The Sea Floor

Geography of the Ocean Basins

  • The world ocean is the dominant feature

  • 61% of the total area of the Northern Hemisphere is ocean

    • About 80% of the Southern Hemisphere.

  • The world ocean is divided into four large basins: Pacific, Atlantic, Indian, and Arctic.



Structure of the Earth

  • Originated 4.5 billion years ago from the Big Bang.

  • Early Earth was likely molten.

    • Due to heat associated with the Big Bang

  • Materials settled by density as they cooled.

  • Heavier materials settled deep.

  • Lighter components formed a thin crust.

  • Over time, oceans and atmosphere began to form.

  • The location of the Earth relative to the Sun allows water to stay liquid.

    • An essential substance for sustaining life



Internal Structure of the Earth

  • Core: innermost layer; solid inner core and liquid outer core; iron-rich.

  • Mantle: middle layer; semi-plastic composition.

  • Crust: outermost layer; thinnest portion of the Earth.



Continental vs. Oceanic Crust

  • Oceanic crust: dark-colored mineral

    • Basalt

    • Denser than continental crust

    • Younger than continental crust (less than 200 million years old).

  • Continental crust: light colored

    • Mostly granite

    • Less dense

    • older



Continental Drift

  • Proposed in 1912 by Alfred Wegener (was thought to be crazy, was institutionalized for a while)

  • All continents had been joined in a single supercontinent named Pangaea

  • Proposed that Pangaea began up 180 million years ago.

  • Not widely accepted at the time



Plate Tectonics

  • Explains Wegner’s continental drift theory

    • The Earth’s surface is covered by a series of lithospheric plates. 



-The ocean floors are constantly moving 

  • Spreading in the center

  • Sinking at the edges

  • regenerated



Convection Currents

  • Beneath the plates assist movement.

  • Heat from the mantle drives these currents.



Evidence for Plate Tectonics

  • Mid-Oceanic Ridges

    • A chain of submarine volcanic mountains that rise from the ocean floor.

  • Geomagnetic Anomalies:

    • At random intervals, the Earth’s magnetic field reverses.

    • “Stripes” of alternating magnetization parallel to the mid-oceanic ridges.

  • Island Arcs

    • Chains of islands are usually situated along the continental side of deep-sea trenches.



Geologic History

  • Pangaea was a continent surrounded by a single-world ocean, Panthalassa.

  • The Tethys Sea separated Eurasia from Africa.

  • These were in place about 200 million years ago.



Marine Sediments

  • Lithogenous sediments: derived from the break-down of rocks (weathering)

  • Biogenous sediments: derived from the skeletons and shells of marine organisms



Continental Margins

  • Boundaries between continental crust and oceanic crust.

  • They generally consist of: 

    • Continental shelf (most landward)

    • Continental slope

    • Continental rise (most seaward)



The Continental Shelf

  • Makes up about 8% of the ocean’s surface are

  • It’s the richest area of the ocean

    • Number of species (biodiversity).

  • Its width varies from 1 km (0.6 mi) to 750 km (470 mi).

  • The shelf ends at the shelf break

    • Usually at depth of 120-400 m (400-1,300 ft).



Continental Slope

  • The “edge” of a continent.

  • Begins at the shelf break

  • Extends to the deep sea floor

  • Steeper than the continental shelf and continental rise.

  • Sediments form it pushed down from the continental shelf and slope.



Deep Ocean Basins

  • Abyssal plains

    • Most of the deep sea floor lies at a depth of 3,000-5,000m (10,000-16,500 feet)

    • Numerous features, including abyssal hills, plateaus, seamounts, and guyots

    • There is a surprising amount of biodiversity, especially around seamounts




Litho = earth

Genic =