Ocean floor and mean sea level
When the tide goes out at the beach, you can walk on sand that was under water a few hours ago. That shows land continues below the sea. The sea floor has hills, plains and deep valleys, just like land.
The ocean floor (समुद्रतळ) is the land under ocean water; its relief is decided by depth from sea level and the shape of the land.
Rule 1: Ocean floor and mean sea level — About 71% of the Earth is water, and the average depth of the oceans is about 3700 m. The ocean bed is uneven, with hills, plateaus, trenches and plains. Mean sea level is the average of the highest high tides and lowest low tides, taken as zero: heights are positive and depths negative. Mount Everest +8848 m; Mariana Trench −11034 m; vertical gap = 8848 + 11034 = 19,882 m. For survey in India the sea level at Chennai is zero.
In the figure : In the figure the mean sea level is taken as zero: heights are positive and depths are negative (Mount Everest +8848 m, Mariana Trench −11034 m).
- Easy : Easy: Sea level is zero.
- Medium : Medium: 8848 + 11034 = 19,882 m between the peak and the trench.
- Exam-type : Exam: Why is depth written negative?
In the figure : In the figure about 71% of the surface of the earth is covered with water and about 29% is land; the average depth of the oceans is about 3700 m.
- Easy : Easy: Water is 71%.
- Medium : Medium: Land = 100 − 71 = 29%.
- Exam-type : Exam: What is the average ocean depth?
Rule 2: A young floor — Ocean-floor rocks are not older than 200 million years, but continental rocks can be 3200 million years old, 3200 ÷ 200 = 16 times older. So the ocean floor is very young; this research led to plate tectonics. Order from the coast: continental shelf → continental slope → abyssal plains (with submerged mountains and plateaus) → deeps and trenches.
In the figure : In the figure the rocks of the ocean floor are not older than about 200 million years, but continental rocks can be up to 3200 million years old, so the ocean floor is young.
Ocean floor and mean sea level
In the app you can move this figure with your finger.
Open in the app- Easy : Easy: Shelf comes first from the coast.
- Medium : Medium: 4 zones from coast to deep.
- Exam-type : Exam: Write the order of ocean-floor features.
71% water. Mean sea level 0. Everest +8848, Mariana −11034. Order: shelf, slope, plain, trench.
Thinking the sea floor is flat. It has hills, plains and trenches.
Memory clue: "मं-उ-मै-ख = मंच, उतार, मैदान, खंदक".
Spot the difference
| Item | Value |
|---|---|
| Water cover | 71% |
| Average depth | 3700 m |
| Ocean-floor age | ≤ 200 million years |
| Continental rock age | Up to 3200 million years |
Practice questions
- 1How much of the Earth is water?
- 2Average depth of oceans?
- 3What is mean sea level?
- 4Which sea level is zero in India?
- 5How old is the ocean floor at most?
- 6Which research led to plate tectonics?
- 7A ship is at −500 m and a hill is at +300 m. What is the difference?
- 8Why do tides show land under the sea?
Show answers (try them yourself first)
- 1About 71%.
- 2About 3700 m.
- 3Average of high and low tides, taken as zero.
- 4Chennai.
- 5200 million years.
- 6Study of the age of the ocean floor.
- 7300 + 500 = 800 m.
- 8At low tide the coastal bed is exposed.
Continental shelf and continental slope
A swimmer at the beach walks a long way in knee-deep water, then suddenly the bottom drops away. That knee-deep stretch is the shelf and the drop is the slope.
The continental shelf (खंडीय मंच) is the shallow, gently sloping submerged coastland; the continental slope (खंडीय उतार) is the steeper part beyond it.
Rule 1: Continental shelf — The land near the coast submerged under the sea; the shallowest part with a gentle slope; depth up to 200 m. Its width varies, narrow in some places and hundreds of km in others. It is very important: sunlight reaches the bed, so algae and plankton grow and extensive fishing grounds form. It gives natural gas, mineral oil and minerals, e.g. Mumbai High in the Arabian Sea.
