This MCQ module is based on: Hydrological Cycle & Ocean Floor Relief
TOPIC 24 OF 29
Hydrological Cycle & Ocean Floor Relief
🎓 Class 11
Social Science
CBSE
Theory
Ch 12 — Water (Oceans)
⏱ ~28 min
🌐 Language: [gtranslate]
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12.1 Earth — The Blue Planet
Can we think of life without water? It is often said that water is life. Water is an essential component of every living thing that walks, swims, flies or grows on this planet. The creatures of the earth are lucky that we live on a water planet — for water is a rare commodity in the rest of our solar system. There is no liquid water on the Sun, none on Mercury, and very little anywhere else nearby. The earth, fortunately, has an abundant supply of water on its surface — and from a satellite high above, the dominant colour of our planet is not green, not brown, but blue. That is why the earth is called the Blue Planet?.
🌐 Why "Blue Planet"?
Roughly 71 per cent of the earth's surface is covered by oceans and seas. Astronauts looking back from space see a sphere dominated by deep blue water bodies, white swirling clouds and only patches of brown and green continents. The colour blue itself comes from preferential absorption of red wavelengths and reflection of blue light by deep water masses.
Next to air, water is the most important element required for the existence of life on earth. But the distribution of water across the planet is highly uneven — some regions enjoy plenty while others have very limited quantity. To understand where water is found, how it moves, and why oceans cover so much of the globe, we begin with the great planetary circulation that links every drop of water — the hydrological cycle.
12.2 The Hydrological Cycle
Water is a cyclic resource. It can be used and re-used because nature constantly circulates it through a closed loop — from ocean to atmosphere, from atmosphere to land, and from land back to ocean. The hydrological cycle? describes the continuous movement of water on, in and above the earth. This cycle has been working for billions of years, and every form of life on earth depends on it. Despite enormous flows in and out, the total volume of renewable water on the earth remains constant — only the demand keeps rising.
📖 Definition — Hydrological Cycle
The hydrological cycle is the circulation of water within the earth's hydrosphere in three different forms — the liquid, the solid (ice) and the gaseous (water vapour) phases. It is also defined as the continuous exchange of water between the oceans, the atmosphere, the land surface and the subsurface, and the organisms that live in or on them.
The Three Phases of the Cycle
The hydrological cycle has three core mechanical processes that move water from one reservoir to another. Together these three phases keep the cycle turning.
1. Evaporation
Liquid water at the ocean and land surface absorbs solar energy and turns into invisible water vapour, rising into the atmosphere. Evapotranspiration (from plants) and sublimation (ice → vapour) are sister processes.
2. Condensation
As moist air rises and cools, the vapour changes back into tiny droplets — forming clouds, fog and mist. Condensation is the link between the gaseous and liquid phases.
3. Precipitation
When droplets coalesce into raindrops or freeze into snow and hail, they fall to the surface as precipitation. About 59 per cent of land precipitation later returns to the atmosphere by evaporation.
Figure 12.1 — The Hydrological Cycle
Components and Processes of the Water Cycle
| Component (Reservoir) | Associated Processes |
|---|---|
| Water storage in oceans | Evaporation, evapotranspiration, sublimation |
| Water in the atmosphere | Condensation |
| Water storage in ice and snow | Precipitation |
| Surface runoff | Snowmelt runoff to streams |
| Groundwater storage | Stream-flow, freshwater storage, infiltration, groundwater discharge through springs |
Where is the Earth's Water? — A Quantitative Snapshot
About 91 per cent of the planetary water is found in the oceans (some sources put the round figure at 97 per cent of all liquid water). The remainder is held as freshwater in glaciers and icecaps, in groundwater sources, in lakes, in soil moisture, in the atmosphere, in streams and within the bodies of living things. Of the precipitation that falls on land, nearly 59 per cent returns to the atmosphere through evaporation from land and oceans together; the rest runs off the surface, infiltrates the ground, or accumulates as glacier ice.
