Water (Oceans) — Practice

Question 1

Why is the salinity of the Baltic Sea much lower (~7‰) than the average ocean (~35‰)?

Accepted Answer

The Baltic Sea has low salinity for two main reasons: (1) HIGH FRESHWATER INPUT — several major rivers (Rhine, Elbe, Vistula, and others from northern Europe) drain into it, adding large amounts of fresh water. (2) LOW EVAPORATION — the Baltic is at high latitudes (cold climate) where evaporation is minimal. Since evaporation is the main mechanism that concentrates salts, low evaporation means salts remain diluted. Limited exchange with the open Atlantic (via the narrow Danish Straits) prevents salty Atlantic water from replenishing it. The result: ~7‰ vs 35‰ average.

Question 2

Describe the major relief features of the ocean floor from the coast to the deepest point.

Accepted Answer

Moving from the coast to the deep ocean, the ocean floor passes through four distinct zones: (1) CONTINENTAL SHELF: gently sloping submarine platform at 0-200 m depth; an extension of the continent. Covers ~8% of ocean floor; important for fisheries, oil, and gas. Width varies — narrow where mountains meet the sea (India's western coast), wide where coastal plains extend. (2) CONTINENTAL SLOPE: steep drop from the shelf edge to the deep ocean floor at 200-3,000 m. Sediment moves down this slope through turbidity currents. (3) DEEP SEA PLAIN (ABYSSAL PLAIN): vast, almost flat expanse at 3,000-6,000 m; covered in fine sediment (ooze); covers ~80% of the ocean floor. Other features here include mid-ocean ridges (underwater mountain chains where new crust is created), seamounts (isolated volcanoes), and guyots (flat-topped seamounts). (4) OCEANIC TRENCHES: deepest parts, 6,000-11,000+ m; narrow, elongated depressions at convergent plate boundaries. The deepest is Challenger Deep in the Mariana Trench (~11,022 m) in the western Pacific.

Question 3

Explain the vertical distribution of temperature in ocean water and discuss the factors that affect the horizontal distribution of salinity.

Accepted Answer

VERTICAL TEMPERATURE DISTRIBUTION: Ocean water is thermally stratified into three layers: (1) Surface Mixed Layer (0-100 m): warm, uniformly mixed by winds and waves. Temperature ~20-25°C in tropics, variable in mid-latitudes. (2) Thermocline (100-1,000 m): rapid temperature decrease with depth. In tropical/subtropical regions, this layer is permanent and steep. In mid-latitudes, it is seasonal (stronger in summer). In polar regions, the thermocline is ABSENT — temperature is cold from surface to bottom (dense cold water has no buoyancy barrier). (3) Deep Layer (below 1,000 m): nearly uniform near-freezing temperature (2-4°C) everywhere on Earth. The cold deep water originates from polar regions where surface water cools, becomes dense, and sinks to the bottom. HORIZONTAL SALINITY DISTRIBUTION (factors): (1) EVAPORATION: increases salinity — highest in subtropical high-pressure zones (~20-30°) where evaporation exceeds precipitation. Red Sea (~41‰) in a subtropical desert zone is very salty. (2) PRECIPITATION: decreases salinity by adding fresh water — equatorial zone has heavy rainfall that dilutes salinity despite high evaporation. (3) RIVER INFLOW: major rivers (Amazon, Ganga, Congo) discharge fresh water near their deltas, locally reducing salinity. (4) OCEAN CURRENTS: transport water of different salinities — warm, salty subtropical water flows poleward; cold, less salty polar water flows equatorward. (5) ICE FORMATION AND MELTING: sea ice formation REJECTS brine, increasing surrounding salinity. Melting adds fresh water, decreasing salinity. Combined effect: maximum salinity at subtropical latitudes (~35-37‰); minimum at equator (~34‰) and polar regions; extreme values in enclosed seas (Dead Sea ~340‰, Red Sea ~41‰, Baltic ~7‰).

Practice — Water (Oceans)

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