Geography: Class 112 — Chapter 12: Water (Oceans)

About 91 per cent of the planetary water is found in the oceans. The remaining is held as freshwater in glaciers, icecaps, groundwater sources, lakes, soil moisture, atmosphere, streams, and within life. Nearly 59 per cent of the water that falls on land returns to the atmosphere through evaporation from over the oceans and other places.

• Continental Shelf: The extended margin of each continent occupied by relatively shallow seas and gulfs. It is the shallowest part of the ocean with an average gradient of 1° or less, ending at the shelf break.
• Continental Slope: Connects the continental shelf and the ocean basins, beginning where the shelf bottom sharply drops. The gradient varies between 2° and 5°, and the depth varies between 200 and 3,000 m.
• Deep Sea Plain: Gently sloping, extremely flat, and smooth regions of the ocean basins, with depths varying between 3,000 and 6,000 m. Covered with fine-grained sediments like clay and silt.
• Oceanic Deeps or Trenches: The deepest parts of the oceans, relatively steep-sided and narrow. They are 3-5 km deeper than the surrounding ocean floor and are associated with active volcanoes and strong earthquakes.

• Mid-Oceanic Ridges: Two chains of mountains separated by a large depression, forming the world's longest mountain ranges under water.
• Seamount: A mountain with pointed summits, rising from the seafloor but not reaching the ocean surface. They are volcanic in origin.
• Submarine Canyons: Deep valleys cutting across continental shelves and slopes, often extending from the mouths of large rivers.
• Guyots: Flat-topped seamounts showing evidence of gradual subsidence.
• Atoll: Low islands found in tropical oceans consisting of coral reefs surrounding a central lagoon.

1. Latitude: Surface water temperature decreases from the equator towards the poles as insolation decreases poleward.
2. Unequal distribution of land and water: Oceans in the Northern Hemisphere receive more heat due to greater contact with land compared to the Southern Hemisphere.
3. Prevailing wind: Offshore winds drive warm surface water away from the coast, causing upwelling of cold water from below. Onshore winds pile up warm water near the coast.
4. Ocean currents: Warm currents raise temperatures in cold areas, while cold currents lower temperatures in warm areas.

Temperature decreases with increasing depth. The boundary region where there is a rapid decrease of temperature is called the Thermocline. Below this zone, which contains about 90% of the total volume of water, temperatures approach 0°C.

The temperature structure in low and middle latitudes forms a three-layer system:

• First Layer: Top layer of warm water, about 500 m thick, with temperatures between 20°C and 25°C.
• Second Layer (Thermocline): Characterized by a rapid decrease in temperature with depth, about 500 - 1,000 m thick.
• Third Layer: Extremely cold layer extending to the deep ocean floor.

• Evaporation and precipitation.
• Freshwater flow from rivers and freezing/thawing of ice in polar regions.
• Wind, by transferring water to other areas.
• Ocean currents, which contribute to salinity variations.

The normal salinity of open oceans ranges between $33^o/_{oo}$ and $37^o/_{oo}$. Land-locked seas like the Red Sea have salinity as high as $41^o/_{oo}$, while estuaries and the Arctic fluctuate between $0 - 35^o/_{oo}$.

Vertically, salinity changes with depth. The distinct zone where salinity increases sharply is called the Halocline. High salinity water is denser and generally sinks below lower salinity water, leading to stratification.