Distribution of Earth's Water — Oceans Dominate
💧 Key Numbers — Total Water on the Earth
- Oceans (saltwater): ~ 97 per cent of all the water on the earth.
- Freshwater in glaciers and icecaps: ~ 2.5 per cent.
- Groundwater: ~ 0.7 per cent.
- Surface water (lakes, rivers, soil moisture, atmosphere): ~ 0.3 per cent.
THINK ABOUT IT — Why is the Renewable Water Constant Yet Crisis is Rising?
L4 Analyse
NCERT notes that the renewable water on the earth is constant while the demand is increasing tremendously, leading to a water crisis in different parts of the world both spatially and temporally. How can you intervene in improving the water quality and augmenting the available quantity of water?
The total volume of renewable freshwater is fixed by the size of the cycle, but humans now extract more than nature replenishes locally. Three responses help. (1) Augment supply — harvest rainwater, recharge groundwater through check-dams, restore traditional tanks and step-wells. (2) Reduce demand — shift to drip irrigation, fix leaking municipal pipes, reuse greywater for gardening and flushing. (3) Protect quality — treat sewage before it enters rivers, control industrial effluents, and stop dumping plastic in waterways. Pollution effectively shrinks the usable share of the cycle. As a student you can reduce personal water-footprint, plant native trees that anchor the cycle, audit your school's water bill, and lobby for piped sewage treatment in your locality. Saving water is therefore not just a moral duty — it is the only way the constant renewable supply can keep pace with a rising population.
12.3 Relief of the Ocean Floor
The oceans are confined to the great depressions of the earth's outer layer. Unlike the continents, which are sharply demarcated, the oceans merge so naturally into one another that it is hard to draw exact boundaries between them. Geographers have divided the oceanic part of the earth into five oceans — the Pacific, the Atlantic, the Indian, the Southern (Antarctic) and the Arctic. Various seas, bays, gulfs and other inlets are simply parts of these large oceans.
Pacific Ocean
The largest and deepest ocean. Contains 32 of the world's 57 explored deeps including the Mariana Trench. Roughly triangular, ringed by the volcanic "Ring of Fire".
Atlantic Ocean
"S"-shaped, smaller than the Pacific. Houses 19 deeps and the famous Mid-Atlantic Ridge. Very busy with global shipping and historic trade routes.
Indian Ocean
Open to the south, almost enclosed in the north by Asia. Holds 6 deeps. Monsoon-driven currents reverse twice a year — unique to this ocean.
Arctic Ocean
The smallest and shallowest. Largely covered by sea-ice. The Siberian shelf within it is the widest continental shelf in the world (~1,500 km).
A major portion of the ocean floor is found between 3 to 6 km below sea level. The "land" under the waters of the oceans — that is, the ocean floor — exhibits complex and varied features as those observed on the continents. The floors are rugged with the world's largest mountain ranges, deepest trenches and the largest plains. These submarine landforms are formed, like those on the continents, by the same trio of agencies — tectonic, volcanic and depositional processes.
Major Divisions of the Ocean Floor
The ocean floor can be divided into four major divisions: (i) the Continental Shelf, (ii) the Continental Slope, (iii) the Deep Sea Plain (Abyssal Plain), and (iv) the Oceanic Deeps or Trenches. Apart from these four, the ocean floor also carries minor but striking relief features — ridges, hills, sea mounts, guyots, atolls and submarine canyons.
Figure 12.2 — Cross-Section of the Ocean Floor (Relief Features)
(i) Continental Shelf
The continental shelf? is the extended margin of each continent occupied by relatively shallow seas and gulfs. It is the shallowest part of the ocean, with an average gradient of 1° or even less. The shelf typically ends at a very steep slope called the shelf break, which marks the transition into deeper water.
📏 Continental Shelves — At a Glance
- Average width: about 80 km, but extremely variable.
- Narrowest: almost absent or very narrow along the coasts of Chile and the west coast of Sumatra (active subduction margins).
