocean is the largest and deepest?

(a) Atlantic Ocean (b) Indian Ocean (c) Arctic Ocean (d) Pacific Ocean

**Q6. Spring tides occur during

(a) First quarter and third quarter moon (b) Full moon and new moon (c) Only during solar eclipse (d) Only during lunar eclipse

**Q7. The S-shaped ocean is the

(a) Pacific Ocean (b) Indian Ocean (c) Atlantic Ocean (d) Arctic Ocean

**Q8. The cold ocean current flowing along the western coast of Africa is the

(a) Gulf Stream (b) Benguela Current (c) Kuroshio Current (d) Brazil Current

**Q9. The continental shelf is the

(a) Deepest part of the ocean (b) Shallowest part of the ocean near the coast (c) Mid-ocean ridge (d) Deepest trench

**Q10. Which of the following factors decreases ocean salinity?

(a) High evaporation (b) Low rainfall (c) Freshwater inflow from rivers (d) High temperature

**Q11. The Great Barrier Reef is located in the

(a) Atlantic Ocean (b) Indian Ocean (c) Pacific Ocean (d) Arctic Ocean

**Q12. Tsunami waves in the open ocean are characterised by

(a) Great height and short wavelength (b) Low height and long wavelength (c) Great height and long wavelength (d) Low height and short wavelength

II. True or False

1. The Pacific Ocean is the smallest of the four major oceans.

2. The Gulf Stream is a cold ocean current.

3. Salinity is higher in regions of high evaporation and low rainfall.

4. Neap tides produce the highest tidal range.

5. Ocean currents flowing from the equator towards the poles are warm currents.

6. The average depth of the ocean is about 3,700 metres.

7. Tide height is affected only by the moon's gravity.

8. The thermocline is the layer of rapid temperature change in the ocean.

9. The Indian Ocean is entirely located in the Southern Hemisphere.

10. The mid-ocean ridge is a depositional feature on the ocean floor.

III. Fill in the Blanks

1. The largest ocean in the world is the ___.

2. The daily rise and fall of sea level is called ___.

3. The cold current that flows along the eastern coast of North America is the ___ Current.

4. The shallowest part of the ocean floor near the coast is called the ___.

5. The average salinity of ocean water is ___ parts per thousand.

6. The smallest ocean in the world is the ___.

7. The deepest point in the ocean is called the ___ Deep in the Mariana Trench.

8. The warm current that moderates the climate of Western Europe is the ___.

9. The rhythmic rise and fall of sea water twice a day is caused by ___.

10. Ocean currents moving from poles towards the equator are called ___ currents.

IV. Match the Following

1 → b — The Gulf Stream is a warm current in the North Atlantic that flows from the Gulf of Mexico northeastward along the eastern US coast and across to Western Europe, significantly warming the climate of countries like the UK and Norway.

2 → a — The Labrador Current is a cold current flowing southward from the Arctic Ocean along the eastern coast of Canada (Labrador). It brings cold water and icebergs south, meeting the warm Gulf Stream off Newfoundland.

3 → c — The Benguela Current is a cold current flowing northward along the southwestern coast of Africa (Namibia and South Africa). It brings nutrient-rich cold water, supporting major fishing grounds.

4 → e — The Kuroshio Current (also called the Japan Current) is a warm current in the North Pacific that flows northward along the eastern coast of Japan, similar in role to the Gulf Stream in the Atlantic.

5 → d — The Challenger Deep is the deepest known point in the Earth's oceans, located at the southern end of the Mariana Trench in the western Pacific Ocean, at approximately 11,034 metres depth.

V. Short Answer Questions (2-3 marks)

Q1. What is the hydrosphere? What percentage of the Earth's water is in the oceans?

The hydrosphere refers to all the water on, under, and above the Earth's surface, including oceans, seas, rivers, lakes, groundwater, glaciers, and atmospheric water vapour. About 97% of the Earth's water is stored in the oceans as saltwater. Only about 3% is freshwater, and of that, about two-thirds is locked in glaciers and ice caps, leaving less than 1% readily accessible for human use.

