Transcript 8_Ocean126_2006

Structure of Ocean Basins
Chapter 4
Continental shelves
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Part of continents
Exposed or not exposed depending on sea level
Cut by submarine canyons
Vary in width depending on type of margin
Accumulate sediment from the continents
(including rivers)
 Fun fact: 25 % of global continental shelf is in the
Bering Sea (very productive fisheries)
Continental Slope
 Separated from the shelf at the shelf break (~ 130
m).
 Continues to an average depth of about 4000 m
(range is 3000-8000 m)
 Most prominent topographic feature on earth
 Marks the end of continents and the beginning of
ocean crust
 Average width of 20 – 100 km
 Average slope of 4o (road grade) - 70 m/km or 370
feet/mile
Continental Rise
 Sediments from the shelf move down the
continental slope and accumulate on the
continental rise.
 Most are transported there by turbidity currents or
through riverine-derived canyons.
 Width of the rise varies and slope is gradual.
 Deep sea currents shape the rises.
 Slope is about 1/8 that of the Continental slope
(about 0.5o or < 10 m/km or < 50 feet/mile)
 Rise absent when slope terminates in a trench
 Conspicuous features of passive (tectonically
inactive) margins so rare in the Pacific
Trenches
 Act as sediment traps!
 Peru-Chile trench – oldest at the S end
(shallower because its full of sediments) and
vice versa
 Feature of active margin/subduction zone
Deep Ocean Basins
 Mainly, abyssal plains and ocean ridges
 Also, trenches, seamounts and guyots
 Include more than half the earth’s surface
Abyssal plain
 True sea floor
 Flat, sediment-covered ocean floor
 Sediments often more than 1000 m thick (not so
featureless underneath sediments)
 Begins at the base of the continental rise
 Very flat except for abyssal hills (which are low, <
1 km high, origin unclear), seamounts and guyots
and islands
 25% of the earth’s surface and 42% of the ocean
bottom is plain
 Most common in the Atlantic and rare in the Pacific
where trenches trap sediments
 Average of about 4000 – 5500 m depth
Abyssal hills
 Not very high and steep
 Protruding ocean floor topography?
Abyssal plain sediments bury old mountains (image generated
by Echo sounding)
Important features of the sea floor
 Trenches
– Extremely deep
– Found in association with plate margins (converging);
mainly in the Pacific because of plates there
 Seamounts and guyots
– Guyots are flat-topped seamounts (eroded)
– Islands are seamounts that break the sea surface
 Coral reefs
– Most common in Pacific with range 40oN and 40oS
– Require warm water, light (deepest living corals at 100
m), salinity (no freshwater forms) and low sediment
load.
– Fragile ecosystems
Trenches
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Convergence zones
Cold at the bottom
3 to 6 km deeper than adjacent seafloor
Trenches curved because of geometry of plate
interactions on a sphere
 Trench steeper on the island/continent side than
the seaward side
 Tectonically active
 Big sediment traps
Island arcs
 Parallel to trenches
 Chain of islands behind a trench
Seamounts and guyots
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Circular or eliptical
More than 1 km in relief
Alone or in groups
Old hot spots or extinct volcanoes formed at
spreading centers
 Guyots have flat tops
Coral reef formation
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Atolls – around volcanoes
Fringe reefs
Corals grow up to keep in the sunlight
Evidence of old reefs at 1400 m
Oceanic Ridges
15 cm/yr at Iceland
Includes diverging and converging boundaries.
Ocean ridge system
 Describe plate boundaries
 Largest and longest mountain range on earth
 Made up of young, basaltic rock at active
spreading centers
 Ridges devoid of sediment
 Oceanic ridges and associated structures account
for 22% of the world’s surface (compare to all land
which is 29%!)
 < 60% of their length is at the center of basins
 Youngest rocks at spreading centers
 Steeper ridges at slower spreading centers
Lavas
 Lava cooling underwater forms pillow lavas
– Solidifies rapidly because of contact with cold
seawater; this also slows flow
– Water pressure keeps gases in magma
“solution” during cooling
 Lave extruded on land
– Cools more slowly; flows longer
– Releases gases (sometimes explosively)
Transform faults
 Fractures in the lithosphere along which
movement has occurred
 Characterized by shallow earthquakes
 A result of the earth being a sphere
 Transform faults are the active parts of fracture
zones
 Fracture zones extend further away from ridge
axis and are evidence of past transform faulting.
 Lithospheric plate on either side of transform faults
move in opposite directions
 Outward sections of fracture zones move in the
same direction
Structure of the mid-Atlantic Ridge between FL and W. Africa
Depressed central rift valley is in blue
Further North and West
Seismic profile of ridge being buried
Hydrothermal vents
 Discovered only in 1977 along the East Pacific
Rise (now found on mid-Atlantic Ridge, in the Sea
of Cortez and on the Juan de Fuca Ridge)
 Really hot (~ 350oC), mineral-rich water (black
smokers). Can form chimneys, deposit minerals,
affects ocean chemistry, etc.
 Seawater descends through fissures to meet hot
rock. Superheated seawater dissolves minerals
and escapes upward.
 Found in Lake Baikal – future ocean?
 Average water temperature is 8 – 16oC (bottom
ocean water is 4oC)
Broad continental shelves
Trenches
Ridges
GBR
Red Sea
Galapagos
Iceland
Take home points
 Continental shelf, break, slope and rise
(relative slopes)
 Abyssal plains
 Mid-ocean ridge system (describes plate
boundaries)
 Hydrothermal vents, coral reefs, seamounts
and guyots
Opening since the Miocene (65 million years ago)