Transcript Plate Tectonic Evidence

Earth History
GEOL 2110
Theory of Plate Tectonics
Part 1:
Toward the Development of the Grand Paradigm
Major Concepts
• Most early models attempting to explain the Earth’s
topographic features (mountains, cratons, ocean basins,
volanic arcs, …) assumed the immobility of the continents
and appealed to a cooling, contracting earth.
• Several lines of evidence that the continents had drifted
apart were presented by Alfred Wegener in 1915, but was
largely dismissed because it lacked a mechanism.
• Paleomagnetic pole studies in the 1950’s followed by
bathymetry and magnetic polarity mapping of the sea-floor
brought on the theory of sea-floor spreading
• With the recognition that the new crust is being made at
mid-oceanic rift zones, the final piece of the puzzle was
realized and the Theory of Plate Tectonics was born.
Mountain-Building and the
Geosyncline Theory
Cross-section through the Appalachians (1842)
Mountain-Building and the
Geosyncline Theory
In seeking an explanation for the deformed nature
of strata in mountain belts, Hall noted that the
stratigraphic units thickened considerably from the
cratons to the mountain belts.
He suggested that the weight of the over-thickened
trough of sediments caused the crust to depress
and eventually crumple.
James Hall (1811-1898)
New York State Geologist
Paleontologist and Geologist
Mountain-Building and the
Geosyncline Theory
Had a similar idea to Hall’s, but suggested that the
over-thickened trough of sediment, which he
termed a “geosyncline”, was the result, not the
cause of downwarping of the continent at their
margins. He believed the warping resulted from
cooling and contraction of the earth.
JD Dana (1813-1895)
Mineralogist, Volcanologist
and Geologist
The Role of Magmatism in Mountain
Belts
• Although granite batholiths
and volcanic rocks were known
to be integral parts of
mountain belts, the role of
magmatism was largely
ignored in the Geosyncline
Theory.
• Early on (1790’s) Hutton had
suggested intrusions of
granites played a major role in
uplift of mountains
Continental Drift and Mountains
The discovery of radioactive heating of the earth in the early 1900’s
not only put Kelvin’s age of the earth into question, but is also
made mountain building by horizontal compression due to a
contracting Earth impossible.
Scientists started to speculate that maybe, just maybe, the
continents could move horizontally – DRIFT.
1908 – American F.B.
Taylor suggested that
drifting of the continents
was responsible for the
great Cenozoic mountain
belts. Speculated that
capture of the moon was
responsible.
Pre-Drift Continental Reconstruction
1912 – Another American
H. Baker postulate the first
reconstruction of what we
now call the Pangea
supercontinent.
He speculated that the
moon was ripped from
Earth to form the Pacific
basin prompting the
continents to drift toward
the void.
Alfred Wegener
Father of the Theory of Continental Drift
1915 – The Origin of Continents and
Oceans (3rd edition, 1922)
Meteorologist by training
Introduced the hypothesis of
Continental Drift
Postulated the existence of the
supercontinent he named Pangea
Evidence:
Fit of the continents
Fossil record
Similar geologic structures
Similar paleoclimates
Alfred Wegener
1880-1930
Fit of the Continents
~250 Ma
Wegener’s 1915 Reconstructions
(brown areas are submerged areas of the continents)
Fossil Evidence
Similar Geologic Structures
Wegener’s Sketch (1931)
The Appalachian-Caledonian Mountains
~250 Ma
Paleoclimate Evidence
~300 Ma
General Dismissal of Wegner’s Theory
• an amateur (not even a geologist!)
• inadequate driving mechanism for horizontal motion
of continents (tidal energy, earth’s rotation)
• uncertainty about the seafloor and earth’s mantle
• some supporters: DuToit (S Africa), Holmes (England),
but many more detractors especially in the US (“utter,
damn rot!” W.B. Scott, American Phil. Society)
Continental Collapse
Continental Drift (Wegner)
from J. Holden
Continental Drift and
Seafloor Spreading
Holmes Suggests a Mechanism
Mantle Convection
Radiochronologist Arthur Holmes suggested in 1928 that
radioactivity heating of the mantle may cause convection which
could be the driving force behind drift
Sial: felsic upper crust
Siam:
mafic
lower
crust
Paleomagnetism:
New Evidence for Continental Drift in the 1950’s
Declination – azimuth
direction to the magnetic
pole
Inclination – angle that
magnetic field is tilted into
the earth
Apparent Polar Wandering
Investigations of the Seafloor
The Final Pieces of the Puzzle
Earthquakes, Volcanoes,
Ocean Ridges, and Ocean Trenches
Ocean Ridges
Earthquakes
Volcanoes
Ocean Trenches
Benioff Zones
Sites of Ocean Floor Subduction and Melting
Hot Spot and the Aging of Volcanic Islands
The evolution of active volcanic
islands to atolls and seamounts
implied that hot spot (mantle
plumes) are fixed relative to a
moving ocean crust.
Radiometric dating verified this
aging.
Hypothesis of Seafloor Spreading
An “Essay in Geopoetry” by Harry Hess (1961)
Magnetic Striping of the Seafloor
Confirmation of the Seafloor Spreading Hypothesis
Vine and Matthews (1963)
Further Confirmation
Youth of the Ocean Crust
Continental Drift + Seafloor Spreading
Theory of Plate Tectonics Emerges and Becomes Accepted
1965 - J. Tuzo Wilson who had been a promoter of the sea
floor spreading hypothesis and continental drift from the very
beginning adds the concept of transform faults to the model.
A symposium on continental drift was held at the Royal Society
of London in 1965 is regarded as the official start of the
acceptance of plate tectonics by the scientific community.
1966 - Wilson published the paper that referred to previous
plate tectonic reconstructions, introducing the concept of
what is now known as the "Wilson Cycle".
1967, at the American Geophysical Union's meeting, a session
was planned to discuss the pros and cons of the new Theory of
Plate Tectonics. By the time the meeting was convened all
those scheduled to speak about the arguments against Plate
Tectonics backed out!
Next Lecture
Theory of Plate Tectonics
Part 2:
Elements of Plate Tectonics