GeographyISC Class 11 · Submarine Relief of the Oceans

Sea-floor Deposits, Marine Resources and Ocean Pollution (ISC Class 11 Geography)

Updated 17 October 2026 · Fact-checked

Sea-floor deposits are terrigenous (land-derived mud on shelf and slope), pelagic organic oozes and red clay on deep plains, volcanic ash and manganese nodules. Marine resources include food, oil, minerals and trade routes within the 200-nautical-mile EEZ. Pollution from oil, plastics, sewage and runoff is tackled by treatment, bans and conventions such as MARPOL.

Understand Sea-floor Deposits, Marine Resources and Ocean Pollution

Deposits.
1. Terrigenous: gravel, sand, silt and mud from land, coarse near the coast and finer offshore, on the shelf and slope. Muds: blue (iron sulphide), red (iron oxide, off the Amazon) and green (glauconite).
2. Pelagic: organic oozes with at least 30 per cent shells and skeletons. Calcareous ooze (globigerina, pteropod) of calcium carbonate dissolves below about 4,000 to 5,000 m. Siliceous ooze (diatom in cold waters, radiolarian in tropical) survives at depth. Red clay: very fine volcanic, wind-blown and meteoric dust, extremely slow, covering vast areas of the Pacific.
3. Volcanic: ash and lava around islands and ridges.
4. Chemical: manganese (polymetallic) nodules with manganese, nickel, copper and cobalt; India explores the Central Indian Ocean Basin.

Marine resources. Food (fish from shelves), minerals and energy (Bombay High oil and gas, placer sands rich in ilmenite and monazite on the Kerala coast, nodules, salt, tidal, wave and thermal energy), trade, climate regulation, medicines, tourism. Exclusive Economic Zone extends 200 nautical miles.

Pollution. Oil spills (Deepwater Horizon, Gulf of Mexico, 2010), plastics and microplastics gathering in gyres, sewage and industrial effluents with heavy metals, agricultural runoff causing algal blooms and dead zones, ship and radioactive waste.

Remedies. Treat waste before discharge; cut single-use plastics (India banned a list of items from 2022); MARPOL; oil-spill response with booms and skimmers; less fertiliser; protected marine areas; clean-ups and awareness.

Sea-floor Deposits, Marine Resources and Ocean Pollution: key facts

Deposits: Terrigenous: shelf, slope
Pelagic: oozes, red clay
Volcanic
Nodules
Ooze rule: At least 30% shells and skeletons | Calcareous ooze absent below about 4,000-5,000 m
EEZ: Extends 200 nautical miles from the coast | Rights over resources
Sediment thickness: $T hi c k n ess = r a t e x t im e ∣ C h ec k mman d y e a r s$

How to answer Sea-floor Deposits, Marine Resources and Ocean Pollution questions

Questions are 3 to 4 marks: classify deposits, explain resources or give pollution causes and remedies.

1Classify deposits by source and depth.
2Link each deposit type to a depth zone.
3List resources with an Indian example.
4Pair every pollution cause with a remedy.

Sea-floor Deposits, Marine Resources and Ocean Pollution in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Slow red clay

Suppose red clay accumulates at 2 mm per thousand years on a deep plain.

How thick is the layer after 5 million years?

Try it first, then show the solutionSolution

5,000,000 years = 5,000 thousand years.
Thickness = 2 x 5,000 = 10,000 mm = 10 m.

Answer: 10 m.

Memory hook: Rate times time.

Algal bloom

Suppose heavy fertiliser runoff reaches a bay and the water turns green, then fish die.

Name the pollution type and a remedy.

Try it first, then show the solutionSolution

Fertiliser causes algal blooms and oxygen-poor dead zones.
Reduce fertiliser use and treat runoff.

Answer: Agricultural runoff; reduce fertiliser use.

Memory hook: Fertiliser feeds algae, starves fish.

Tricks to remember Sea-floor Deposits, Marine Resources and Ocean Pollution

Short memory aids. Say each one aloud once, then use it on a question the same day.

Calcium dissolves deep
Calcareous ooze vanishes below 4,000-5,000 m; siliceous survives.
Use it when: Ooze.
Pollution pairs
Oil: booms; plastic: bans; sewage: treatment.
Use it when: Remedies.

Sea-floor Deposits, Marine Resources and Ocean Pollution: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Deposit identification · 1 mark · MCQBoard level

Very fine reddish-brown clay on the deepest plains is called:
(a) terrigenous mud
(b) calcareous ooze
(c) red clay
(d) manganese nodule

Try it first, then show the solutionSolution

Pacific.

Answer: (c) Red clay.

Example 2: Nodules · 3 marks · Short answerBoard level

What are manganese nodules and where does India explore them?

Try it first, then show the solutionSolution

Nodules of manganese, nickel, copper and cobalt formed by slow precipitation from sea water.
They are chemical deposits.
India explores the Central Indian Ocean Basin.