- Widest in the world: the Siberian shelf in the Arctic Ocean stretches up to 1,500 km.
- Depth: as shallow as 30 m in some areas; up to ~600 m elsewhere; usually ends near a 200 m break.
- Sediment cover: brought down by rivers, glaciers and wind, and redistributed by waves and currents.
- Economic value: richest fishing grounds, and massive sedimentary deposits over geological time become fossil fuel source rocks (oil and gas).
(ii) Continental Slope
The continental slope connects the continental shelf and the deep ocean basins. It begins where the bottom of the shelf sharply drops off into a steep slope. The gradient of the slope region varies between 2° and 5°, and the depth of the slope region varies between 200 m and 3,000 m. The slope boundary indicates the true edge of the continents — beyond it, oceanic crust takes over from continental crust. Submarine canyons and trenches are commonly observed in this region.
(iii) Continental Rise & Deep Sea Plain (Abyssal Plain)
At the foot of the continental slope, fine sediment that has tumbled down the slope accumulates as a long, gentle apron called the continental rise. Beyond it stretches the great deep sea plain? — also called the abyssal plain. Deep sea plains are gently sloping areas of the ocean basins. They are the flattest and smoothest regions of the world, with depths varying between 3,000 and 6,000 m. These plains are covered with fine-grained sediments such as clay and silt, slowly raining down from the surface waters above over millions of years.
(iv) Oceanic Deeps or Trenches
The trenches? are the deepest parts of the oceans. They are relatively steep-sided, narrow basins. They are some 3-5 km deeper than the surrounding ocean floor. Trenches occur at the bases of continental slopes and along island arcs, and are associated with active volcanoes and strong earthquakes — that is why they are so significant in the study of plate movements. 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.
🏆 The Deepest Place on Earth — Mariana Trench
The Mariana Trench in the western Pacific Ocean reaches a depth of more than 11,000 m at its lowest point (the Challenger Deep). If Mount Everest (8,848 m) were placed at the bottom of the Mariana Trench, its summit would still be more than 2 km below the ocean surface. This depth scale is shown in the chart below.
Mariana Trench Depth vs Other Reference Points
| Ocean | Position / Shape | Number of Deeps | Famous Feature |
|---|---|---|---|
| Pacific | Largest, roughly triangular, "Ring of Fire" | 32 | Mariana Trench (>11,000 m) |
| Atlantic | "S"-shaped, between Americas and Eurafrica | 19 | Mid-Atlantic Ridge (Iceland) |
| Indian | Open south, monsoon currents | 6 | Ninety East Ridge |
| Arctic | Smallest, ice-covered | — | Siberian Shelf (1,500 km wide) |
| Southern (Antarctic) | Encircles Antarctica | — | Antarctic Circumpolar Current zone |
12.4 Minor Relief Features of the Ocean Floor
Apart from the four major divisions, several minor but significant relief features dominate different parts of the oceans. Some are volcanic in origin; some are tectonic; some are depositional — but each adds a distinct character to the seabed.
Mid-Oceanic Ridges
A mid-oceanic ridge? is composed of two parallel chains of mountains separated by a large central depression (the rift valley where new crust is being formed). The mountain ranges can have peaks as high as 2,500 m, and some even reach above the ocean's surface as islands. Iceland, a part of the Mid-Atlantic Ridge, is a famous example. The global mid-oceanic ridge system is the longest mountain chain on the planet — running for about 65,000 km zig-zagging across all the major oceans.
Seamounts
A seamount? is a mountain with a pointed summit, 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 chain — an extension of the Hawaiian Islands in the Pacific Ocean — is a celebrated example, marking the trail of a hot-spot beneath a slowly moving Pacific plate.
Submarine Canyons
Submarine canyons? are deep valleys, some comparable in scale to the Grand Canyon of the Colorado river on land. They are sometimes found cutting across the continental shelves and slopes, often extending from the mouths of large rivers. The Hudson Canyon off the east coast of the United States is the best-known submarine canyon in the world. Many of these canyons are carved by turbidity currents — dense, sediment-laden underwater avalanches that periodically scour the slope.
