The ocean floor and its relief
A fishing boat near the coast casts its net in shallow water, but far out the water suddenly turns deep. The sea floor has hills and valleys just like the land.
The landforms of the ocean floor are called ocean relief (submarine relief). Near the coast lies the continental shelf (shallow edge), then the continental slope (steep drop), then the abyssal plain (wide, flat, deep floor). The mid-ocean ridge is a long underwater mountain chain built by volcanic activity, and trenches are the deepest parts.
In the figure : The order from the coast to the deep sea.
Rule 1: Learn the order from the coast outwards
- Easy : The shelf is the nearest and shallowest part, and the slope comes after it.
- Medium : After the slope ends, the flat abyssal plain begins.
- Exam-type : The deepest trench is the Mariana Trench in the Pacific Ocean.
Rule 2: Relief affects what people can do at sea
- Easy : Sunlight reaches the shallow shelf, so it holds more fish.
- Medium : Oil and gas may lie under the shelf.
- Exam-type : A seamount is an underwater volcanic mountain that does not reach the surface; if its top is worn flat it is called a guyot.
The sea floor has mountains, plains and valleys too, and they shape fishing, oil and gas, and ocean exploration.
Mixing up the ridge and the trench; the ridge is a high mountain chain and the trench is the deepest part.
Memory clue: S-S-P-T — Shelf, Slope, Plain, Trench (from the coast inwards). Shallow shelf = fish.
Spot the difference
| Feature | Meaning | Key fact |
|---|---|---|
| Continental shelf | Shallow submerged edge of a continent | Fish, oil and gas |
| Mid-ocean ridge | Mountain chain built by volcanoes | Longest mountain range on Earth |
| Trench | Deepest part of the ocean | Mariana Trench, Pacific |
Practice questions
- 1What is the continental shelf?
- 2Which is the deepest trench?
- 3What is an abyssal plain like?
- 4Why does the continental shelf have more fish?
- 5What is the difference between a seamount and a guyot?
- 6How is a mid-ocean ridge formed?
- 7Arrange from the coast inwards: trench, shelf, plain, slope.
- 8A boat catches 40 fish per net on the shelf. How many fish will it get in 6 nets?
Show answers (try them yourself first)
- 1The shallow, submerged edge of a continent.
- 2The Mariana Trench in the Pacific Ocean.
- 3Wide, flat and deep.
- 4Sunlight reaches the shallow water and food is plentiful.
- 5A seamount has a pointed top; a guyot has a top worn flat.
- 6By lava piling up from volcanic activity under water.
- 7Shelf, slope, plain, trench.
- 840 × 6 = 240 fish.
Waves and tides
On a beach the waves keep rolling in, and twice a day the water climbs up and slips back. Waves come from wind, while tides come from the pull of the Moon and Sun.
A wave is the rise and fall of the water surface, mostly caused by wind. The distance between two neighbouring crests is the wavelength. High tide is when the water level is highest and low tide when it is lowest; in most places this happens twice a day.
In the figure : Waves and the two kinds of tides.
Rule 1: Waves shape and wear down the coast
- Easy : If two crests are 10 m apart, the wavelength is 10 m.
- Medium : A cliff wears into a crack, cave, arch, sea stack and finally a stump.
- Exam-type : A giant wave caused by an undersea earthquake is called a tsunami.
Rule 2: Tell spring tides from neap tides
- Easy : On full moon and new moon days the Sun, Earth and Moon line up, so a spring tide occurs.
- Medium : At half moon the pulls act in different directions, so the gap between high and low tide is smaller (neap tide).
- Exam-type : If high tide is at 4 m and low tide at 1 m, the tidal range is 4 − 1 = 3 m.
High tide lets big ships enter harbours, helps fishing, and tidal energy can make electricity.
Saying that tides are caused by wind; they are caused by the gravitational pull of the Moon and Sun.
Memory clue: Spring = Straight line (full and new moon), Neap = Now half moon.
Spot the difference
| Type | When | Result |
|---|---|---|
| Spring tide | Full and new moon | Very high highs, very low lows |
| Neap tide | At half moon | Smaller gap between high and low |
| Wave | Caused by wind | Erodes the coast and builds beaches |
Practice questions
- 1What is wavelength?
- 2What mainly causes waves?
- 3When does a spring tide occur?
- 4Why is high tide important for ships?
- 5Why is the tidal gap smaller at half moon?
- 6Name any three landforms made by waves.
- 7High tide is at 5 m and low tide at 2 m. What is the tidal range?
- 8One wave crest passes every 5 seconds. How many crests pass in 1 minute?
Show answers (try them yourself first)
- 1The distance between two neighbouring crests.
- 2Wind blowing across the sea.
- 3On full moon and new moon days.
- 4Rising water lets ships enter harbours easily.
- 5The pulls of the Sun and Moon act in different directions.
- 6Cave, arch, sea stack (or a beach).
- 75 − 2 = 3 m.