Q2. Name the four major oceans and state one characteristic of each.

The four major oceans are: (1) Pacific Ocean — the largest and deepest ocean, covering about one-third of the Earth's surface and containing the Mariana Trench; (2) Atlantic Ocean — the second largest, S-shaped, with the busiest shipping routes between Europe and the Americas; (3) Indian Ocean — the third largest, the warmest ocean, bordered by Asia, Africa, and Australia; (4) Arctic Ocean — the smallest and shallowest ocean, located around the North Pole and partially covered by ice year-round.

Q3. What are ocean currents? Name two warm currents and two cold currents.

Ocean currents are continuous, directed movements of ocean water driven by prevailing winds, differences in water temperature and salinity, Earth's rotation (Coriolis effect), and the shape of coastlines. Two warm currents are: (1) Gulf Stream (North Atlantic) and (2) Kuroshio Current (North Pacific). Two cold currents are: (1) Labrador Current (North Atlantic) and (2) Benguela Current (South Atlantic, off SW Africa).

Q4. What are tides? Name the two types of tides.

Tides are the periodic rise and fall of sea level caused by the gravitational pull of the moon and the sun, combined with the centrifugal force of the Earth-moon system. The two main types are: (1) Spring tides — occur during full moon and new moon when the sun, moon, and Earth are aligned, producing the highest high tides and lowest low tides; (2) Neap tides — occur during the first and third quarter moons when the sun and moon are at right angles, producing the smallest tidal range.

Q5. Describe the ocean floor relief features — continental shelf, slope, and rise.

The ocean floor has three major relief features parallel to the coast: (1) Continental shelf — the gently sloping, shallow area (0-200 m depth) extending from the coast seaward. It is the most productive zone for fishing and mineral resources. (2) Continental slope — the steeply descending zone from the edge of the continental shelf down to about 2,000-3,000 m depth, marking the true edge of the continent. (3) Continental rise — the gently sloping area at the base of the continental slope, formed by the accumulation of sediment transported down the slope, leading to the deep ocean floor (abyssal plain).

Q6. What factors affect the salinity of ocean water?

The salinity of ocean water is affected by several factors: (1) Evaporation — higher evaporation increases salinity by removing freshwater (e.g., Red Sea ~40 ppt). (2) Precipitation — heavy rainfall decreases salinity by diluting seawater (e.g., equatorial regions ~34 ppt). (3) Freshwater inflow — rivers flowing into the sea dilute salinity near their mouths (e.g., Baltic Sea ~7 ppt). (4) Temperature — higher temperatures increase evaporation, raising salinity. (5) Ice formation — when seawater freezes, salts are excluded, increasing salinity in surrounding water. Ice melting has the opposite effect.

Q7. What is the Coriolis effect and how does it influence ocean currents?

The Coriolis effect is the apparent deflection of moving objects (including ocean currents and winds) caused by the Earth's rotation. In the Northern Hemisphere, currents are deflected to the right, and in the Southern Hemisphere, to the left. This deflection creates large circular current systems called gyres. For example, in the North Atlantic, the Coriolis effect causes the Gulf Stream to curve northeastward, forming a clockwise gyre. The Coriolis effect is why ocean currents generally flow in a circular pattern rather than in straight lines from equator to poles.

Q8. What are mid-oceanic ridges?

Mid-oceanic ridges are continuous underwater mountain ranges that run through all the major oceans, forming the longest mountain chain on Earth (over 65,000 km). They are formed at divergent plate boundaries where magma rises from the mantle, creating new oceanic crust. The Mid-Atlantic Ridge is the best-known example, running through the centre of the Atlantic Ocean from the Arctic to the Southern Ocean. These ridges are sites of intense volcanic activity and hydrothermal vents.

