Drainage System

The Hydrological Cycle is the continuous, solar-powered circulation of water among the hydrosphere, atmosphere, and lithosphere through evaporation, transpiration, condensation, precipitation, and runoff. Oceans hold 97.25% of all water on Earth!

The 4 Major Divisions of the Ocean Floor (Bathymetry):

1. 1. Continental Shelf:
   • The shallow, gently sloping seaward extension of the continental landmass under seawater (average gradient $1^\circ$ or less; depth up to 200 meters).
   • Width varies dramatically: virtually absent along active Pacific margins (Chile, $<5 \text{ km}$), but extraordinarily broad along the Siberian Arctic shelf (up to 1,500 km wide!).
   • Economic Primacy: Sunlit photic zone enables abundant phytoplankton growth, creating the world's richest commercial fishing grounds (Grand Banks of Newfoundland, Dogger Bank). Shelves contain massive deposits of marine sedimentary offshore petroleum and natural gas (e.g., Mumbai High in India, North Sea, Persian Gulf).
2. 2. Continental Slope:
   • Connects the outer edge of the continental shelf to the deep ocean floor. Steep gradient ($2^\circ - 5^\circ$ or more); depth ranges from 200 m to 3,000 m.
   • Marked by deep, V-shaped transverse valleys called Submarine Canyons carved by high-density underwater turbidity currents (e.g., Hudson Canyon).
3. 3. Continental Rise: Gently sloping apron of accumulated sediment at the foot of the slope (gradient $0.5^\circ - 1^\circ$).
4. 4. Deep Sea Abyssal Plain: Extremely flat, extensive, sediment-blanketed ocean floors at depths of 3,000 to 6,000 meters, covering over 50% of the Earth's total surface area.

• Mid-Oceanic Ridges: An interconnected underwater mountain chain spanning over 70,000 km across global oceans, formed by divergent plate tectonics where basaltic magma creates new oceanic crust (e.g., Mid-Atlantic Ridge).
• Ocean Deeps / Trenches: Narrow, steep-sided, V-shaped chasms plunging 3 to 5 km deeper than the surrounding abyssal plain, formed along subduction zones:
  • Challenger Deep in the Mariana Trench: The deepest point on Earth, plunging to an astonishing depth of 11,022 meters (over 11 km) in the Pacific Ocean! (Mount Everest could be dropped inside and still have 2 km of water above its peak!).
• Seamounts & Guyots:
  • Seamount: An isolated submerged volcanic peak rising steeply from the sea floor that does not reach the ocean surface (e.g., Emperor Seamounts).
  • Guyot: A flat-topped, submerged extinct volcanic mountain whose peak was eroded flat by ancient surface wave action before sinking (subsiding) beneath the sea.
• Atolls: Ring-shaped coral reefs encircling a central shallow lagoon without an island in the middle.

A. Factors Controlling Ocean Water Temperature:

• Latitude: Surface temperature decreases systematically from an average of $27^\circ\text{C}$ near the Equator to freezing $0^\circ\text{C}$ at the Poles.
• Land and Sea Contrast: Oceans in the Northern Hemisphere receive more heat and have higher average temperatures ($19^\circ\text{C}$) than oceans in the Southern Hemisphere ($16^\circ\text{C}$) due to contact with vast landmasses.
• Prevailing Winds: Offshore winds push warm surface water away from coasts, causing cold deep water to well up (Upwelling). Onshore winds pile up warm surface water along coasts.
• Ocean Currents: Warm currents (Gulf Stream) raise coastal temperatures; cold currents (California, Canaries, Labrador) lower coastal temperatures.

B. Vertical Temperature Stratification & The Thermocline:

Ocean temperature decreases with depth across three distinct vertical thermal layers:

1. Layer 1 (Epipelagic / Surface Layer, 0 to 500 m): Warm surface layer ($20^\circ - 25^\circ\text{C}$) mixed by wave turbulence and winds.
2. Layer 2 (The Thermocline, 500 to 1,000 m): A critical boundary layer characterized by a rapid, dramatic plunge in temperature with increasing depth! (Water temperature drops precipitously from $20^\circ\text{C}$ down to $4^\circ\text{C}$).
3. Layer 3 (Deep Cold Layer, beyond 1,000 m): Extremely cold water hovering near freezing ($1^\circ - 3^\circ\text{C}$), extending down to the ocean floor.

Ocean Salinity: The total amount of solid dissolved mineral salts present in 1,000 grams (1 kilogram) of seawater, expressed in parts per thousand (ppt or ‰):

• Average global oceanic salinity is 35‰ (35 parts per thousand).
• Dominant Dissolved Salts: Sodium Chloride ($NaCl$, 77.8%), Magnesium Chloride ($MgCl_2$, 10.9%), Magnesium Sulfate ($MgSO_4$, 4.7%), Calcium Sulfate ($CaSO_4$, 3.6%).

Factors Controlling Salinity:

1. Rate of Evaporation: High evaporation in hot, cloudless subtropical high-pressure belts concentrates salt, increasing salinity.
2. Inflow of Freshwater: Heavy river discharge (Ganga, Amazon) or melting glacial ice dilutes salinity. (The Bay of Bengal has lower salinity ~30‰ due to colossal river runoff from the Ganga and Brahmaputra, while the Arabian Sea has higher salinity ~36‰ due to high evaporation and few rivers!).
3. Precipitation: Heavy equatorial rainfall dilutes salinity near the equator.

Highest Salinity Water Bodies on Earth: