Download Ocean Basins – Chapter 3

Ocean Basins
• When early exploration of the oceans began,
most scientists believed that the ocean floor
was completely flat and featureless
• Furthermore, scientists believed that the
deepest parts of the oceans were located in
their center
• Soon, however, scientific exploration (and
modern technology) unraveled the mysteries
of the ocean floor
Ocean Basins
• As scientists mapped the ocean floor, they
found that the terrain of the sea floor was in
fact, highly variable and included deep
troughs, ancient volcanoes, submarine
canyons, and tremendous mountain ranges
• Moreover, they discovered that some of the
deepest parts of the oceans are actually close
to land, and NOT in the middle!
The ocean floor is mapped by
bathymetry
• The discovery and study of the ocean floor
contours is called bathymetry (“bathy” =
deep; “meter” = measure)
• Early bathymetric studies involved using a
rope and stone, or piano wire and an anchor
• The Challenger expedition of the 19th century
made 492 bathymetric recordings alone!; but
the process is long and tedious
From great tragedy came great
innovation
• The sinking of the Titanic in 1912 stimulated
research that finally ended the slow, laborious
weight-on-a-line efforts
• By 1914, a former employee of Thomas Edison
invented the “Iceberg Detector and Echo
Depth Sounder”
– Emitted a powerful underwater sound pulse
ahead of the ship, and listened for the return echo
from a submerged portion of the iceberg
Echo sounders bounce sound off the
seabed
• In 1922, an echo sounder based on this design
was used aboard the USS Stewart, a US Navy
vessel which made the first continuous profile
across an ocean basin (Atlantic Ocean)
• By the late 1920’s, the German research vessel
Meteor – using an improved echo sounder –
made 14 profiles across the Atlantic Ocean
– Revealed the mid-Atlantic Ridge and the obvious
coincidence with coastlines on both sides of the
Atlantic stimulated discussions of plate tectonics!
03_05
An echo sounder record
To Land 
Echo Sounders are not entirely accurate
Multibeam
Echo Sounding
Fig. 4-3a, p. 79
Satellites Can Also Be Used to Map
Seabed Contours
• Seafloor features directly
influence Earth’s gravitational
field
•Deep areas such as trenches
correspond to a lower
gravitational attraction, while
large undersea objects such as
seamounts exert a stronger
gravitational pull, causing the
ocean surface to bulge upward
•Satellites use microwave beams
to measure sea level to within
4cm (1.5 in) of accuracy!
multibeam
sounding
satellite
• Satellite mapping of the
ocean floor is more
accurate and provides
greater resolution than
multi-beam sounding
Bathymetry can identify mid-oceanic
ridge systems
BATHYMETRY – OCEAN FLOOR
CONTOURS
• The transition between the thick granitic rock
of the continents and the relatively thin basalt
of the deep sea floor marks the boundary
between continental margin and ocean basin
(raised up because
hot = buoyant)
Continental Margins
• Continental margins are shallow water areas
close to the continents; they literally are the
submerged outer edges of a continent
(raised up because
hot = buoyant)
Ocean Basins
• Ocean basins are deep-water areas farther
from land, beyond the continental margin
(raised up because
hot = buoyant)
Continental margins vs. Ocean
basins
Continental margins may be active
or passive
• Continental margins are classified as being
either active or passive depending on their
proximity to plate boundaries
• Continental margins facing the edges of
diverging plates are passive because relatively
little earthquake or volcanic activity is
associated with them
• Continental margins near the edges of
converging plates are active because of their
greater earthquake and volcanic activity
Passive margins
• Passive margins are not located on plate
boundaries; rather, they are imbedded within
the interior of a lithospheric plate, and
therefore are not in close proximity to a plate
boundary
– Why they lack tectonic activity
– Usually produced the rifting of continental
landmasses and continued sea floor spreading
– Also called “Atlantic-type”; e.g., east coast of US
Passive margins
Continental and oceanic crust are located on the
same lithospheric plate
Active Margins
• Active margins are associated with plate
boundaries, and are marked by a high degree
of tectonic activity
– “Pacific-type”; e.g., west coast of US
• Two types: convergent and transform
depending on whether the margins are
associated with convergent (oceanic and
continental) plate boundaries, or transform
plate boundaries, respectively
Active margins
Continental crust distinct (separate) from oceanic crust
Continental shelves are seaward
extensions of the continents
• The shallow, submerged extension of a
continent is called the continental shelf
• Continental shelves are underlain by granitic