VI. Detailed Answer Questions (5 marks)

Q1. Describe the major features of the ocean floor.

The ocean floor has diverse relief features similar to those on land, divided into four major zones:

Continental Shelf (0-200 m): The shallowest part of the ocean floor, extending from the coastline to about 200 m depth. It is a gently sloping extension of the continent. It is economically important because it contains rich fishing grounds, mineral deposits (petroleum, natural gas), and supports coral reefs. About 90% of the world's fish catch comes from continental shelves.

Continental Slope (200-3,000 m): The steep descent from the edge of the continental shelf down to the deep ocean floor. The slope angle is typically 2-5 degrees but can be steeper. Submarine canyons (deep, steep-sided valleys) often cut across the continental slope, funnelling sediment to the deep ocean.

Deep Ocean Floor (3,000-6,000 m): The vast, flat abyssal plains are the flattest areas on Earth, covered by fine sediment. Other features include: (1) Mid-oceanic ridges — long underwater mountain ranges formed by divergent plate boundaries (e.g., Mid-Atlantic Ridge). (2) Seamounts — underwater volcanic mountains that do not reach the surface. (3) Guyots — flat-topped seamounts that once reached the surface and were eroded by waves. (4) Trenches — the deepest parts of the ocean, formed at convergent plate boundaries where one plate subducts beneath another (e.g., Mariana Trench at 11,034 m).

Oceanic Trenches (>6,000 m): Long, narrow, deep depressions in the ocean floor, typically located near subduction zones. They are the deepest parts of the oceans. Major trenches include the Mariana Trench (Pacific), the Puerto Rico Trench (Atlantic), and the Java Trench (Indian Ocean).

Q2. Explain ocean currents — types, causes, and their effects on climate.

Ocean currents are continuous, directed movements of seawater that flow over long distances. They are classified into two types based on temperature:

Warm Currents flow from the equator towards the poles, carrying warm water to higher latitudes. Examples: Gulf Stream (Atlantic), Kuroshio Current (Pacific), North Atlantic Drift.

Cold Currents flow from the poles towards the equator, carrying cold water to lower latitudes. Examples: Labrador Current (Atlantic), Benguela Current (Atlantic), Peru (Humboldt) Current (Pacific), Oyashio Current (Pacific).

Causes of Ocean Currents: 1. Prevailing winds — trade winds and westerlies drag surface water along, creating currents. 2. Temperature differences — warm water is less dense and rises, cold water is denser and sinks, creating convection currents. 3. Salinity differences — high-salinity water is denser and sinks, driving vertical circulation (thermohaline circulation). 4. Coriolis effect — Earth's rotation deflects currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, creating circular gyres. 5. Shape of coastlines — landmasses redirect and channel currents. 6. Gravitational pull — the moon and sun influence tidal currents.

Effects on Climate: Warm currents raise the temperature and humidity of adjacent coastal regions, while cold currents lower them. The Gulf Stream keeps Western Europe (especially the UK and Norway) significantly warmer than other regions at similar latitudes. Cold currents like the Peru (Humboldt) Current cool the western coast of South America and contribute to the formation of coastal deserts (Atacama Desert). Where warm and cold currents meet, fog forms and rich fishing grounds develop (e.g., Grand Banks off Newfoundland where the Gulf Stream meets the Labrador Current). Cold upwelling currents bring nutrients to the surface, supporting major fisheries.

Q3. Explain tides — causes, types, and significance.

Tides are the periodic rise and fall of sea level caused primarily by the gravitational pull of the moon and the sun, combined with the centrifugal force generated by the Earth-moon system.

Causes: The moon's gravitational pull is the primary cause of tides because, although the sun is much larger, the moon is much closer to Earth and exerts a stronger tidal force. The moon pulls the water towards itself, creating a bulge (high tide) on the side facing the moon. A second bulge occurs on the opposite side due to the centrifugal force of the Earth-moon system. Between these bulges, sea level drops (low tide). The sun also contributes: during alignment (spring tides), its gravitational effect adds to the moon's; during right angles (neap tides), it partially cancels it.