- 860 ÷ 5 = 12 crests.
Ocean currents
Like rivers, the sea has streams of water flowing in set directions. They help explain why one coast freezes and another does not.
An ocean current is a steady stream of water flowing in a set direction over the sea surface. Winds, the Earth's rotation (the Coriolis force), differences in temperature and salinity, and the shape of continents cause it. A current from a warm region is a warm current and one from a cold region is a cold current.
In the figure : Kinds of currents and why they matter.
Rule 1: Currents change the temperature of the coast
- Easy : A warm current keeps a coast warmer.
- Medium : The Gulf Stream (warm) keeps Norway's waters warm, so sea ice does not form easily there.
- Exam-type : The Labrador Current off Canada's east coast is cold, so icebergs and sea ice form there.
Rule 2: Fishing is best where warm and cold currents meet
- Easy : The water there is full of nutrients.
- Medium : Tiny organisms grow on the nutrients and fish feed on them.
- Exam-type : Moving water bends to the right in the Northern Hemisphere and to the left in the Southern Hemisphere (Coriolis force).
On maps, warm currents are drawn in red and cold currents in blue.
Thinking a warm current is warm everywhere; its type depends on the region it comes from.
Memory clue: Warm = Red, Cold = Blue; Meeting = more fish.
Spot the difference
| Current | Type | Effect |
|---|---|---|
| Gulf Stream | Warm | Keeps Norway's waters warm |
| Labrador Current | Cold | Icebergs and sea ice |
| Warm + cold meeting | Mixed | Rich fishing ground |
Practice questions
- 1What is an ocean current?
- 2Give any two causes of ocean currents.
- 3In which colour are warm currents shown on maps?
- 4Why does ice not form easily on Norway's coast?
- 5Why is fishing good where warm and cold currents meet?
- 6What is the Coriolis force?
- 7What effect does the Labrador Current have on Canada's coast?
- 8A current flows 24 km a day. How far does it go in 5 days?
Show answers (try them yourself first)
- 1A steady stream of water moving in a set direction over the sea surface.
- 2Winds and the Earth's rotation (or temperature and salinity differences).
- 3Red.
- 4The warm Gulf Stream keeps the water there warm.
- 5The water is rich in nutrients, so fish gather there.
- 6Due to Earth's rotation, moving things bend right in the Northern and left in the Southern Hemisphere.
- 7Its cold water helps sea ice and icebergs form.
- 824 × 5 = 120 km.
Ocean currents
In the app you can move this figure with your finger.
Open in the appCyclones, tsunamis and early warning systems
In a coastal village a phone alert arrives and families head for a safe shelter. This section shows where that alert comes from.
A cyclone is a large rotating storm that builds over warm ocean water. A tsunami is a giant wave, usually raised by an earthquake under the sea. An early warning system spots the danger in advance so people can be moved to safety in time.
In the figure : The four steps of a tsunami warning.
Rule 1: A warning moves through four steps
- Easy : First the earthquake is detected, then the wave is confirmed.
- Medium : Coastal tide gauges and radar track the wave.
- Exam-type : After the Indian Ocean tsunami of 26 December 2004, many countries built such systems; in India INCOIS runs the tsunami early warning centre.
Rule 2: Technology needs awareness too
- Easy : The GEMINI device gives fishermen satellite information on their mobile phones.
- Medium : Cyclones in this region get names from a fixed list (for example Biparjoy 2023, Remal 2024).
- Exam-type : People on Simeulue island used old folk stories to recognise tsunami signs and saved lives; traditional knowledge helps too.
A warning system must work all the time and people must understand what a warning means.
Thinking a tsunami is caused by wind; it is mostly caused by an earthquake under the sea.
Memory clue: D-C-T-W — Detect, Confirm, Track, Warn.
Spot the difference
| Hazard | Cause | Protection |
|---|---|---|
| Cyclone | Disturbance over warm sea | Shelters, early forecasts, evacuation |
| Tsunami | Undersea earthquake | Seismic stations, seafloor sensors, alerts |
| Fishermen's risk | High waves, strong winds | GEMINI device and mobile app |
Practice questions
- 1What is a cyclone?
- 2What causes a tsunami?
- 3Name the four steps of a tsunami warning.
- 4Who is the GEMINI device for and what does it do?
- 5Why is public awareness needed along with warnings?
- 6What does the Simeulue island example teach us?
- 7About how many lives were lost in the 2004 tsunami, according to the book?
- 8After a warning, 600 villagers must be shared equally among 3 shelters. How many go to each?
Show answers (try them yourself first)
- 1A big rotating storm that forms over warm ocean water.
- 2An earthquake under the sea.
- 3Detect, confirm, track, warn.
- 4For fishermen; it passes satellite information to their phones.
- 5People act correctly only if they understand the warning.
- 6Traditional stories and knowledge can save lives in a disaster.
- 7About 2,30,000.
- 8600 ÷ 3 = 200 people.
