Plate Tectonics

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Plate Tectonics

• Part 1 History and
• Geophysical Techniques

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2000
Morgan, McKenzie and Parker plate tectonics
Vine Mathews seafloor spreading
Arthur Holmes thermal convection drives drift
Alfred Wegener similarity of rocks and fossils
1900
Eduard Suess existence of Gondwanaland
Antonio Snider maps depicting continental drift
Charles Lyell principle of uniformitarianism
William Smith stratigraphic succession of rocks
1800
James Hutton no vestige of a beginning, no
prospect of an end
1700
Nicolaus Steno principal of superposition
Bishop Ussher earth created 0900 10/26/4004 BC
Francis Bacon matching of Atlantic shorelines
1600
Xenophanes of Colophon fossils are ancient life
500 BC
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 Knowledge is power.Nam et ipsa scientia
potestas est.              Meditationes Sacræ. De
Hæresibus. Novum Organum - commented on
similarity of the west coasts of S. America and
Africa noted configurations of Old and New
Worlds were similar "both of which are broad and
extended towards the N narrow and pointed
towards the south.
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Siccar Point, Scotland
The unconformable contact of Old Red Sandstone on
vertical Silurian rocks noted by James Hutton,
who in 1788 first recognized the significance of
its meaning in relationship to necessity for an
old age for the earth
http//www.glg.ed.ac.uk/courses/field/siccarpt/
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 speculated that the Atlantic was formed by
catastrophic event, "a flow of eddying
rivers...directed first to the NE, then towards
the NW, and back again to the NE. What we call
the Atlantic Ocean is nothing else than a valley
scooped out by the sea."
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avant
apres
In 1858, geographer Antonio Snider-Pellegrini
made these two maps showing his version of how
the American and African continents may once have
fit together, then later separated. Left The
formerly joined continents before (avant) their
separation. Right The continents after (aprés)
the separation. (Reproductions of the original
maps courtesy of University of California,
Berkeley.)
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2000
Morgan, McKenzie and Parker plate tectonics
Vine Mathews seafloor spreading
Arthur Holmes thermal convection drives drift
Alfred Wegener similarity of rocks and fossils
1900
Eduard Suess existence of Gondwanaland
Antonio Snider maps depicting continental drift
Charles Lyell principle of uniformitarianism
William Smith stratigraphic succession of rocks
1800
James Hutton no vestige of a beginning, no
prospect of an end
1700
Nicolaus Steno principal of superposition
Bishop Ussher earth created 0900 10/26/4004 BC
Francis Bacon matching of Atlantic shorelines
1600
Xenophanes of Colophon fossils are ancient life
500 BC
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Alfred Wegener a man ahead of his time
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Continental Drift

• Alfred Wegener (1912)
• German meteorologist - devoted much of his life
  to championing Drift. He pooled the existing
  pre-Drift geologic data suggesting that the
  continuity of the older structure, formations and
  fossil faunas and floras

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Continental Drift

• As noted by Snider-Pellegrini and Wegener, the
  locations of certain fossil plants and animals on
  present-day, widely separated continents would
  form definite patterns (shown by the bands of
  colors), if the continents are rejoined.

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Glossopteris Leaves
Wegener studied the distribution of animals and
fossil land plants to help him in his
interpretations. Wegener found that the plant
Glossopteris had left behind leaf remains which
were relatively common in the Southern Hemisphere
continents.
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Glossopteris Leaves
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Glossopteris Leaves
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Tillites and Striations
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Mesosaurus
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Mesosaurus
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Mesosaurus
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Wegners Theory

• Postulated that all continents were joined into
  a single land mass termed Pangaea ("All the
  Earth").

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Wegeners Theory

• More appropriate to consider Pangaea as two super
  continents Laurasia (Laurentia and Asia) - No.
  America, Greenland, Europe, and Asia and Gondwana
  (Land of the Gonds (India)) - S. America,
  Antarctica, Africa, Madagascar, India, Australia

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Wegeners Theory

• Super continents were separated by Tethys Ocean

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Wegners Theory
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Wegeners Theory

• Resistance to Wegener was personal and trivial in
  many ways
• 1) Wegener's CD rejected the static earth
  position (Scientific orthodoxy)
• 2) drew on several different lines of evidence
  which he was not an expert

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Wegners Theory

• Resistance to Wegener was personal and trivial in
  many ways
• 3) some details were not correct leading others
  to discard the whole hypothesis
• 4) lack of reasonable mechanism which was the
  single largest obstacle (Wegner postulated that
  centripetal forces acting on the higher standing
  continents caused them to drift.)

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Earthquakes
In 1954, French seismologist J.P. Rothé published
this map showing the concentration of earthquakes
along the zones indicated by dots and
cross-hatched areas. (Reproduced with permission
of the Royal Society of London.)
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Geophysics

• Paleomagnetics
• Seismics
• Heat Flow
• Gravity

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Paleomagnetics

• Rocks have magnetic properties because some
  minerals have magnetic domains (particularly
  those with Fe).

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Paleomagnetics

• At high temperatures, the ferromagnetic minerals
  can realign with a changing magnetic field
  because the energy level is high enough to keep
  the molecules in motion. As the temperature
  decreases below the Curie Point or Temperature,
  the energy level is not high enough to allow the
  magnetic minerals to realign and hence has a
  fixed magnetic field.

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Paleomagnetics

• Paleomagnetic measurements include the Delination
  and Inclination (Dip) of the magnetic field
  recorded in the rock during formation (Fig. 8-11.
  Inclination of the paleomagnetic field tells us
  about the paleolatitude of the rock during
  formation.

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Using the Inclination for Paleo-latitude
determination
tan I 2 tan L
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Slide showing geomagnetic field
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Paleomagnetics
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Geomagnetic reversals

• First reported by Brunhes (1909) and Matuyama
  (1928).
• Magnetic Poles were reversed

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Magnetic Anomalies

• Difference between the measured magnetic
  intensity and that predicted.

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Magnetic Anomalies

• Reykajanes Ridge

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Magnetic Anomalies

• Model
• When mapped, the anomalies produce a
  zebra-striped pattern of parallel positive and
  negative bands. The pattern was centered along,
  and symmetrical to, the mid-ocean ridge.
• A hypothesis was presented in 1963 by Fred Vine
  and Drummond Matthews to explain this pattern.
  They proposed that lava erupted at different
  times along the rift at the crest of the
  mid-ocean ridges preserved different magnetic
  anomalies.

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Magnetic Anomalies

• Model

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Magnetic Anomalies

• Model

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Magnetic Anomalies
positive
negative
From Raff and Mason, 1961, Bull GSA
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The remarkable symmetry about the axis of the
Pacific-Antarctic spreading center
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Sea floor age map
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Evolution of the paleomagnetic timescale
(1963-1969)
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Southern Mid-Atlantic Ridge
Observed
Model
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Paleomagnetics

• Apparent Polar Wander

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Paleomagnetics

• Apparent Polar Wander

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Sea Floor Spreading

• Sandwell and Smith

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The concept of sea floor spreading (after Hess,
1962)
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Sea Floor Spreading

• Harry Hess explored the Pacific ocean during
  WWII.
• Key Observations from ocean exploration
• Guyots
• Ocean sediment thickness
• Small number of underwater volcanoes

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Guyots

• Flat-topped Volcanic Seamounts

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Guyots

• Flat-topped Volcanic Seamounts

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Guyots
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Sediment Thickness

• Figure on sediment Isopach