Guyots
A guyot? is a flat-topped seamount. They show clear evidence of gradual subsidence through stages — once they were volcanic islands above the sea-level, then their tops were planed flat by wave erosion, and finally they sank below the surface as the ocean floor cooled and subsided. It is estimated that more than 10,000 seamounts and guyots exist in the Pacific Ocean alone.
Atolls
Atolls are low islands found in the tropical oceans, consisting of coral reefs surrounding a central depression. The central depression may be a part of the sea (called a lagoon), or it may sometimes form an enclosure of fresh, brackish or even highly saline water cut off from the open ocean. Atolls of the Maldives, the Lakshadweep and the Tuamotus are perfect examples.
| Feature | Origin | Defining Trait | NCERT Example |
|---|---|---|---|
| Mid-oceanic Ridge | Tectonic (sea-floor spreading) | Twin mountain chains with central rift, peaks up to 2,500 m | Mid-Atlantic Ridge / Iceland |
| Seamount | Volcanic | Pointed-summit mountain rising 3,000-4,500 m, does not break surface | Emperor Seamount (Hawaii extension) |
| Guyot | Volcanic, then eroded & subsided | Flat-topped submerged mountain | 10,000+ in the Pacific |
| Submarine Canyon | Erosional (turbidity currents) | Deep valley cutting across shelf and slope | Hudson Canyon |
| Atoll | Biogenic (coral reefs) | Ring of coral with central lagoon | Maldives, Lakshadweep |
LET'S EXPLORE — Ranking the Ocean Floor by Depth
L3 Apply
Arrange the following features in order from shallowest (closest to sea level) to deepest, and write next to each one the typical depth range as given in the chapter: continental rise, abyssal plain, continental slope, Mariana Trench, continental shelf, mid-oceanic ridge crest.
Shallowest → deepest:
- Mid-oceanic ridge crest — peaks rise 2,500 m above the surrounding floor, sometimes breaking the surface (Iceland) → effectively 0 m to a few hundred metres for the highest peaks.
- Continental shelf — average end-depth ~ 200 m (range 30 - 600 m); gradient about 1°.
- Continental slope — 200 m to 3,000 m; gradient 2-5°.
- Continental rise — gentle apron at the foot of the slope, ~ 2,000 - 4,000 m.
- Abyssal plain — 3,000 to 6,000 m.
- Mariana Trench — > 11,000 m (Challenger Deep) — the deepest known point on Earth's surface.
Notice how the trench is roughly 5 km below the surrounding abyssal plain — exactly the "3-5 km deeper than surrounding ocean floor" figure given by NCERT.
MAP WORK — Locate Six Submarine Features in the World Atlas
L3 Apply
Open your atlas to the world physical map and locate the following submarine features. For each, write down the name of the ocean it lies in: (1) Mariana Trench, (2) Mid-Atlantic Ridge, (3) Hudson Canyon, (4) Emperor Seamount chain, (5) Siberian Shelf, (6) Ninety East Ridge.
(1) Mariana Trench → Pacific Ocean (west, near Guam). (2) Mid-Atlantic Ridge → Atlantic Ocean (runs north-south through the middle, cuts through Iceland). (3) Hudson Canyon → Atlantic Ocean (off the east coast of the United States, near New York). (4) Emperor Seamount chain → Pacific Ocean (north-west, extending from the Hawaiian Islands towards Kamchatka). (5) Siberian Shelf → Arctic Ocean (north of Russia). (6) Ninety East Ridge → Indian Ocean (runs almost exactly along the 90° E meridian).
🎯 Competency-Based Questions — Hydrological Cycle & Ocean Floor Relief
Case Stem. A class-11 student is reading a satellite image of the Pacific Ocean. The image shows a continental shelf about 50 km wide off the west coast of South America, dropping rapidly to a long, narrow trench (the Peru–Chile Trench) more than 8,000 m deep. Further west the image shows a flat abyssal plain at about 4,500 m depth, with two volcanic seamounts rising to 3,500 m below the surface, and one flat-topped guyot. Use this scenario for Q1–Q4.
