LECTURE NOTES:
OCEANOGRAPHY (MARSC 100),
SNYDER, L.
CH. 4: CONTINETAL MARGINS & OCEAN BASINS
BATHYMETRY: Study of Ocean floor
•Historically by soundings
•Lowering weighted ropes to touch bottom
•(85 BCE – 1914)
•Laborious
•Time consuming
HMS Challenger (1870’s): Steam-power winch, 492 soundings = real view of sea floor
Echo Sounding (sonar): 1914 – Echo Depth Sounder
•Icebergs
•Ship-mounted sound pulse system
•Measure down & back times of sound pulses
•Point by point
Continuous Echo Sounders:
Repeatedly "ping" seafloor with narrow cone of sound as ship moves on surface
Multibeam Sonar:
•~121 beams from ship’s hull
•Continuous “swath” of ocean bottom
•200 vessels = 125 yrs. to chart seafloor
Sidescan Sonar:
•Towed in water
•Closer to ocean bottom
•Feature Images
Satellite Altimetry:
•Can’t measure ocean depth directly
•Gravitational pull of seafloor features warp surface
•Estimate topography from sea surface deformations
Ocean Floor Topography: As varied as the continents
Ocean Is 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
1. Continental Margins Can Be:
•Passive (Atlantic): Divergent plate boundaries, few volcanoes & earthquakes
•Active (Pacific): Convergent plate boundaries, many volcanoes & earthquakes
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)
b. Continental Slope: steeply sloping transition from shelf to ocean floor
•Structurally part of continents
•Average incline = 4°
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 (“turbidity current”):
Continuous sediment cascade, Some fed by large rivers
c. Continental Rise: accumulated sediment at base of continental slope
•
Created by: Turbidity current deposits at mouths of submarine canyons2.
Deep-Ocean Basins:
•Accumulated sediment (up to 5 km) over basaltic rock (5 km)
•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
•If lack a cover of sediments, expose small hills (Abyssal Hills): Formed by
seafloor spreading•New crust (magma) coats hills
•Usually < 200 m. (650 ft.)
b.Seamounts: submerged volcanoes
•Form at hotspots or spreading centers
erosion)•Alone or in chains (10-100)
•Guyot: flat-topped seamount (Wave
c. Oceanic Ridges: mountain chain at active spreading centers (Diverging plates)
•Where new oceanic lithosphere is made
•Fracture zones (Transform Faults): occur where plate segments are offset= slide
horizontally
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.
•1st discovered (1977) in Alvin By Ballard & Grassle (Wood’s Hole OI)
1.
Cold H2O seeps through seafloor cracks2. H2O is heated (350°) by molten rock
below seafloor
•Chemically reacts with minerals
3.
Hot H2O & minerals rises to surface
4.
Hydrothermal fluid exits chimney & mixes with cold seawater
e. Ocean Trenches: Long, narrow, steep- sided depressions in deep seafloor
•Occur where a converging plate is subducted
•Deepest part of ocean
•Many earthquakes & tsunamis originate in 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.)
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 •Like diving in deep end of
swimming pool: feel Hydrostatic pressure against ear drums