The hydrological cycle
The hydrological cycle is the non-stop movement of water on, above and below the Earth. The total water stays about the same; it only changes place and form (liquid, ice, vapour).
- Evaporation from oceans and lakes, and transpiration from plants, send vapour into the air.
- Condensation forms clouds.
- Precipitation brings water down as rain or snow.
- Runoff (rivers), infiltration and groundwater flow carry it back to the sea.
About 97% of the Earth's water is in the oceans; about 2% is ice sheets and glaciers; less than 1% is fresh water in rivers, lakes, soil and underground. Using water carefully matters because usable fresh water is so small.
Relief of the ocean floor
Oceans are divided by the continents into the Pacific, Atlantic, Indian, Southern and Arctic Oceans. Their floor has four major divisions:
- Continental shelf: the gently sloping, shallow edge of a continent under the sea, usually down to about 200 m. Width varies from almost nothing to over 100 km. It is rich in fish, and sediments here form oil and gas deposits.
- Continental slope: the steep drop from the shelf edge to depths of 2–3 km. Deep valleys called canyons cut into it.
- Deep sea (abyssal) plain: the wide, very flat floor at 3–6 km, covered with fine mud.
- Oceanic deeps or trenches: the deepest, long and narrow parts, 3–5 km deeper than the floor around them, near plate edges with volcanoes and earthquakes. The Mariana Trench in the Pacific is about 11 km deep.
Minor relief features
- Mid-ocean ridges: long chains of underwater mountains where plates move apart; the peaks can rise above the sea as islands (like Iceland).
- Seamount: a lone underwater volcano with a pointed top that does not reach the surface.
- Guyot: a flat-topped seamount (its top was cut down by waves long ago).
- Submarine canyons: deep valleys cut into the shelf and slope, often at river mouths.
- Atoll: a ring-shaped coral island around a lagoon in warm seas (Lakshadweep has atolls).
- Banks and shoals: raised, shallow parts of the shelf — good fishing grounds.
Temperature of ocean water
The sea is heated by the Sun, mostly at the surface. Water heats and cools slowly, so the sea's temperature changes less than land's.
Factors
- Latitude: surface temperature falls from the equator to the poles.
- Unequal land and water: northern oceans touch more land and are a little warmer than southern oceans.
- Prevailing winds: offshore winds push warm surface water away, letting cold water rise (upwelling); onshore winds pile up warm water.
- Ocean currents: warm currents raise and cold currents lower the temperature.
Vertical distribution (three layers)
- Top layer (~0–500 m): warm, 20–25 °C in the tropics; well mixed.
- Thermocline (~500–1,000 m): temperature falls fast with depth.
- Deep layer: very cold, near 0–4 °C, down to the floor.
Near the poles the surface is already cold, so there is only one cold layer. The average surface temperature of the oceans is about 27 °C near the equator and falls about 0.5 °C for each degree of latitude away from it.
Horizontal distribution
The highest surface temperatures are in the tropics, not exactly at the equator; enclosed seas in hot regions, like the Red Sea and the Persian Gulf, are warmer than the open ocean.
Salinity of ocean water
Salinity is the total amount of dissolved salts in sea water, written as parts per thousand (‰): grams of salt in 1,000 grams of water. The average is about 35‰. Common salt (sodium chloride) is the largest part. The line joining places of equal salinity is an isohaline.
Factors affecting salinity
- Evaporation: more evaporation → higher salinity (hot, dry areas).
- Fresh water: rain, river water and melting ice → lower salinity.
- Wind moves water and mixes it.
- Ocean currents carry saltier or fresher water to other places.
Distribution
- Open oceans: 33–37‰. Highest in the subtropics (~20°–30°: hot, dry, strong evaporation); lower near the equator (heavy rain) and near the poles (melting ice).
- Enclosed seas in hot dry lands are very salty: Red Sea ~41‰; the Dead Sea (a salt lake) ~238‰; Lake Van in Turkey ~330‰.
- Seas with many rivers or ice-melt are less salty: Baltic Sea and the Bay of Bengal (Ganga–Brahmaputra).
- Arabian Sea is saltier than the Bay of Bengal (more evaporation, less river water).
- With depth: salinity changes sharply in a layer called the halocline; salty water is denser and sinks.
Try it at home
Dissolve 35 g of salt (about 7 teaspoons) in 1 litre of water: that is average sea water. Pour it into a wide plate and keep it in the sun for a few days. As water evaporates, the salt left behind shows how evaporation raises salinity. Put an egg in plain water (it sinks) and in very salty water (it floats): salty water is denser.
Key formulas and definitions
- Water cycle: Evaporation → Condensation → Precipitation → Runoff/Infiltration → Sea
- Water share: oceans ~97%, ice ~2%, other fresh water < 1%
- Floor from coast: Shelf (to ~200 m) → Slope → Deep sea plain (3–6 km) → Trench (Mariana ~11 km)
- Ocean layers: warm top (~0–500 m) → thermocline (~500–1,000 m) → cold deep water (0–4 °C)
- Salinity (‰) = grams of salt per 1,000 g of sea water; average ≈ 35‰
- Salinity ↑ with evaporation; ↓ with rain, rivers, melting ice
Worked examples
1. A 2 kg sample of sea water leaves 70 g of salt when dried. Find its salinity.
2 kg = 2,000 g. Salinity = 70 ÷ 2,000 × 1,000 = 35‰.
2. How much salt is in 10 kg of Red Sea water at 41‰?
Salt = 41 × 10,000 ÷ 1,000 = 410 g.
3. Why is the Bay of Bengal less salty than the Arabian Sea?
Large rivers like the Ganga and Brahmaputra pour huge amounts of fresh water into the Bay, and it gets heavy rain. The Arabian Sea gets less fresh water and has more evaporation.
4. Why is salinity higher at 25° N than at the equator, even though the equator is hotter?
The equator gets rain almost every day, which adds fresh water. Near 25° the air is dry and clear (subtropical high), so evaporation is stronger and rain is low.
5. A seamount and a guyot both rise from the ocean floor. How do you tell them apart?
A seamount has a pointed top. A guyot has a flat top, because waves once cut it down when it was near the surface before it sank.
Common mistakes
- Thinking the deepest part of the ocean is in the middle. Trenches are usually near continents and island arcs, at plate edges.
- Saying salinity is highest at the equator. It is highest in the subtropics; the equator gets heavy rain.
- Writing salinity as a percent. It is per thousand (‰): 35‰ = 3.5%.
- Thinking deep ocean water is warm in the tropics. Below the thermocline it is cold everywhere.