Download Continental Margins and Ocean Basins

CONTINENTAL MARGINS
AND
OCEAN BASINS
Bathymetry and
Topography
Bathymetry
 Discovery and study of ocean floor contours
 Bathy = deep
 Meter = measure
History of Bathymetry
 85 B.C – Posidonius
 Rope and rock
 Depth measurements up to 2 km
 1818 – Sir James Clark Ross
 Rope and rock
 Depth measurements up to 4.5 km
 1870’s – HMS Challenger
 Steam-powered winch to lower/lift rock
 Made 492 bottom soundings
 Confirmed Matthew Maury’s discovery of the Mid-
Atlantic Ridge
History of Echo Sounders
 Developed by Reginald A. Fessenden
 Started by developing the “Iceberg Detector and
Echo Depth Sounder”
 1922 – USS Stewart
 First continuous profile across an ocean basin
 1925-1927 – Meteor
 14 profiles across the Atlantic
Single Beam Echo Sounders
History of Echo Sounders
 Echo sounding wasn’t perfect
 Exact position of ship wasn’t always known
 Speed of sound varies with temperature, salinity,
density, and pressure
 Couldn’t pick up the small features of the sea floor
 Even though echo sounders weren’t perfect,
the first comprehensive chart of the ocean
floor was created by 1959
New Technology
 New techniques were developed
 Multibeam echo sounders
 Satellite Altimetry
 Made possible by faster computers and
improved sensors
Multibeam Echo Sounders
 Similar to a single beam echo sounder
 Can have as many as 121 beams radiating
from ship
 Beams fan out at right angles to the direction
of travel
 Covers 120˚ arc
Multibeam Echo Sounders
 Typically, sound pulses are sent to seafloor
every 10 seconds
 Listening devices record the echo
 Ships follow a “mowing the lawn” pattern
 Allows researchers to build a complete map of an
area
 Less than 200 ships contain this technology
Multibeam
Echo Sounder
Satellite Altimetry
 Cannot measure depth directly
 Measure small variations in the elevation of
surface water
 Disregards waves, tides, and currents
 Ocean can vary in height by as much as 200m
 Gravity causes mounds of water to form over
large underwater structures
 The gravity of the structure “pulls” water toward
it from the side, which forms this mound
 Geosat, TOPEX/Poseidon, Jason-1
Satellite Altimetry
Satellite Altimetry
Topography of the Ocean
 Topography – graphic representation of a
place or region on a map, indicating relative
positions and elevations
 The lighter (less denser) continental
lithosphere floats above the level of the
heavier (denser) oceanic lithosphere
 Topography of the ocean floor is a result of
dynamic balance of lithospheric plates and
the jostling of the tectonic plates
Topographic Chart of Indian Ocean
Topography
 Continents are made of thick granite rock, while
the deep seafloor is made of basalt
 The transition of basalt marks the “true edge” of
the continent
 Divides ocean floor into 2 major provinces
 Continental Margin – the submerged outer edge of the
continent
 Ocean Basin – the deep seafloor beyond the
continental margin
Continental Margins
Continental Margins
 A continental margin is the submerged edge
of the continent
 Greatly influenced by tectonic activity
 2 types of margins
 Passive margins – aka Atlantic-type margins
 Very little earthquake and volcanic activity
 Active Margins – aka Pacific-type margins
 Large amounts of earthquake and volcanic activity
Continental Shelves
 Shallow submerged extension of the
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continent
Made of granite
Much more like the continent than the
seafloor
Can have hills, depressions, sedimentary
rocks, mineral and oil deposits
7.4% of Earth’s ocean area
Continental Shelves
 Formed as fragments of Pangea were carried
away from each other by seafloor spreading
 Material on a shelf comes from erosion of
land
 Width of a shelf depends on:
 Proximity to plate boundary
 Ocean currents
 Sediment accumulation
 Rivers
Passive-Margin Shelves
 Broad
 Gradual slope – drops about 9 ft per mile
 Average about 220 miles wide
 End at an average depth of 460 ft
 Typically smooth and flat due to sediment
buildup
 Very few features
Active-Margin Shelves
 Tend to be narrower
 Usually contains volcanoes, transform faults,
and other geographic features
 Many drop into a trench
Active vs Passive Margins
Argentina
Active
Passive
Shelf Break
 Marks the abrupt transition from continental
shelf to continental slope
 Surprisingly constant at about 460 ft
worldwide
 Exception: Antarctica’s shelf break is 1000—1300
ft deep
Continental Slope
 The transition between the continental shelf and
the deep-ocean floor
 Formed of sediments that reach the built-out
edge of the shelf and are transported over the
side
 Slope of 370 feet per mile
 Slopes of active margins are generally steeper
than passive margin slopes
 Averages about 12 miles wide
 Ends at the continental rise at an average depth
of 12,000 ft
Submarine Canyons
 Cut into the continental shelf and slope
 More than 100 exist
 Often ends on the deep-sea floor
 Creates a sea fan on the sea floor
 Created by turbidity currents
 Continuous cascade of sediments which causes
erosion of the shelf and slope
Submarine Canyon and Sea Fan
Monterey Canyon
Continental Rise
 Only found along passive margins
 Accumulation of sediments at the base of the
continental slope
 Sediments usually get there by turbidity currents
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Sediments come from the shelf
Varies in width from 63-630 miles
Slope is very gradual
Shape of rise is affected by deep-ocean
currents
Can you label: shelf, break, slope, rise,
submarine canyon, sea fan???