Types of Tides: 1. Spring Tides: Occur during the full moon and new moon when the sun, moon, and Earth are aligned. Their gravitational forces combine, producing the highest high tides and the lowest low tides. Despite the name, "spring" refers to the tides "springing forth," not the season. 2. Neap Tides: Occur during the first and third quarter moons when the sun and moon are at right angles to each other. Their gravitational effects partially cancel out, producing the smallest tidal range — lower high tides and higher low tides.

Significance of Tides: 1. Navigation — high tides allow ships to enter shallow harbours and estuaries. 2. Fishing — tides bring nutrient-rich water to coastal areas, supporting fish populations. 3. Energy — tidal energy can be harnessed to generate electricity (e.g., tidal power plants). 4. Coastal ecology — tides shape coastal landforms and support intertidal ecosystems. 5. Desalination — tidal movement helps mix and circulate ocean water.

Q4. Compare the four major oceans — Pacific, Atlantic, Indian, and Arctic.

Pacific Ocean:

• Area: ~165.25 million sq km (largest, covers one-third of Earth's surface)
• Average depth: ~4,280 m; Deepest point: Challenger Deep (Mariana Trench) ~11,034 m
• Shape: Roughly oval, bordered by the Ring of Fire (volcanoes and earthquakes)
• Features: Contains the most trenches, islands (Oceania), and the Great Barrier Reef
• Currents: Kuroshio (warm), California (cold), Peru/Humboldt (cold), East Australian (warm)

Atlantic Ocean:

• Area: ~106.46 million sq km (second largest)
• Average depth: ~3,646 m; Deepest point: Milwaukee Deep (Puerto Rico Trench) ~8,376 m
• Shape: Distinctive S-shape, narrowest between South America and Africa
• Features: Mid-Atlantic Ridge runs through its centre; busiest shipping routes
• Currents: Gulf Stream (warm), Labrador (cold), Canary (cold), Benguela (cold), Brazil (warm)

Indian Ocean:

• Area: ~70.56 million sq km (third largest)
• Average depth: ~3,741 m; Deepest point: Sunda Deep (Java Trench) ~7,725 m
• Shape: Triangular, bordered by Asia (north), Africa (west), Australia (east), Southern Ocean (south)
• Features: Warmest ocean; monsoon winds reverse currents seasonally; rich in marine biodiversity
• Currents: Agulhas (warm), West Australian (warm), South Indian (part of gyre)

Arctic Ocean:

• Area: ~14.06 million sq km (smallest)
• Average depth: ~1,205 m (shallowest); Deepest point: Fram Basin ~5,450 m
• Shape: Nearly circular, centred around the North Pole
• Features: Partially covered by ice year-round; connected to the Atlantic via the Greenland Sea; increasing ice melt due to climate change
• Currents: Norwegian (warm), East Greenland (cold); largely influenced by Atlantic inflow

Q5. Explain the distribution of temperature and salinity in the oceans.

Temperature Distribution: Ocean temperature varies with latitude, depth, and season. At the surface, the hottest waters are near the equator (25-28 degrees Celsius), and the coldest are near the poles (below 0 degrees Celsius). Temperature decreases with depth — the surface layer (0-200 m) is warm and well-mixed by waves and currents. Below this, the thermocline (200-1,000 m) is a zone of rapid temperature decrease. Below 1,000 m, the deep ocean is uniformly cold (1-3 degrees Celsius). Seasonal variation affects only the surface layer, primarily in temperate regions. Warm currents raise temperatures in adjacent areas, while cold currents lower them.

