Earthquakes and Mid-Ocean Ridge Spreading - PowerPoint

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					Earthquakes and Mid-Ocean
     Ridge Spreading
      Guest Scientist:
      Dr. Maya Tolstoy
       12 March 2005
   What Is the Mid-Ocean Ridge?
• World’s longest chain of mountains–
  nearly 40,000 km of basaltic ridges
  winding from the Artic through the Atlantic,
  Indian, and Pacific Oceans
• Almost entirely submerged– notable
  exceptions include Iceland and such
  archipelagos as the Galapagos (Pacific)
  and Azores (Atlantic)
  How Was the MOR Discovered?
• Early sailors were not very interested in
  water deep enough to float a boat
• First necessity to know details of sea
  floors came with efforts to lay down trans-
  Atlantic telegraph cables
• “HMS Challenger” (1872 – 1876) used
  weighted piano wire to “sound” the ocean
  depths and produced first general idea
      HMS Challenger circumnavigated the
      world from 1872 - 76 in the first major
          scientific study of the oceans.




•   http://www.wshs.fcps.k12.va.us/academic/science/bjewell/ocean/hhocean/final/chall.htm
What Revealed More Details? SONAR

• Beginning in the 1920s, SOund Navigation
  and Ranging was developed to produce
  rapid images of what lay beneath a ship
• Also called “ECO-SOUNDING,” SONAR
  emits sound signals and detects the
  returning echo to calculate depths and
  shapes beneath the surface
• Extensively used by WW II shipping
P(ic)assow
        Heezen and Tharp's
       "physiographic maps"
• SONAR was widely employed in WW II, and
  many records became available after the war.
• Dr. Bruce Heezen and Marie Tharp here at
  Lamont developed techniques beginning in the
  1950s to change these 2-D records into 3-D
  physiographic charts, a drawing technique
  developed by their Columbia professors E.
  Raisz and A. K. Lobeck.
  Through publication
arrangements with the
National Geographic
Society, these
physiographic images
completely changed the
way in which people
envision the ocean floors.
   By the early 1970s,
almost all of the oceans
were depicted in such
maps.
           Evolution of a theory
     During the past century, understanding of
    the ocean floors changed dramatically.

•   “Continental Drift”
•   “Convection” in the upper mantle
•   “Sea-floor Spreading”
•   “Plate Tectonics”
Ocean Basins and Plate Tectonics
• Francis Bacon (1561 – 1626) noted “fit” of
  South America and Africa
• Alfred Wegener (1880 – 1930) and Frank
  Taylor (1860 – 1939) independently
  proposed continental drift
• Challenged existing theories, could not
  provide satisfactory mechanism
 Early Scientific Studies and Sea-Floor
                Spreading
• Bruce Heezen and Maurice Ewing here at
  Lamont first connected earthquake
  patterns with the MOR in the mid-1950s
• Harry Hess introduced the concept of sea-
  floor spreading (1960)
• By the late 1960s, the theory of PLATE
  TECTONICS was beginning to be widely
  accepted
          Supporting Evidence
• Earthquake and volcano patterns
• SONAR and other observation techniques
• Geomagnetic patterns
  -- “normal” and “reverse” polarity
   -- “mirror” images on opposite side of MOR
  -- 170 reversals over past 76 million years
• Increasing age away from MOR rift valley
• Greatest heat flow along MOR
• Increasing thickness of sediments away from
  MOR
    3 Types of Plate Boundaries
• Divergent



• Convergent



• Transform



     http://www.cotf.edu/ete/modules/msese/earthsysflr/plates4.html
Today we often represent the plate boundaries with maps like this one




             http://www.platetectonics.com/book/page_2.asp
New shipboard and airborne techniques for
mapping the ocean floors include “side scan
sonar” and high-resolution seismic profiling.
The next slide provides examples of such
images.
•   http://marine.usgs.gov/fact-sheets/fs172-97/mapping.html
  Interesting Features of the MOR
• Central Rift Valley
• Basaltic composition (quite different from
  lava flows on continents)
• Discontinuous nature of the system
  (Menard and Heezen found right-angle off-
  sets, 1960)
• Transform faults (Wilson, 1965)
• Propagating rifts (segments defined by two
  transform faults)
        Additional Discoveries
• Variations in spreading rate:
     East Pacific Rise (60 – 170 mm/year)
     Mid-Atlantic Ridge (30 mm/year)
• Fast-spreading ridges generally had crests
  several hundred m high and 5 – 20 km
  wide
• Slow-spreading ridges have rift valleys
  several km deep and 20 – 30 km wide
       Unexpected Discoveries
• Connections between ridge and the
  source region for their lava may remain
  unchanged for tens of millions of years
• Segments can migrate along the ridge,
  lengthen or shorten
• Variations in activities among types of
  discontinuities
• Less than 5% of sea floor has been
  mapped
• Small manoeuvrable research submersibles,
  such as the “ALVIN” operated by Woods
  Hole Oceanographic Institution, provide
  access to deep-sea features not otherwise
  accessible.
http://www.comptons.com/encyclopedia/CAPTIONS/18005895_P.html
The Most Amazing Discoveries!
• “Black Smokers” or
  hydrothermal vents
• Spewing forth very
  hot water, sulphur,
  other minerals
• Basis for chemo-
  synthetic ecosystems
  unknown before the
  late-1970s.


   http://www.amnh.org/nationalcenter/expeditions/blacksmokers/smoker2.html
                         Amazing deep-
                         sea organisms!




http://www.amnh.org/nationalcenter/expeditions/blacksmokers/life_forms.html
        Next: Dr. Maya Tolstoy
Following a break, we
  will hear from Maya
  Tolstoy about her
  research and career
  as marine scientist.
Although participation in
  “Aliens of the Deep”
  has brought “new
  fame,” Dr. Tolstoy has
  participated in many
  discoveries about the
  ocean floors.
Examples of Questions Dr. Tolstoy and
   Colleagues Have Investigated
• Is there any correlation between tides and
  microearthquakes?
 http://www.ldeo.columbia.edu/~tolstoy/pdf/axial_tides.pdf

• Can seismometers be used to identify lava
  eruptions?
 http://www.ldeo.columbia.edu/~tolstoy/pdf/Tolstoy_Gakkel.pdf

• What can be learned by studying sound in
  the oceans?
 http://www.ldeo.columbia.edu/res/pi/tphase/
http://www.womenoceanographers.org/