lecture 5

Overview of Oceanic Statistics

• Earth as an Ocean World:

  • Age: ~$4$ billion years

  • Total Area: $331,441,932$ km² ($127,970,445$ miles²)

  • Volume: $1,303,155,354$ km³ ($312,643,596$ miles³)

  • Average Temperature: $3.9$ °C ($39$ °F)

  • Average Salinity: $34.482$ g/kg ($0.56$ ounce/lb)

  • Average Depth: $3,682$ m ($12,081$ ft)

  • Greatest Depth: $10,994$ m ($36,070$ ft) in the Challenger Deep ($Mariana Trench$)

  • Average Land Elevation: $840$ m ($2,772$ ft)

• Contextual Facts:

  • Hypsographic Curve: A graph that shows the proportion of Earth's surface at various elevations/depths, highlighting that land is significantly higher on average than the sea floor is deep.

  • The ocean contains $97\%$ of Earth's water, with most of the remaining $3\%$ locked in ice caps and glaciers.

  • Division of Oceans: Earth is divided into five major basins:

    • Pacific: The largest and deepest, shrinking due to subduction.

    • Atlantic: Growing due to seafloor spreading at the Mid-Atlantic Ridge.

    • Indian: Primarily in the Southern Hemisphere.

    • Southern: Surrounds Antarctica, defined by the Antarctic Circumpolar Current.

    • Arctic: The smallest and shallowest.

Continental Margins and Seafloor Features

• Types of Margins:

  • Passive Margins: Found along most Atlantic coasts; they are geologically quiet, lacking plate boundaries, and characterized by wide continental shelves.

  • Active Margins: Found primarily in the Pacific; associated with plate boundaries (subduction or transform), characterized by narrow shelves, steep slopes, and offshore trenches.

• Classification of Features:

  • Continental Shelf: The gently sloping submerged extension of the continent. It varies in width from almost non-existent to over $1,500$ km. This area is biologically rich due to sunlight penetration.

  • Shelf Break: The abrupt transition where the gentle slope of the shelf becomes the steep continental slope.

  • Continental Slope: The steep gradient ($1$ to $25$ degrees) that leads to the deep-ocean floor.

  • Continental Rise: At the base of the slope, a thick wedge of sediment deposited by turbidity currents. Found primarily at passive margins.

  • Abyssal Plain: The flattest places on Earth, covered by thick layers of fine-grained pelagic sediment (ooze and clay) that mask the rugged volcanic crust beneath.

  • Seamounts and Guyots: Volcanic peaks. Seamounts are pointed; Guyots (tablemunts) are flat-topped due to wave erosion before they subsided below sea level.

  • Mid-Ocean Ridge: A continuous underwater mountain range over $65,000$ km long. It features a central Rift Valley where magma rises to create new crust.

  • Hydrothermal Vents: Located near ridges, where mineral-rich, superheated water supports ecosystems via chemosynthesis (energy from chemicals rather than light).

Mapping and Studying the Seafloor

Bathymetry Techniques

• Echo Sounding (Sonar):

  • Single-beam: Emits a single pulse to measure depth directly beneath the ship.

  • Multibeam Echo Sounders: Use a fan of sound beams to map a wide swath (up to $10$-$20$ km) of seafloor in great detail.

  • Formula for Depth Calculation:

    $Depth = V \times T \times \frac{1}{2}$

    where $V$ is the speed of sound in seawater (approx. $1,500$ m/s) and $T$ is the round-trip travel time.

• Satellite Altimetry: Uses radar to measure the height of the sea surface. Because gravity pulls water toward massive seafloor features (like mountains), the sea surface mimics the topography of the ocean floor.

Importance of Studying Ocean Basins

• Ocean Circulation: The shape of the basins directs the "Great Ocean Conveyor Belt" (Thermohaline Circulation), which redistributes heat from the equator to the poles.

• Marine Sediments: Often called "The Memory of the Ocean," these sediments contain microfossils ($Foraminifera$) that reveal past Earth temperatures and atmospheric compositions.

• Tectonic Insights: The discovery of the Mid-Atlantic Ridge and magnetic "stripes" on the seafloor provided the definitive evidence for the theory of Plate Tectonics and Seafloor Spreading.