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OCEAN BASINS
CHAPTER 4
Study Plan
• The Ocean Floor is mapped by
bathymetry
• Ocean-floor Topography varies with
Location
• Continental Margins may be Active or
Passive
• The Topology of Deep-Ocean Basins
differs from that of the Continental Margin
• The Ground Tour
The Ocean Floor is mapped by
bathymetry
• “bathymetry”: ocean floor contours
• 85 BCE in Greece by Posidonius
• 1818 – Sir James Clark Ross
– Soundings of the South Atlantic – 4893
meters
• 1870s – HMS Challenger
– Confirmed the existence of the Mid-Atlantic
Ridge
Echo Sounders bounce Sound
off the Seabed
• 1922 – USS Stewart – first continuous
ocean basin profile of the Atlantic
• 1925-1927: Meteor made 14 profiles in
the Atlantic
• Echo sounding was not perfect
– Temperature, pressure, salinity
Multibeam Systems combine
many Echo Sounders
• Bounce sound off the seafloor at specific
intervals to measure the depth of the
ocean
• Too time consuming… it would take 125
years to map the entire ocean!!!!
Satellites can be used to Map
Seabed Contours
• Geosat satellite measures ocean water
elevation with a resolution of 0.03 meter
(1 inch)
• Gravity can reveal undersea mountains or
canyons
Ocean-Floor Topography varies
with Location
• Continental margin: the submerged edge
of the continent
• Ocean basin: the deep-sea beyond the
margin
Continental Margins may be
Active or Passive
• Passive margin: margins facing a
divergent plate
• Active margin: margins facing a
convergent plate boundary
Continental Shelves are
Seaward Extensions of the
Continents
• Continental shelf: shallow, submerged
extension of a continent
• Passive vs. active margins
Continental Slopes connect
Continental Shelves to the
Deep-Ocean Floor
Submarine Canyons form at the
Junction between Shelf and
Continental Slope
• Submarine canyon: cut in the wedge of a
submerged fan
• What causes these canyons?
• Turbidity currents and earthquakes
Continental Rises form as
Sediments Accumulate at the
Base of the Continental Slope
• Occur along passive margins as aprons of
sediments
The Topology of Deep-Ocean
Basins
• Deep-ocean basins are blanketed by 3
miles of sediments overlying basaltic rocks
• Oceanic ridge: mountainous chain of
YOUNG and HOT basaltic rocks
• Stretches 65,000 km
Hydrothermal vents
• 1977: discovered by Ballard and Grassle of
the Woods Hole Oceanographic Institution
– Chimneys
– East Pacific Rise
– 350 degrees Celsius
Abyssal Plains
• Flat, featureless expanses of sedimentcovered ocean floor
Volcanic Seamounts and Guyots
• Seamounts: volcanoes that do not reach
the surface
• Guyots: flat-topped seamounts
Trenches
• Trench: arc-shaped depression in the
deep ocean floor
• Deepest on Earth
• Mariana’s Trench 11,022 meters (36, 163
feet)
The Ground Tour