Answer: Metal-rich chemical nodules; Central Indian Ocean Basin.

Example 3: Accumulation sum · 4 marks · Long answerHarder board level

Red clay accumulates at 2 mm per thousand years. Find the thickness after 5 million years and the time to reach 25 m. Compare with a shelf deposit growing 1 mm per year.

Try it first, then show the solutionSolution

5 million years = 5,000 thousand; thickness = 2 x 5,000 = 10,000 mm = 10 m.
25 m = 25,000 mm; time = 25,000/2 x 1,000 = 12.5 million years.
Shelf deposit: 1 mm per year gives 5,000,000 mm = 5,000 m in 5 million years.
So the shelf grows 500 times faster (1,000 mm per thousand years against 2).

Answer: 10 m; 12.5 million years; shelf deposit is 500 times faster.

Example 4: Oozes · 4 marks · Long answerHarder board level

Distinguish calcareous and siliceous oozes on composition, location and depth range.

Try it first, then show the solutionSolution

Calcareous: calcium carbonate; siliceous: silica.
Calcareous: warm moderately deep water; siliceous: cold polar (diatom) and tropical (radiolarian) waters.
Calcareous dissolves below about 4,000-5,000 m.
Siliceous survives at great depths.

Answer: Calcareous ooze (calcium carbonate) is absent below about 4,000-5,000 m; siliceous (silica) survives.

Example 5: Pollution case · 5 marks · Case-basedHOTS / competency

Case: A tanker spill coats a bay while plastic fills a gyre. (a) Name each pollution type. (b) Give a remedy for each. (c) Name the international convention on ship pollution.

Try it first, then show the solutionSolution

(a) Oil spill and plastic pollution.
(b) Booms and skimmers for oil; reducing and banning single-use plastics.
(c) MARPOL.

Answer: (a) Oil, plastics (b) booms; bans (c) MARPOL.

Where students lose marks in Sea-floor Deposits, Marine Resources and Ocean Pollution

The slips examiners see most often, and what to write instead.

Mistake: Placing calcareous ooze in the deepest plains
Depth limit ignored.
Do this: Calcium carbonate dissolves below about 4,000-5,000 m.
Usually costs: 1 mark.
Mistake: Naming no remedy
Pollution described only.
Do this: Pair each cause with a remedy.
Usually costs: 1 to 2 marks.
Mistake: Unit slip in thickness
mm and m confused.
Wrong working
12,000 m
Right working
12,000 mm = 12 m
Do this: Convert 1,000 mm to 1 m.
Usually costs: 1 mark.

Exam tips

Classify deposits by depth zone.
Give an Indian resource example.
Pair pollution and remedy.
Mention MARPOL and EEZ.

Practice questions on Sea-floor Deposits, Marine Resources and Ocean Pollution

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The Exclusive Economic Zone extends:
(a) 12 nautical miles
(b) 50 nautical miles
(c) 200 nautical miles
(d) 500 nautical miles
Hint
Resource rights.
Answer
(c) 200 nautical miles.
Q2 · 3 marks · Short answerHarder board level
A sediment layer grows at 4 mm per thousand years. Find its thickness after 3 million years.
Hint
Rate times time.
Answer
3 million years = 3,000 thousand; 4 x 3,000 = 12,000 mm = 12 m.
Q3 · 3 marks · Short answerHarder board level
Explain why terrigenous deposits are found mainly on the shelf and slope.
Hint
Rivers, coast.
Answer
They come from land through rivers, glaciers, wind and coastal erosion, so they are coarse near the coast and finer offshore, settling mainly on the shelf and slope.
Q4 · 4 marks · Long answerHarder board level
Explain four uses of marine resources with one Indian example.
Hint
Food, oil, minerals, trade.
Answer
Fish from the shelf; oil and gas such as Bombay High; ilmenite and monazite sands on the Kerala coast; sea trade as the cheapest bulk route; also climate regulation and tourism.
Q5 · 4 marks · Case-basedHOTS / competency
Case: Fertiliser runoff, sewage and plastic bags reach a bay. (a) Name three pollution types. (b) Explain one effect of each. (c) Suggest two common remedies.
Hint
Dead zones, entanglement.
Answer
(a) Agricultural runoff, sewage, plastics. (b) Dead zones, disease and heavy-metal poisoning, entanglement of animals. (c) Treat sewage and waste; reduce single-use plastics.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Pelagic deposit in Pacific?Show answer
Red clay.
2.EEZ width?Show answer
200 nautical miles.
3.Oil field off Mumbai?Show answer
Bombay High.
4.Gyre plastic mass called?Show answer
Great Pacific Garbage Patch.
5.Ship pollution convention?Show answer
MARPOL.

Sea-floor Deposits, Marine Resources and Ocean Pollution: frequently asked questions

Why does calcareous ooze vanish at depth?

Calcium carbonate dissolves there.

Are nodules renewable?

They form extremely slowly.

What is a dead zone?

Oxygen-poor water from algal blooms.