In the figure : In the figure from the coast towards the deep sea: continental shelf, continental slope, abyssal plain (with submerged hills, islands and plateaus) and trenches.
- Easy : Easy: The shelf is the shallowest.
- Medium : Medium: Shelf depth 200 m is only about 5% of the 3700 m average (200 ÷ 3700 × 100 ≈ 5.4%).
- Exam-type : Exam: Why are fish plentiful on the shelf?
Rule 2: Continental slope — After the shelf ends, the bed becomes steeper. Depth is from 200 m to 3600 m, a span of 3600 − 200 = 3400 m. It is narrow. Its lower boundary is taken as the boundary of the continents.
In the figure : In the figure shelf: up to 200 m deep with a gentle slope. Slope: 200 m to 3600 m deep, steeper and narrow; its lower boundary is the boundary of the continents.
- Easy : Easy: Slope depth is 200 to 3600 m.
- Medium : Medium: Steeper than the shelf.
- Exam-type : Exam: Compare shelf and slope.
In the figure : In the figure on the shallow continental shelf sunlight reaches the bed, so plankton grows and fish feed on it; the shelf also gives natural gas, mineral oil (Mumbai High) and minerals.
- Easy : Easy: Mumbai High gives oil.
- Medium : Medium: Sunlight → plankton → fish.
- Exam-type : Exam: List two benefits of the shelf.
Shelf: up to 200 m, gentle, fish, oil. Slope: 200–3600 m, steep; boundary of continents.
Mixing shelf and slope. Shelf is shallow and gentle; slope is steep.
Memory clue: "मंच = मासे + तेल; उतार = सीमा".
Spot the difference
| Feature | Shelf | Slope |
|---|---|---|
| Depth | Up to 200 m | 200–3600 m |
| Gradient | Gentle | Steep |
| Use | Fishing, oil | Edge of continents |
Practice questions
- 1What is the continental shelf?
- 2Its depth?
- 3Why is it good for fishing?
- 4Name an oil field on a shelf.
- 5Depth of the continental slope?
- 6What is the lower boundary of the slope taken as?
- 7A slope goes from 200 m to 3000 m. How deep is it in total?
- 8Why must we protect the shelf from pollution?
Show answers (try them yourself first)
- 1Submerged coastland near the shore.
- 2Up to 200 m.
- 3Sunlight reaches the bed and plankton grows.
- 4Mumbai High.
- 5200 m to 3600 m.
- 6The boundary of the continents.
- 73000 − 200 = 2800 m.
- 8It is rich in fish and resources.
Abyssal plains, submerged mountains, islands and plateaus
Under a calm lake, there may be a hidden hill whose top peeps out as a small island. The ocean has the same, only huge.
Abyssal plains (अथांग मैदाने) are the flat ocean bed beyond the slope, with submerged hills, mountains and plateaus; peaks above the sea are marine islands.
Rule 1: Plains, ridges, islands — The abyssal plain is flat with submerged hills and plateaus. Submerged mountains are hundreds of km wide and thousands of km long: the Mid-Atlantic Ridge is the longest continuous range, up to 65,000 km (about 1.6 times the Earth's circumference of nearly 40,000 km). Peaks that rise above the sea are marine islands: Iceland (Atlantic), Andaman and Nicobar (Bay of Bengal). Flat, wide summits are submarine plateaus: the Chagos Plateau (Indian Ocean).
In the figure : In the figure peaks of submerged hills that rise above the sea are marine islands (Iceland); flat and extensive summits are submarine plateaus (Chagos Plateau).
- Easy : Easy: Iceland is a marine island.
- Medium : Medium: Chagos is a plateau, not an island.
- Exam-type : Exam: How are marine islands formed?
In the figure : In the figure examples: Iceland (Atlantic Ocean), Andaman and Nicobar Islands (Bay of Bengal), Chagos Plateau (Indian Ocean), Mariana Trench (Pacific Ocean).
- Easy : Easy: Andaman is in the Bay of Bengal.
- Medium : Medium: Four examples in four oceans or seas.
- Exam-type : Exam: Name the longest submerged range.