continental crust
– Much more like the continent in composition than
the ocean floor; continental shelves contain hills,
depressions, sedimentary rocks and mineral
and/or oil deposits that are similar to those found
on nearby dry land
Continental shelves
• Taken together, continental shelves make up
7.4% of Earth’s ocean area
• The width of any given continental shelf is
determined by:
– Its proximity to a plate boundary
• Shelves at passive margins are wide, while shelves at
active margins are quite narrow
– Localized water current speed
• Fast-moving ocean currents prevent sediment from
accumulating
– Sea level; determines exposure or submersion
Continental shelves (examples)
Continental slopes connect continental
shelves to the deep ocean floor
• The continental shelf extends from the
continent to the shelf break, an abrupt
transition between the continental shelf and
the continental slope
• The depth of water at the shelf break is
surprisingly constant; 140 meters (460 feet)
• The continental slope is steeper than the
shelf, and end at the deep ocean basin
• The continental slope is composed of
sediments transported from the continent
Features of the continental margin
• At passive margins (only), a continental rise can
also be found at the base of the continental slope
– Covered by a blanket of accumulated sediment
Submarine canyons
• The continental slope and the continental
shelf may exhibit submarine canyons; Vshaped canyons carved by rivers that often
terminate in a fan-shaped wedge of sediment
• >100 submarine
canyons nick the
edge of nearly all
continental
shelves
• Hudson Canyon,
off the coast of
NY and NJ, is a
prime example;
located off the
Hudson River
Turbidity Currents
• Submarine canyons are primarily caused by
erosion of sediments
• Scientists believe that submarine canyons
originate from turbidity currents; underwater
avalanches of muddy water mixed with rocks
and other debris
– Strong, erosive currents that move down-slope
under the force of gravity (denser than water)
Ocean Basins
• The structure of the ocean floor is quite
different than that of the continental margins
• Here, the seafloor is blanketed with up to 5km
(3 miles) of sediment overlying basaltic rocks
• Deep-ocean basins constitute more than half
of the Earth’s surface
• If the oceans evaporated, the oceanic ridges
would be the Earth’s most remarkable
features
Oceanic ridges
• The deep-ocean floor consists of oceanic ridge
systems and adjacent sediment-covered
plains. In addition, deep ocean basins may be
rimmed by trenches or masses of sediment
• Flat expanses are interrupted by islands, hills,
active and extinct volcanoes
• Oceanic ridges are mountainous chains of
young, basaltic rock at the active spreading
center of an ocean
Oceanic ridges
• Ocean ridges girdle the globe like seams
surrounding a softball
Oceanic ridges
• Oceanic ridges literally cover 40,000 miles of
Earth (that’s more than 1.5x the
circumference of the Earth)
• In some places, these ridges actually project
upward to the surface to form islands, such as
Iceland, the Azores, and Easter Island
Hydrothermal Vents: The ‘What’
• Some of the most exciting features of the
ocean basins are hydrothermal vents
• Hydrothermal vents are fissures (volcanic
vents) from which geothermally-heated water
rise
• First discovered in 1977 near the Galapagos
Islands near the East Pacific Rise; now
believed to be relatively common along midoceanic ridges
Hydrothermal Vents: The ‘How’
• Seawater descending through cracks in the
ridge floor comes into contact with the very
hot rocks associated with active seafloor
spreading; the superheated water dissolves
minerals and escapes upward through the
vents
• This superheated water should be released as
steam, but exists in liquid form because of the
tremendous pressures exerted at this depth!
Hydrothermal Vents: The ‘Who’
• The temperature of water rushing out of a
particular hydrothermal vent determines its
appearance:
– Warm-water vents: <30°C (86°F); emit clear water
– White smokers: 30-350°C (86-662°F)!!!; emit white
water because of the presence of light-colored
compounds such as Barium, Calcium, and Silicon
– Black smokers: >350° (662°F)!!!; emit black water
because of the presence of dark-colored metal
sulfides, such as Iron, Nickel, Copper, and Zinc
• The average temperature in the vicinity of
hydrothermal vent activity is ~8-16°C (4661°F), much warmer than usual for oceanbottom water (~3-4°C)
Hydrothermal Vents
• In Iceland, hydrothermal vents exist on dry
land
– Iceland rests on a mid-oceanic ridge (Mid-Atlantic
Ridge) that has lifted above sea level
• Underwater, hydrothermal vents host a
unique assembly of organisms, fueled
by the chemicals dissolved in the vent
fluids
– Chemosynthetic Archaea form the base
of the food chain
Our present knowledge of vent systems