Ch. 20 The Ocean Basins

1
Ch. 20 The Ocean Basins

• Ch. 20.2 Features of the Ocean Floor

2
Two Major Divisions of the Ocean Floor

• 1. Continental Marginsshallower portions made
  up of continental crust and a thick wedge of
  sediment.
• 2. Deep Ocean Basinmade up of oceanic crust and
  a thin sediment layer.

3
Continental Margins

• The line that divides continental crust from
  oceanic crust is almost always offshore.
• Continental Shelfpart of a continents edge
  covered by ocean water. Slopes gently away from
  the shore.
• Average depth of about 60 meters.
• Width of the continental shelf varies, but
  averages about 70 km wide.

4

• Affected by changes in sea level.
• During ice ages, sea levels fall, and more
  continental shelf is exposed to weathering and
  erosion.

5
Continental Slope

• Steeper slope at edge of the continental shelf.
• Continental slopes base is the boundary between
  the continental and oceanic crust.
• Ocean depth rapidly increases along the
  continental slope.

6
(No Transcript)
7

• May be cut by deep V-shaped valleys called
  submarine canyons, which could have been caused
  by river flows or turbidity currents (underwater
  sediment landslides).
• Along the base of the continental slope is a
  raised wedge of sediments called the continental
  rise.

8

• stop

9
Deep Ocean Basins

• Contain higher mountains and flatter plains than
  any found on the continents!
• Trenchesdeepest feature of the earths surface.
  Mariana Trench is over 11,000 meters deep.
  Associated with subduction zones and all of their
  features, such as earthquakes, volcanic island
  arcs, and volcanic mountain ranges.

10
Abyssal Plains

• Vast flat areas in the deep ocean basins where
  depth is often greater than 4 km.
• Flattest regions on earth, covering about half of
  the deep ocean basins.
• Covered with sedimentthin in the Pacific, but
  much thicker in the Atlantic Ocean, which is not
  bordered by trenches.

11
(No Transcript)
12
Mid-Ocean Ridges

• Underwater mountain ranges that run along all
  ocean floors.
• Result from divergent plate boundaries.
• A rift valley runs along the crest (top) of the
  ridges.
• New magma rises up through the rift.

13

• Warm, new crust is less dense than cold, older
  crust away from the ridge, so the colder, denser
  rock sinks, and elevation decreases as you move
  away from the ridge.
• Fracture zones result from different parts of the
  ridge separating at different ratesthey are
  faults that run perpendicular to the ridge.

14
(No Transcript)
15
Seamounts

• Submerged volcanic mountains at least 1000 m
  high.
• Abyssal hillssubmerged volcanic mountains less
  than 1000 m.
• Both are usually associated with hotspots.
• Seamounts that rise above the surface become
  volcanic islands.
• Guyots (GEE-oze)sunken volcanic islands that
  have had their tops eroded flat by wave action.

16
(No Transcript)