Plain, ridge, island, plateau. Mid-Atlantic Ridge up to 65,000 km.
Thinking all islands are made by volcanoes only. Many are peaks of submerged mountains.
Memory clue: "शिखर वर = बेट; माथा सपाट = पठार".
Spot the difference
| Landform | Example |
|---|---|
| Marine island | Iceland |
| Marine island | Andaman and Nicobar |
| Submarine plateau | Chagos |
| Longest range | Mid-Atlantic Ridge |
Practice questions
- 1What is an abyssal plain?
- 2What landforms are on it?
- 3Which is the longest submerged range?
- 4What is a marine island?
- 5Name two marine islands.
- 6Which plateau is in the Indian Ocean?
- 7A ridge is 65,000 km. How many times does it span 13,000 km?
- 8Why is a plateau called flat-topped?
Show answers (try them yourself first)
- 1The flat ocean bed beyond the slope.
- 2Submerged hills and plateaus.
- 3Mid-Atlantic Ridge.
- 4A submerged hill's peak above sea level.
- 5Iceland and the Andaman Islands.
- 6Chagos.
- 765,000 ÷ 13,000 = 5.
- 8Its summit is wide and level.
Marine deeps and trenches
A crack in the road is a small gap; a canyon is a huge one. On the sea floor, narrow and deep gaps are called trenches, and the earth there is restless.
Marine deeps and trenches (महाखोलगटे व खंदक) are deep, narrow, steep landforms; the Mariana Trench is the deepest.
Rule 1: Trenches — Shallower ones are deeps; deeper ones extending far are trenches, thousands of metres deep. The Mariana Trench in the Pacific is about 11,034 m deep; even Mount Everest (8848 m) would be submerged with 11034 − 8848 = 2186 m of water above its peak.
In the figure : In the figure Mount Everest (8848 m) would be completely submerged in the Mariana Trench (about 11,034 m deep), the deepest trench of the world, in the Pacific Ocean.
- Easy : Easy: Mariana is the deepest.
- Medium : Medium: 11034 − 8848 = 2186 m above Everest's peak.
- Exam-type : Exam: Where is the Mariana Trench?
Rule 2: Active zones — Mid-oceanic ridges and trenches are the geologically most active parts of the sea bed: many active volcanoes and earthquakes. Quakes and eruptions there cause tsunamis on nearby coasts. At trenches the oceanic plate sinks and melts; away from ridges rocks get older. This is why the ocean floor is young.
In the figure : In the figure new rock forms at the mid-oceanic ridges and becomes older away from them; at the trenches the oceanic plate sinks and melts, so the ocean floor is young.
- Easy : Easy: New rock forms at ridges.
- Medium : Medium: Rock is oldest far from ridges.
- Exam-type : Exam: Why is the ocean floor young?
In the figure : In the figure ridges and trenches are geologically active (volcanoes, earthquakes); their eruptions and quakes produce tsunamis in the nearby coastal areas.
- Easy : Easy: Quakes cause tsunamis.
- Medium : Medium: Coasts near trenches are at risk.
- Exam-type : Exam: Name two active features.
Mariana −11034 m. Ridges and trenches: volcanoes, quakes, tsunamis.
Thinking the deep sea is calm. Ridges and trenches are the most active.
Memory clue: "रांग बनवते, खंदक गिळतो".
Spot the difference
| Feature | What happens |
|---|---|
| Mid-oceanic ridge | New rock forms |
| Trench | Plate sinks and melts |
| Both | Volcanoes, quakes, tsunamis |
Practice questions
- 1What is a trench?
- 2Which is the deepest trench?
- 3In which ocean?
- 4How deep is it?
- 5What do quakes on the sea bed cause?
- 6Where does new ocean rock form?
- 7A trench is 10,000 m deep and a mountain is 8000 m high. How much water covers the peak if the mountain stands in it?
- 8Why do rocks get older away from ridges?
Show answers (try them yourself first)
- 1A deep, narrow, steep valley on the ocean bed.
- 2Mariana Trench.
- 3Pacific.
- 4About 11,034 m.
- 5Tsunamis.