Salinity Distribution: Salinity refers to the amount of dissolved salts in seawater, averaging about 35 parts per thousand (ppt). Distribution varies based on several factors: (1) Evaporation — high evaporation increases salinity (Red Sea ~40 ppt, Mediterranean ~38 ppt). (2) Precipitation — heavy rainfall decreases salinity (equatorial regions ~34 ppt). (3) Freshwater inflow — river mouths have low salinity (Baltic Sea ~7 ppt, Bay of Bengal ~33 ppt). (4) Ice formation and melting — freezing excludes salts, raising local salinity; melting ice dilutes it.

The highest salinity is found in subtropical regions (20-30 degrees latitude) where evaporation exceeds precipitation. The lowest salinity is near river mouths and in polar regions with ice melt. The Red Sea has one of the highest salinities (~40 ppt) due to high evaporation and no major river inflow, while the Baltic Sea has one of the lowest (~7 ppt) due to abundant freshwater inflow and low evaporation.

Common Mistakes

1. Confusing warm and cold currents: Warm currents flow from the equator towards the poles (e.g., Gulf Stream). Cold currents flow from the poles towards the equator (e.g., Labrador Current). Remember the direction of flow to determine the type.

2. Saying the Arctic Ocean is the deepest: The Arctic Ocean is the shallowest ocean with an average depth of about 1,205 m. The Pacific Ocean is the deepest, with an average depth of about 4,280 m and containing the Mariana Trench.

3. Confusing spring tides and neap tides: Spring tides have the greatest tidal range (highest highs, lowest lows) and occur during full/new moons. Neap tides have the smallest tidal range and occur during quarter moons. "Spring" does not refer to the season.

4. Thinking salinity is highest at the equator: Equatorial regions actually have moderate salinity (~34-35 ppt) because heavy rainfall dilutes the water despite high evaporation. The highest salinity is in subtropical regions (20-30 degrees) where evaporation far exceeds precipitation, and in enclosed seas like the Red Sea (~40 ppt).

5. Confusing continental shelf and continental slope: The continental shelf is the shallow, gently sloping area (0-200 m) near the coast. The continental slope is the steep descent from the shelf edge to the deep ocean floor. The shelf is where most fishing and oil extraction occurs.

6. Stating that tides are caused only by the moon: Tides are caused by the gravitational pull of both the moon and the sun, plus the centrifugal force of the Earth-moon system. The moon has about twice the tidal effect of the sun because it is much closer, but the sun's contribution is still significant.

7. Confusing the Mariana Trench with the Challenger Deep: The Challenger Deep is the specific point (approximately 11,034 m) within the Mariana Trench that is the deepest known point in the ocean. The Mariana Trench is the entire trench, about 2,550 km long.

Exam Checklist

• [ ] Can define the hydrosphere and state the percentage of water in oceans (~97%)
• [ ] Can name and compare the four major oceans (Pacific, Atlantic, Indian, Arctic) with key facts
• [ ] Can describe the ocean floor relief features (shelf, slope, rise, abyssal plain, ridge, trench)
• [ ] Can distinguish between warm and cold currents with examples of each
• [ ] Can explain the causes of ocean currents (winds, temperature, salinity, Coriolis effect)
• [ ] Can describe the effects of ocean currents on climate with specific examples
• [ ] Can explain the causes and types of tides (spring and neap tides)
• [ ] Can discuss factors affecting ocean salinity with examples (Red Sea, Baltic Sea)
• [ ] Can explain temperature variation in oceans (surface vs depth, latitude, thermocline)
• [ ] Can describe the mid-oceanic ridge system and its formation
• [ ] Can identify key ocean features: Mariana Trench, Challenger Deep, Great Barrier Reef
• [ ] Can explain the significance of tides for navigation, fishing, and energy

Internal Links

• [Class 9 Social Science — Hydrosphere Guide](/school/tamil-nadu-state-board/class/9/social/hydrosphere)
• [All Class 9 Social Solutions](/school/tamil-nadu-state-board/class/9/social/solutions)
• [Class 9 Social Science Subject Guide](/school/tamil-nadu-state-board/class/9/social)