Ocean Floor and Ocean Water
NCERT Class 11 - Fundamentals of Physical Geography, Chapters 13 and 14
Ocean Floor and Ocean Water
Oceans hold about 97 per cent of all the water on earth. Their floor is not a featureless plain but has mountains, plains and trenches larger than anything on land, and their water varies in temperature and salinity in ways that drive currents and fisheries.
The hydrological cycle
- Water moves in a closed cycle through evaporation, transpiration, condensation, precipitation, runoff and infiltration.
- Only about three per cent of all water is fresh, and most of that is locked in glaciers and ice caps; most of the remainder is groundwater, and only a very small share is in rivers and lakes.
- The cycle is driven by solar energy and gravity, and the total quantity of water on earth stays constant.
Major relief features of the ocean floor
| Feature | Character |
|---|---|
| Continental shelf | Gently sloping submerged edge of the continent, usually to about 200 metres depth |
| Continental slope | Steeper descent from the shelf to the deep ocean floor |
| Continental rise | Gentle apron of sediment where the slope meets the deep plain |
| Abyssal or deep sea plain | Vast level plain covered with fine sediment; the largest part of the ocean floor |
| Oceanic deeps or trenches | Long, narrow and extremely deep depressions, usually near land or island arcs |
- The continental shelf is the most valuable part of the ocean floor - it holds the world's richest fishing grounds because sunlight reaches the bottom and plankton grow abundantly, and it is the source of most offshore oil and gas.
- Trenches are the deepest parts of the ocean and are associated with subduction zones; the deepest known is in the Pacific Ocean.
- Minor features include the mid-oceanic ridge, underwater mountains called seamounts, flat-topped seamounts called guyots, submarine canyons cut into the shelf and slope, atolls which are ring shaped coral reefs, and banks and shoals which are good fishing grounds.
Temperature of ocean water
- The surface is heated by the sun, so temperature decreases with depth, but not evenly.
- The layer of rapid decrease, roughly between 100 and 1000 metres, is called the thermocline; below it the water is uniformly cold.
- The highest surface temperatures are found slightly north of the equator rather than exactly on it, because the northern hemisphere has more land.
- Temperature falls from the equator towards the poles, and enclosed seas in the tropics are warmer than open oceans at the same latitude.
- The diurnal range of ocean temperature is very small, because water heats and cools far more slowly than land.
Salinity
- Salinity is the total salt content of ocean water, expressed in parts per thousand; the average for the open ocean is about 35 parts per thousand.
- It increases where evaporation is high and decreases where there is heavy rainfall, large river inflow or melting ice.
- Salinity is therefore highest in the subtropics, lower at the equator because of heavy rain, and lower again near the poles because of melting ice.
- Enclosed seas show extremes - the Red Sea is far saltier than the open ocean because it is hot, enclosed and fed by no large river, while the Baltic Sea is much less salty because it receives heavy river inflow.
- The Dead Sea and similar inland lakes reach extraordinary salinity because water leaves only by evaporation.
- High salinity increases the density of water, and differences in density are one of the forces that drive deep ocean circulation.
हिंदी संकेत: महाद्वीपीय मग्न तट (continental shelf) सबसे उपयोगी भाग है - यहीं विश्व के सबसे अच्छे मत्स्य क्षेत्र और अपतटीय तेल-गैस मिलते हैं। यह सबसे अधिक पूछा जाने वाला बिंदु है।
Exam me kaise aata hai
Match-the-following on relief feature and its character is standard. Factual questions ask the average salinity, the depth limit of the continental shelf, and what a guyot is. Application questions ask why the Red Sea is saltier or why fishing grounds lie on the shelf.
UPSC / State PSC ke liye
Connect the shelf to economic geography - fisheries, offshore hydrocarbons and the exclusive economic zone all rest on it, which is why maritime boundary disputes concern shelves. On salinity, the rule to state is that evaporation raises it and fresh water inflow lowers it, and every regional example follows from that rule.
Yahan confuse hote hain
✗ Salinity is highest at the equator | ✓ It is highest in the subtropics; heavy rain lowers it at the equator
✗ The abyssal plain is the smallest part of the ocean floor | ✓ It is the largest part
✗ Ocean temperature falls evenly with depth | ✓ It falls rapidly in the thermocline and then remains nearly uniform
Ek nazar me
- Oceans hold about 97 per cent of the earth's water; only about three per cent is fresh.
- Ocean floor - continental shelf, slope, rise, abyssal plain and trenches.
- The shelf holds the richest fishing grounds and most offshore oil and gas.
- Minor features - mid-oceanic ridge, seamount, guyot, submarine canyon, atoll, bank and shoal.
- Temperature falls rapidly in the thermocline and then stays nearly uniform.
- Average open ocean salinity is about 35 parts per thousand.
- Salinity is highest in the subtropics; the Red Sea is very salty and the Baltic much less so.