Ocean Basins and Ocean Water Properties

  • Ocean Origins:

    • Earth's water primarily comes from two sources:
    • Outgassing from Earth's interior through volcanic activity.
    • Comet or asteroid impacts bringing volatiles, including water, from space.

  • Ocean Basins:

    • Formed in depressions made of basalt, which is less dense than continental crust made of granite.
    • Oceanic crust is generally 4-7 km thick; continental crust averages 20-40 km.
    • Oceans cover approximately 70% of the Earth's surface and have an average depth of almost 4 km.
    • The deepest point is the Challenger Deep in the Mariana Trench, approximately 11 km deep.

  • Seafloor Structure:

    • Studied through direct sampling (dredging) and remote sensing (sonar).
    • Mid-oceanic ridges are major features, forming a 80,000 km long chain, elevated 2-3 km above abyssal plains.

  • Sea-Floor Spreading:

    • Influences sea levels; slower spreading leads to lower sea levels and vice versa.
    • Hydrothermal vents, known as "black smokers," form when heated water dissolves minerals and then precipitates them upon exit.

  • Underwater Ecosystems:

    • Black smokers host unique ecosystems based on chemosynthesis.
    • Organisms such as red tubeworms depend on bacteria that derive energy from chemicals.

  • Plate Boundaries:

    • Convergent boundaries create deep trenches; subduction zones associated with volcanic island arcs (e.g., along the Pacific Ocean).
    • Accreted terranes occur when island arcs are added to continental landmasses instead of being subducted.

  • Island Formation:

    • Islands may form over "hot spots," such as the Hawaiian Islands, which are volcanic in nature and age as they move away from the hotspot gaining distance.
    • Seamounts are underwater volcanic mountains; guyots are flat-topped seamounts due to erosion.
    • Atolls form when islands subside and reefs continue to grow.

  • Ocean Floor Layers:

    • Layer 1: Sediment accumulation over time.
    • Layer 2: Pillow basalts formed from lava cooling in contact with seawater.
    • Layer 3: Basalt dikes and gabbro formed from slower cooling magma.

  • Continental Margins:

    • Active margins coincide with tectonic plate boundaries, while passive margins are further away.
    • Can include features such as canyons from turbidity currents and carbonate platforms in warm shallow seas.

  • Ocean Water Properties:

    • Five major oceans: Pacific, Atlantic, Indian, Arctic, and Antarctic.
    • Ocean water salinity averages 3.5% (35‰), primarily composed of sodium and chloride ions.
    • Surface salinity and temperature vary based on latitude; thermocline (rapid temperature drop), halocline (salinity change), and pycnocline (density change) are important layers in the ocean.