Deep Ocean Topography

1
Deep Ocean Topography

• Mid-Ocean Ridges
• and Hydrothermal Vents
• Sarah Fawcett

2
Mid-Ocean Ridges

• Linear mountain chains.
• Some of the largest features on Earth.
• 5km-2.6km deep.
• Roughly symmetrical in cross section.
• Thousands of kilometers wide.
• Volcanoes, earthquakes, hills and mountains.

3
Location of Ridges

• Mid-Atlantic Ridge
• 1855 - Fontaine Maury identified shallow middle
  ground.
• 1950s - Heezen Ewing proposed a continuous
  mountain range.
• East Pacific Rise
• Largest oceanic ridge.
• 1870s - Challenger Expedition
• 1950-60s - described by Heezen and Ewing.

4
Formation of Ridges

• Divergent tectonic plate motions.
• Tensional forces thinning of oceanic crust and
  upwelling of magma, forming ridges.

5
Seafloor Spreading

• Lava buried by sediment as seafloor spreads away
  from ridge.
• Spreading Rates
• Slow - 10mm/yr (Southwest Indian Ridge)
• Fast - up to 160mm/yr (East Pacific Rise)
• Correlation between global spreading rates and
  transgression of ocean waters onto the
  continents.
• Early Cretaceous - Global spreading rates
  uniformly high
• Marine sediments

6
Seafloor Dating

• Paleomagnetic dating
• Curie Point
• Use spreading rate to calculate age of rock.
• Age of the Seafloor
• Gets older further from the ridge.

7
Hydrothermal Vents

• Localized discharges of heated seawater.
• Cold water percolates down into the crust through
  fissures.
• Heated water rises and seeks a path to the
  surface.
• Bursts into the ocean as hot as 400ºC but intense
  pressure from overlying ocean prevents it from
  boiling.
• Accounts for amount of the Earths heat loss.

8
Growth of Vents

• Chimneys
• Minerals leached from the crust - Zn, Fe, Cu.
• Rapid growth rate.
• Black Smokers
• Hottest vents.
• Iron monosulfide.
• White Smokers
• Cooler vents.
• Compounds of Ba, Ca and Si.

9
Discovery of Vents

• 1977 on East Pacific Rise
• Near Galapagos Islands
• ALVIN
• Research submersible.
• Woods Hole Oceanographic Institute.
• Viewports, searchlights, mechanical arm, cameras.
• First temperature measurement.

10
Location of Hydrothermal Vents
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Life at Hydrothermal Vents

• Harsh environment, yet abundant life
• Tubeworms
• Crabs
• Shrimp
• Clams
• Anemones
• CHEMOSYNTHETIC BACTERIA
• No photosynthesis
• Bacteria convert sulfur to energy by
  chemosynthesis, forming base of foodchain.
• Animals eat bacteria or bacteria live inside
  their bodies.
• Origin of Life?

12
Flow at Vents
13
Baker, Cormier, Langmuir and Zavala

• Hydrothermal plumes along segments of contrasting
  magmatic influence, 15º20 - 18º30N, East
  Pacific Rise Influence of axial faulting.
  Geochemistry Geophysics Geosystems. Volume 2.
  September 2004. AGU and the Geochemical Society.
• Theory Greater incidence of hydrothermal vents
  on faster spreading ridge segments, not always
  the case - Tectonic forces can dominate.
• Segment of ridge between Orozco and Rivera
  transform faults (15º18N - 18º30N).
• 133 rock cores.
• Comparison of hydrothermal environment of three
  adjacent but distinctly different segments.
• Prediction17ºN segment should have less
  extensive hydrothermal plumes than16º segments -
  slower spreading rate.
• Opposite is true.

14

• 17ºN segment plume incidence mean of super
  fast spreading segments on the southern EPR.
• Local permeability environment in the region
  controls amount of hydrothermal activity
• 16ºN segment little indication of faulting,
  model for fast spreading rates, may have
  hydrothermal activity suppressed by volcanic
  flows that act as an impermeable cap over much of
  the segment.
• Conclusion
• Tectonic forces can control the extent and nature
  of hydrothermal activity.
• Documented for several sites on the Mid-Atlantic
  Ridge.
• On a global scale, however this portion of the
  ridge follows the existing global correlation
  between plume incidence and spreading rate.