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The three pillars of Earth history
Stratigraphy
Isotope geochemistry
Paleomagnetism & plate tectonics
The last 200 million years of earth history
revealed by paleomagnetism and sea floor
spreading*
*Link to Quicktime movie
Alfred Wegener, a great SES scientist!
(1880-1930)
- Thermodynamics of the Atmosphere (1911)
- The Origin of Continents and Oceans
(1915,1929)
- Climate and Geological Pre-history (1924)
quantitative fitting of
the shapes of W.
Africa and E. South
America
The geology matches
very closely
Proterozoic
orogenic belts
>2000 Ma cratons
North America,
Greenland, and
Africa can be
tied together
And the southern hemisphere Gondwana continents
Africa
Madagascar
India
South America
Antarctica
Australia
Late Paleozoic
Gondwana
glaciation
Paleozoic and
Mesozoic
mountain belts
Fossil distributions
The evidence for continental drift was compelling and
well established during the first half of the 20th
Century
but it was not accepted by many geologists (almost all
U.S. geologists) until the advent of plate tectonics in
the late 1960’s-early 1970’s!
Why??
Why??
Wegener was aggressively attacked and then ignored
- because of his lack of geological credentials
- his geophysical theories for the dynamics of
continental drift were off track
Why??
The focus of geologists was on complex continental
geology at local to regional scales, not global scales.
Why??
The obvious importance of vertical motions in
continental geology, leading to the "fixest" (versus
"mobilist") synthesis that was the reigning theory for
much of the first half of the last century.
Why??
the ocean basins were largely “terra
incognita” before WWII
WWII-Cold War explosion of
technology and observations
 Marine geophysics
 Isotope geochemistry
 Global seismological
networks
 Digital computers
Plate tectonics
Revolution
(1962 – 1972)
Simplified view of Earth’s tectonic plates
Plate boundary types
divergent
boundary
convergent
boundary
lithospheric
plate
asthenosphere
Plate boundary types
divergent
boundaries
Transform
fault boundary
lithospheric
plate
asthenosphere
Relative motions across modern plate boundaries
cm/year
Continental
rifting:
formation
of an ocean
Continental
rifting in the
Middle East:
Red Sea and
Gulf of Aden
Convergent boundary: subduction zones
South American
Andes
island
arc
The key breakthroughs:
• Paleomagnetism
• Seafloor Spreading
• Contrasting oceanic
and continental crustal
structure
• Seismicity of plate
boundaries
Plate Tectonics
revolution
The key breakthroughs:
Geomagnetism: major key to
Earth’s history
• Paleomagnetism
• Seafloor Spreading
• Contrasting oceanic
and continental crustal
structure
• Seismicity of plate
boundaries
Plate Tectonics
revolution
Age of ocean floor recorded by Seafloor Spreading
“tape recorder”
seafloor
age, Ma