Ocean Basins
The Basics
 Very different topography than the
continental margin
 Covered with a layer of sediments up to 3
miles thick
 Made of basaltic rock (rather than granite)
 Cover more than 50% of the Earth’s surface
Mid-Ocean Ridges
 Mountainous chain of young basaltic rock at
the active spreading center of the ocean
 Stretches more than 40,000 miles
 Usually devoid of sediment layers
 Rise approximately 1.25 miles above seafloor
Mid-Ocean Ridge System
Mid-Atlantic Ridge
Mid-Ocean Ridges
 Youngest rock is located at the ridge center
 As the lithosphere cools, it shrinks and
subsides
 Sometimes crosses land – called a rift valley
or rift zone
 Iceland – plates are spreading over 6 inches/year
 Ridges are offset by transform faults causing
a stair step look
Iceland
Hydrothermal Vents
 First vents were found near the Galapagos
Islands along the East Pacific Rise in 1977
 Rocky chimneys that can reach up to 20m
 Spew mineral-laden water of temperatures up to
660º F
 Water descends through cracks in the floor until
it comes into contact with very hot rocks
associated with active seafloor spreading
 The superheated water dissolves minerals and
gases and escapes upward through vent
 Minerals give the water the smoky look
Black Smokers
Hydrothermal Vents
 Many are nicknamed “black smokers”
because the water looks like smoke
 Thought to play an important role in:
 Chemical composition of seawater
 Composition of the atmosphere
 Formation of mineral deposits
Hydrothermal vent that is not a black smoker
Abyssal Plains
 Covers 25% of the earth’s surface
 Flat featureless expanses of sediment
covered ocean floor
 Largest: Canary Abyssal Plain – 350,000 sq miles
 Lie between the continental margins and the
oceanic ridges
 Usually between 12,000-18,000 ft below the
surface
Abyssal Plains
Abyssal Plains
 Extraordinarily flat – varies by no more than a
few meters over an entire area
 Flatness is caused by the thick layer of
sediment that can be more than ½ mile thick
 This masks many small features of the basaltic
floor
 Most sediments are terrestrial in origin and
were transported by turbidity currents and
wind
Abyssal Plains
Abyssal Hills
 The sediment layer isn’t thick enough near
ridges , so features of the basaltic floor start
to show
 Plains become punctuated by abyssal hills
which are small, sediment covered extinct
volcanoes or intrusions of molten rock and
are usually less than 650 ft tall (from basalt to
tip)
 Associated with seafloor spreading and occur
more frequently with faster spreading centers
Seamounts
 A volcanic projection that does not rise above
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the sea surface
Circular or elliptical in shape
More than 0.6 miles high
Many form at hotspots, but others have
formed at spreading centers
Movement of the plates has moved them
away from their original location
About 10,000 occur in the Pacific Ocean
Seamounts
Seamounts
Guyots
 Flat-topped seamount that once was tall
enough to approach or penetrate the sea
surface
 Flat top suggests they were eroded by wave
action when near the surface
 Like seamounts, guyots are formed near
spreading centers and are transported
outward
 Generally confined to the west-central Pacific
Guyot
Trenches
 Arc-shaped depression in the ocean floor
 Deepest places on Earth
 Mariana Trench – 36,163 ft deep (almost 7 miles),
44 miles wide, 1,600 miles long
 Occur where a converging oceanic plate is
subducted
 Among the most active geologic features
 Can cause great earthquakes and tsunamis
 No continental rise when a trench occurs
Trenches
Trenches
Trenches
Island Arcs
 Curving chains of volcanic islands and
seamounts
 Found parallel to the concave edges of
trenches
 Caused by the build up of pressure in the
Earth’s interior due to subduction
 Causes active volcanoes to form which eventually
reach the surface to become islands
 Examples: Aleutian Islands, Mariana Islands,
South Sandwich Islands, Kuril Islands
Island Arcs
Aleutian
Islands