Marine resources and the Blue Economy
The fish in the market, goods arriving by ship, tourists on the beach and the conch in a puja all come from the sea.
Valuable things found in the sea are marine resources. Living resources are fish, sea plants and biodiversity; non-living resources are minerals, oil and gas, sand and gravel, and tidal and wave energy. The Blue Economy means using ocean resources so that jobs and income grow while the sea stays healthy.
In the figure : What the sea gives us.
Rule 1: Sort a resource as living or non-living
- Easy : A pomfret fish is a living resource.
- Medium : Crude oil under the sea is a non-living resource.
- Exam-type : Fish living over the shelf (0–200 m depth) are coastal fish, and those beyond are oceanic fish.
Rule 2: Use must be sustainable
- Easy : Catching too many fish reduces the fish stock.
- Medium : Cutting pollution and protecting marine ecosystems are goals of the Blue Economy.
- Exam-type : The Blue Economy links to UN Sustainable Development Goal 14, 'Life Below Water'; about 90 percent of world trade by volume goes by sea.
The sea gives food, energy, jobs and culture, but it must be used responsibly.
Thinking the Blue Economy just means catching as many fish as possible; it aims at growth while keeping the sea healthy.
Memory clue: Living = lives (fish), Non-living = lies below (oil, minerals).
Spot the difference
| Resource | Type | Use |
|---|---|---|
| Fish | Living | Food and jobs |
| Oil and gas | Non-living | Fuel |
| Tidal and wave energy | Non-living (renewable) | Making electricity |
Practice questions
- 1What are marine resources?
- 2Give one example of a living resource.
- 3Give one example of a non-living resource.
- 4What is the Blue Economy?
- 5Why is overfishing harmful?
- 6What is Sustainable Development Goal 14 about?
- 7State the cultural importance of the conch and cowrie shells.
- 8A port receives 30 ships every month. How many ships arrive in 12 months?
Show answers (try them yourself first)
- 1Materials and features of the sea that have economic or natural value.
- 2Fish (or coral, sea plants).
- 3Crude oil (or gas, sand).
- 4Using ocean resources for growth while keeping the sea healthy.
- 5Fish numbers fall and fishers' livelihoods are threatened.
- 6'Life Below Water' — reducing marine pollution and protecting ecosystems.
- 7Sacred symbols of prosperity linked to the Samudra Manthana story; cowries were once used as money.
- 830 × 12 = 360 ships.
Maritime zones and the rules of the sea
Roads have traffic rules, and so does the sea. Maritime zones answer who controls which part of the sea and where fishing is allowed.
Sea zones are measured from a country's baseline: the territorial sea to 12 nautical miles, the contiguous zone to 24 nautical miles and the Exclusive Economic Zone (EEZ) to 200 nautical miles. Beyond lies the high seas, which belong to no single country. These rules are set by the international treaty UNCLOS.
In the figure : Maritime zones growing outwards from the baseline.
Rule 1: A country's rights differ in each zone
- Easy : In the territorial sea a country has almost full control.
- Medium : A foreign ship may pass peacefully without stopping; this is called innocent passage.
- Exam-type : In the EEZ a country has special rights over fish, oil and gas, but does not fully own the water.
Rule 2: Convert nautical miles into kilometres
- Easy : 1 nautical mile = 1852 m, that is about 1.85 km.
- Medium : 12 nautical miles = 12 × 1.852 = about 22.2 km.
- Exam-type : 200 nautical miles = 200 × 1.852 = about 370.4 km.
On the high seas a ship follows the laws of its flag state, the country where it is registered.
Saying the EEZ is sea fully owned by the country; the country only has special rights over resources there.
Memory clue: 12 – 24 – 200 miles, in that order. Ship rules: COLREGs = avoid collisions, MARPOL = avoid pollution.
Spot the difference
| Zone | Distance from baseline | Country's rights |
|---|---|---|
| Territorial sea | 12 nautical miles | Almost full control |
| Exclusive Economic Zone | 200 nautical miles | Special rights over resources |
| High seas | Beyond all EEZs | Equal rights for all countries |
Practice questions
- 1What is a baseline?
- 2How many nautical miles does the territorial sea extend?
- 3What is innocent passage?
- 4What rights does a country have in its EEZ?
- 5Who owns the high seas?
- 6How do COLREGs and MARPOL differ?
- 7How many km is 24 nautical miles? (1 nautical mile = 1.852 km)
- 8A ship sails at 15 nautical miles per hour for 4 hours. How many nautical miles does it cover?
Show answers (try them yourself first)
- 1The coastline-based line from which the distances of sea zones are measured.
- 212 nautical miles.
- 3A foreign ship passing peacefully through a country's waters without stopping.
- 4Special rights over resources such as fish, oil and gas.
- 5No single country; it is part of the global commons.
- 6COLREGs set rules to avoid collisions; MARPOL prevents pollution from ships.
- 724 × 1.852 = about 44.4 km.
- 815 × 4 = 60 nautical miles.