Download Ch. 20 The Ocean Basins

Ch. 20 The Ocean Basins
Ch. 20.2 Features of the Ocean
Floor
Two Major Divisions of the Ocean
Floor
• 1. Continental Margins—shallower
portions made up of continental crust and
a thick wedge of sediment.
• 2. Deep Ocean Basin—made up of
oceanic crust and a thin sediment layer.
Continental Margins
• The line that divides continental crust from
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oceanic crust is almost always offshore.
Continental Shelf—part of a continent’s 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.
• Affected by changes in sea level.
• During ice ages, sea levels fall, and more
continental shelf is exposed to weathering
and erosion.
Continental Slope
• Steeper slope at edge of the continental
shelf.
• Continental slope’s base is the boundary
between the continental and oceanic
crust.
• Ocean depth rapidly increases along the
continental slope.
• 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.
• stop
Deep Ocean Basins
• Contain higher mountains and flatter
plains than any found on the continents!
• Trenches—deepest feature of the earth’s
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.
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 sediment—thin in the Pacific,
but much thicker in the Atlantic Ocean,
which is not bordered by trenches.
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.
• 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
rates…they are faults that run
perpendicular to the ridge.
Seamounts
• Submerged volcanic mountains at least 1000 m
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high.
Abyssal hills—submerged 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.