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CONTINENTAL CONTINENTAL CONTINENTAL CONTINENTAL
MARGINS & OCEAN MARGINS & OCEAN MARGINS & OCEAN MARGINS & OCEAN
BASINSBASINSBASINSBASINS
CH. 4CH. 4CH. 4CH. 4
Bathymetry
•Study of Ocean floor
•Historically by soundings
•Lower weighted ropes to touch bottom
•(85 BCE – 1914)
•Laborious
•Time consuming
•HMS Challenger
•Steam-power winch•492 soundings = real view of sea floor
1870’s
Multibeam Sonar:
•~121 beams from ship’s hull
•Continuous “swath” of ocean bottom
•200 vessels = 125 yrs. to chart seafloor
Florida
Coast
details
Sidescan Sonar:
•Towed in water
•Closer to seafloor
•Better images
Satellite Altimetry:
•Can’t measure ocean depth directly
•Gravitational pull of seafloor features warp surface
•Estimate topography from sea surface deformations
Pulls H2O
toward it
from sides
Ocean Floor Topography
As varied as continents
Mid-Atlantic Ridge
Ocean Divided Into 2 Zones:
1. Continental margins (CM): submerged, outer edge of continents
• Shelf, Slope, & rise
2. Ocean Basins: seafloor beyond CM
• Abyssal Plain, Trenches, & Ridges
Continental Margin Ocean Basin Continental Margin
Continental Margins Can Be:
• Passive (Atlantic): Divergent plate boundaries, few volcanoes & earthquakes
• Active (Pacific): Convergent & Transform plate boundaries, many volcanoes & earthquakes
West East
S. American Plate
Continental Margin Types:
a. Continental Shelf: shallow, gently sloping extension of continents
• Formed by continental erosion & sediment accumulation (river transport)
• Wide at passive margins (up to 1280 km = 800 mi.)
• Narrow at Active Margins (few km)
Continental Margin Types:
b. Continental Slope: steeply sloping transition from shelf to seafloor
• Structurally part of continents
Shelf break (~140 m)
Submarine Canyons:
• Can cut into Continental Slope & Shelf:
• Earthquakes shake loose mass of sediment that plunges down shelf
OR
•Continuous small amount of debris erodes shelf
Hudson Canyon
(New Jersey)
Earthquake
generated (1929)
“turbidity current”
Continuous sediment
cascade
Some fed by large
rivers
Turbulence mixes
sediments into water
Continental Margin Types:
c. Continental Rise: accumulated sediment at base of continental slope
• Created by:
• Turbidity current deposits at mouths of submarine canyons
•Descending sediment from slope
Deep-Ocean Basins:
• Accumulated sediment (5 km/3 mi deep) over basaltic rock
• Variable topography
Ocean Basin Features:
a.Abyssal Plains: broad flat regions built by sediment deposition
• Depth = 3,700 – 5,500 meters (2.5-3.6 mi.)
•Covers seafloor basaltic rock
•Common in Atlantic
Abyssal Plains:
• That lack a cover of sediments, expose small hills (Abyssal Hills)
• Formed by seafloor spreading
•New crust (magma) coats hills
•Usually < 200 m. (650 ft.) high
Ocean Basin Features:
b. Seamounts: submerged volcanoes
•Form at hotspots or spreading centers
•Guyot: flat-topped seamount
• Wave erosion
Alone or in chains (10-100)
Ocean Basin Features:
c. Oceanic Ridges: mountain chain at active spreading centers
• Where new ocean lithosphere is made
Ocean Ridges:
• Fracture zones (Transform Faults): occur where plate segments are offset
Youngest Rock= slide horizontally
Iceland’s Rift
Valley
Diverging plates
Ocean Basin Features:
d.Hydrothermal Vents: Seafloor Hot spring
•Continuously spews HOT (350°C = 660°F), mineral-rich water
•Most at depths ≥≥≥≥ 2,100 m. (7,000 ft)
•Areas of seafloor spreading (Mid-Ocean Ridges)
•All seawater circulates through seafloor crust
• ~10 million yrs. “Black Smoker”
Central portion of
Ocean Ridge
Cool water
1
seafloor
•Chemically reacts with minerals
3. Hot H2O & minerals rises to surface
4. Hydrothermal fluid exits chimney & mixes with cold seawater
Metals combine
with sulfur = black
metal sulfides
Cold H2O seeps through seafloor cracks
2. H2O is heated (350°) by molten rock below
Hydrothermal Vents Around World
• 1st discovered (1977) in Alvin
•By Ballard & Grassle (Wood’s Hole OI)
1111stststst Vent Vent Vent Vent DiscoveredDiscoveredDiscoveredDiscovered
Ocean Basin Features:
e. Ocean Trenches: Long, narrow, steep–sided depressions in deep seafloor
•Occur where converging plates are subducted
•Deepest part of ocean
•Where many earthquakes & tsunamis originate
Ocean Trenches:
• Hot rising magma creates volcanic island arcs (Japan) adjacent to trench
Where are Oceanic Trenches?
• Most occur in W. Pacific
• Deepest spot in ocean: Challenger Deep (Marianas Trench) = 11,022 m (36,163 ft.)
Shinkai 6500
Alvin (Challenger Deep)
Deep Sea Research & Diving
•Deeper you dive = more H2O is on top of you
• Greater pressure on body
•Weight of H2O above you: Hydrostatic Pressure
•Deep end of swimming pool
•Hydrostatic pressure against ear drums
Shinkai
6500 Deepest Diver
Alvin (Challenger Deep): 16,000 lbs/in2
7 mi.
of
H2O