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Earthquakes and Mid-Ocean Ridge Spreading

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Additional Discoveries. Variations in spreading rate: East ... Unexpected Discoveries ... The Most Amazing Discoveries! 'Black Smokers' or hydrothermal vents ... – PowerPoint PPT presentation

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Title: Earthquakes and Mid-Ocean Ridge Spreading


1
Earthquakes and Mid-Ocean Ridge Spreading
  • Guest Scientist
  • Dr. Maya Tolstoy
  • 12 March 2005

2
What Is the Mid-Ocean Ridge?
  • Worlds 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)

3
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

4
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/bj
    ewell/ocean/hhocean/final/chall.htm

5
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

6
P(ic)assow
7
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.

8
  • 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.

9
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

10
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

11
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

12
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

13
3 Types of Plate Boundaries
  • Divergent
  • Convergent
  • Transform

http//www.cotf.edu/ete/modules/msese/earthsysflr/
plates4.html
14
Today we often represent the plate boundaries
with maps like this one
http//www.platetectonics.com/book/page_2.asp
15
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.
16
  • http//marine.usgs.gov/fact-sheets/fs172-97/mappin
    g.html

17
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)

18
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

19
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

20
  • 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/1800
    5895_P.html

21
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/bla
cksmokers/smoker2.html
22
Amazing deep-sea organisms!
http//www.amnh.org/nationalcenter/expeditions/bla
cksmokers/life_forms.html
23
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.

24
Examples of Questions Dr. Tolstoy and Colleagues
Have Investigated
  • Is there any correlation between tides and
    microearthquakes? http//www.ldeo.columbia.edu/t
    olstoy/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
    /

25
  • http//www.womenoceanographers.org/
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