- 6At mid-oceanic ridges.
- 710,000 − 8,000 = 2,000 m.
- 8New rock forms at ridges and moves outward.
Marine deposits
The beach after a festival holds coconut shells, plastic bottles and flowers. Flowers rot soon, but the bottles stay for years. The sea floor collects such things too.
Marine deposits (सागरी निक्षेप) are materials that settle on the sea bed: from continents, from volcanoes and living things, and from humans.
Rule 1: Natural deposits — (1) From the continents: pebbles, clay and soil brought by rivers and glaciers settle mainly on the continental shelf; the Ganga's sediments are seen up to about 2000 km into the sea. (2) Volcanic and organic: lava, ash, fine soil, remains of marine plants and animals. Fine clay with 30% or more remains of sea organisms is marine ooze, found in the deep sea away from the coast (in a 500 g sample, 30% = 150 g). Layers pressed by sea water form sedimentary rocks. These deposits tell us about marine life and minerals.
In the figure : In the figure deposits: from the continents (pebbles, clay and soil brought by rivers and glaciers), volcanic and organic (ash, lava and remains of marine life; marine ooze has 30% or more) and human-induced (sewage, plastics, waste).
- Easy : Easy: Rivers bring soil to the sea.
- Medium : Medium: Ooze needs at least 30% sea-life remains.
- Exam-type : Exam: What is marine ooze?
Rule 2: Human-induced deposits — Sewage, solid waste, radioactive material, waste chemicals and plastics are hazardous to the hydrosphere and extremely harmful to marine life; though small in quantity, their nuisance value is high. Seas have higher biodiversity than land, so we must not dump waste in them. Plants and dead animals are perishable; plastics, glass and metals are non-perishable and stay long. We should run coast-cleaning campaigns.
In the figure : In the figure plants and dead animals are perishable; plastics, glass and metals are non-perishable and stay for a long time, so we must not dump waste in the sea.
- Easy : Easy: Plastic is non-perishable.
- Medium : Medium: A flower rots in days; a bottle lasts years.
- Exam-type : Exam: Why not dump waste in the sea?
Deposits: continental, volcanic and organic, human-induced. Ooze ≥ 30%. Plastic stays long.
Thinking the sea can absorb any waste. Non-perishable waste harms marine life.
Memory clue: "खंड - ज्वाला/जीव - माणूस" = तीन निक्षेप.
Spot the difference
| Deposit | Examples |
|---|---|
| From continents | Pebbles, clay, soil |
| Volcanic and organic | Lava, ash, ooze |
| Human-induced | Sewage, plastics, waste |
Practice questions
- 1Name the three kinds of marine deposits.
- 2Where do river sediments settle mainly?
- 3What is marine ooze?
- 4Where is ooze found?
- 5Give two human-induced deposits.
- 6Which wastes are non-perishable?
- 7A 800 g sample has 40% organic remains. How many grams?
- 8How can students help keep coasts clean?
Show answers (try them yourself first)
- 1From continents, volcanic and organic, human-induced.
- 2On the continental shelf.
- 3Fine clay with 30% or more remains of sea organisms.
- 4In the deep sea away from the coast.
- 5Plastics and sewage.
- 6Plastics, glass and metals.
- 7800 × 40 ÷ 100 = 320 g.
- 8Join cleaning campaigns and not litter.
Quick revision
- 71% of the earth is water. Average ocean depth ≈ 3700 m. Mean sea level = 0. Everest +8848 m, Mariana −11034 m. Chennai = zero for survey in India.
- Shelf (up to 200 m, gentle, fishing, Mumbai High) → slope (200–3600 m, steep, boundary of continents) → abyssal plain → deeps and trenches.
- Marine islands = peaks of submerged mountains (Iceland, Andaman and Nicobar). Plateau = flat summit (Chagos). Mid-Atlantic Ridge ≈ 65,000 km.
- Mariana Trench (Pacific) is the deepest. Ridges and trenches have volcanoes, earthquakes and tsunamis.
- Deposits: from rivers and glaciers, volcanic and organic (ooze ≈ 30%), human-induced.