Covering 71% of Earth’s surface, oceans dominate the globe with significant variations in size, depth, and characteristics. The Pacific Ocean is the largest, encompassing 35.4% of oceanic area, followed by the Atlantic (18.4%), Indian (14.5%), and Arctic (3%). The Northern Hemisphere is termed the continental hemisphere due to its 40% land coverage, whereas the Southern Hemisphere is known as the oceanic hemisphere, with oceans covering 81%. The oceans’ average depth is 4.3 km, with the Pacific’s Mariana Trench being the deepest point at 11.033 km. Ocean water contains numerous constituents, with chloride being the most abundant. The relief features, temperature, salinity, currents, and various formations like coral reefs play a vital role in the oceanic ecosystem.
Learning Outcomes:
Understand the global distribution of oceans.
Identify oceanic relief features and their characteristics.
Examine the factors affecting ocean temperature and salinity.
Analyze the role of ocean currents in climate and ecosystems.
Comprehend the significance of coral reefs and their global distribution.
Relief Features of the Ocean
Ocean relief features are identified using tools like SONAR and eco-sounders. They vary based on the slope from the continental edges to the deep oceanic basins.
Continental Shelf
Shallow, submerged areas at continental margins, averaging 150-200 m in depth and a slope of 1°-3°:
Shelf Break: Marks a rapid increase in slope toward deeper waters.
Composed of continental crust with materials like granite.
Shelves are wider along low-lying coasts (e.g., Arctic Siberia) and narrower near mountains (e.g., Western North America).
Rich in plankton, making them some of the world’s richest fishing grounds (e.g., Grand Bank).
Continental Slope
Steeper zones extending to abyssal plains, with slopes varying from 2° to 5°:
Depth ranges from 200 m to 2000 m.
Characterized by submarine canyons and mounds.
Andesite Line: The boundary between the continental shelf and slope.
Continental Rise
A sudden elevation forming a V-shaped notch, with the trench at the bottom of the ‘V.’
Ocean Basin
Comprising the abyssal floor, tectonic rises, and seamounts:
Abyssal Plains: Flat, smooth surfaces.
Seamounts & Guyots: Volcanic features; guyots are flat-topped, submerged platforms.
Ocean Deeps or Trenches
Deepest parts, narrow and steep-sided:
Associated with volcanoes and earthquakes.
Major Trenches: Mariana Trench (Pacific), Puerto Rico (Atlantic), Java (Indian).
Submarine Canyons
Deep valleys cutting across continental shelves, often extending from river mouths (e.g., Hudson Canyon).
Temperature of Oceans
Ocean water temperature significantly affects sedimentation and marine life. The global mean temperature is 17.4°C:
Varies regionally: Pacific (19.1°C), Indian (17.0°C), Atlantic (16.9°C).
Warm Currents: Transport heat to higher latitudes; Cold Currents: Lower temperatures at lower latitudes.
Insolation directly influences temperature. Salinity affects the temperature-absorbing capacity.
Freezing Point: Ocean water freezes at –1.872°C; saltier water freezes at even lower temperatures.
Thermohaline Circulation: Driven by differences in temperature and salinity, resulting in vertical water movement.
Horizontal Temperature Distribution
Average surface temperature: 26.7%.
Decreases with latitude, typically by 0.5°F per latitude.
Higher temperatures recorded in the Northern Hemisphere.
Vertical Temperature Distribution
Maximum temperature at the surface, decreasing with depth.
Rapid fall up to 200 m; slows beyond.
At 4000 m, temperatures hover around 1.6°C.
Salinity of Oceans
Salinity is the amount of dissolved solids in seawater, affecting waves, currents, temperature, and marine organisms:
Average Salinity: 35.2%.
Factors Influencing Salinity: Evaporation, precipitation, river inflow, and oceanic circulations.
Higher salinity increases water density and affects freezing and evaporation rates.
Salinometer: Measures salinity; Isohaline: Lines on maps indicating equal salinity.
Salinity Distribution: Generally decreases from the equator to the poles. Highest at subtropical high-pressure belts due to evaporation.
World’s Highest Saline Bodies
Lake
Salinity
Lake Van (Turkey)
33.8%
Dead Sea
33.7%
Great Salt Lake
32.0%
Important Note: Precipitation reduces salinity, hence lower salinity in equatorial waters compared to tropical regions.
Ocean Currents
Ocean currents are defined directional movements of water influenced by temperature, wind, Earth’s rotation, and salinity differences:
Rotation: Causes water bulging at the equator, generating gradients that initiate cyclic movements.
Pressure Zones: High pressure lowers sea level, creating upwelling of cold currents.
Salinity: Movement occurs from areas of lower to higher salinity at the surface.
Types of Currents: Warm (equatorial origin) and Cold (polar origin).
Drifts: Movements with lower velocity than currents, influenced by surface winds.
Effects of Ocean Currents
Distribute heat, influencing weather and climate.
Affect monsoon patterns (e.g., Indian subcontinent).
Support marine life by transporting nutrients and oxygen.
Impact human activities like fishing and navigation.
Important Note: Desertification on continents’ western sides often results from cold currents inhibiting evaporation.
Ocean Tides
Tides are the regular rise and fall of sea levels due to gravitational forces of the Moon and Sun:
Two main tidal bulges occur: One due to the Moon’s attraction, the other due to centrifugal forces.
Tides are stronger on wide continental shelves and in bays.
Types of Tides
Spring Tides: Occur during New and Full Moon, when the Sun, Moon, and Earth align, creating the highest tides.
Neap Tides: Occur during the first and third quarters of the month when the Sun and Moon are at right angles, resulting in lower tides.
Periodic Tides: Vary with the Moon’s proximity (apogean and perigean) and Earth’s position (aphelion and perihelion).
Coral Reefs
Coral reefs are biodiversity-rich ecosystems formed by the accumulation of coral polyps‘ skeletons:
Symbiotic Association: Coral polyps and algae, with coral secreting a calcium carbonate skeleton.
Sensitive to temperature changes, serving as environmental indicators.
Coral Bleaching
Loss of algae leads to whitening and potential death of corals.
Increasing due to global warming.
Types of Coral Reefs
Fringing Reefs: Narrow, close to the shore, separated by a lagoon (e.g., Gulf of Mannar).
Barrier Reefs: Wide platforms with a deep lagoon (e.g., Great Barrier Reef).
Atolls: Circular reefs surrounding a lagoon (e.g., Fiji Atoll).
Distribution and Importance
Cover approximately 284,300 km², with 91.9% in the Indo-Pacific.
Provide food, shelter, coastal protection, and medicinal resources.
Important Note: Coral reefs are the foundation of marine ecosystems and are critical for human life sustainability.
MCQ with Answer
Which of the following factors primarily influences ocean currents?