Q1. The shelf along the west coast of South America is much narrower than the world average of 80 km. The most likely reason is —
L3 ApplyAnswer: (b). NCERT explicitly notes that shelves are "almost absent or very narrow along some of the margins like the coasts of Chile and the west coast of Sumatra" — both classic active subduction zones where an oceanic plate dives under a continental plate, leaving no room for a broad sediment apron.
Q2. The Peru-Chile Trench is associated with strong earthquakes and active volcanism. Why does NCERT classify trenches as the most "significant" feature in the study of plate movements?
L4 AnalyseTrenches mark active plate boundaries. They are produced where one tectonic plate is subducting under another. The dragging plate triggers strong earthquakes, and the partial melting of the descending plate feeds the line of active volcanoes that runs parallel to the trench (e.g. the Andes behind Peru-Chile). So trenches are simultaneously the deepest, most seismically active and most volcanically active stretches of the ocean — a triple signature of plate movement.
Q3. The two volcanic seamounts rise to 3,500 m below sea level but never break the surface. The flat-topped guyot in the same area shows that —
L5 EvaluateThe guyot was once an island. Its flat top is the signature of long wave-erosion at the sea surface. After erosion levelled the volcanic peak, the cooling oceanic crust subsided and dragged the island below the waves. So a guyot is a "fossilised" island — direct evidence that the ocean floor itself has been moving slowly downwards for millions of years. The two seamounts may be younger volcanoes that have not yet reached the surface, or that have already partially subsided without being eroded.
HOT Q. Trace, step by step, the journey of one molecule of water that begins as a wave breaking on the beach in Mumbai. Use at least four named processes from the hydrological cycle.
L6 CreateSample journey: (1) The water molecule absorbs heat from the tropical Sun and undergoes evaporation, leaving the Arabian Sea as vapour. (2) Carried by the south-west monsoon wind, the vapour rises against the Western Ghats and cools through condensation, forming clouds. (3) The droplets coalesce and fall as precipitation over the Sahyadri slopes. (4) On the ground the water either infiltrates the soil to recharge groundwater, becomes surface runoff in a stream, or is taken up by plant roots and released back as vapour through evapotranspiration. (5) Eventually the runoff water joins a river such as the Krishna and flows back to the Bay of Bengal — closing the loop of the cycle.
⚖️ Assertion–Reason Questions — Class 11
Options:
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
Assertion (A): The earth is called the Blue Planet.
Reason (R): Roughly 71 per cent of the earth's surface is covered by oceans, and water at depth absorbs red wavelengths and reflects blue wavelengths back to space.
Answer: (A) — Both statements are true and R correctly explains A. The dominant blue colour seen from space is a direct consequence of the vast water cover and the optical behaviour of deep water.
Assertion (A): The continental shelves are the world's richest fishing grounds and the source of fossil fuels.
Reason (R): Massive sedimentary deposits accumulate on the shelves over geological time, and the shallow, sunlit waters above them sustain phytoplankton on which fish populations depend.
Answer: (A) — Both true and R is the correct explanation. NCERT specifically notes that shelf sediments "received over a long time… become the source of fossil fuels", while the shallow gradient (~1°) lets sunlight reach the seabed and drive primary productivity that feeds fisheries.
Assertion (A): A guyot is a flat-topped submerged mountain.
Reason (R): Guyots are formed when a sea-mount is forced upward by tectonic uplift until its peak reaches the ocean surface.
Answer: (C) — A is true but R is false. The flat top of a guyot is the result of wave erosion when the seamount was once an island, followed by gradual subsidence of the cooling ocean floor — the reverse process from tectonic uplift. NCERT phrases this as "evidences of gradual subsidence through stages to become flat-topped submerged mountains".
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