Transcript The plate tectonic revolution part I.

The Plate Tectonics
Revolution
A paradigm shift
in the Earth
Sciences
Topics:
 Wegener’s Theory of continental drift
 Seafloor spreading: the Vine-MatthewsMorley hypothesis
 The plate-tectonics revolution
 What is a plate? How many are there?
 J. Tuzo Wilson and his cycles
 Triple junctions
 Promoted continental drift
with publication of The Origin
of Continents and Oceans in
1915
 Alternative to land bridges
as theory to explain similar
fossils on different continents
Alfred Wegener (1880-1930)
 Supporting evidence:
Appalachian mountains match
Scottish Highlands; similar
rocks in South Africa and Brazil
 Coined term Pangea (Greek,
meaning “all the Earth”)
Appalachians (eastern N.A.)
Alfred Wegener
Scottish Highlands
In 1962, U.S. Navy marine
geologist Harry Hess
proposed that ocean crust is
much younger than
continental crust, and forms
by “seafloor spreading”
Harry Hess
Raff, A. D. and R. G. Mason.
1961. Magnetic survey off the
west coast of North America,
40º N. latitude to 52º N.
latitude. Geological Society of
America Bulletin 72:1267-1270.
 Magnetic surveys
revealed a distinctive
striped pattern in the
oceans
In the meantime,
scientists in U.S.A.
and Australia
discovered that the
Earth’s magnetic field
has reversed many
times
The Vine-Matthews Hypothesis
Magnetic Stripes =
Seafloor Spreading +
Magnetic Field Reversals
 Basalt extruded at
mid-ocean ridges is
magnetized in the
direction of the Earth’s
magnetic field
Vine, F. J. and D. H.
Matthews. 1963. Magnetic
anomalies over oceanic
ridges. Nature 199:947-949.
… but did they discover this
first?
Vines-MatthewsMorley hypothesis
 several months before Vines
and Matthews, Larry Morley
(Geological Survey of Canada) had
submitted a paper to Nature with
the same conclusion
 His paper was rejected as “too
speculative”
 Revisionist history:
Vines-Matthews hypothesis is
now known as the
Larry Morley
The new global tectonics of the 1970’s
Earth’s ultimate recycling program
 New oceanic
lithosphere is created
at mid-ocean ridges
 Oceanic lithosphere
is destroyed at deepsea trenches
(subduction zones)
 This process
largely explains the
global distribution of
earthquakes and
volcanoes
What is the lithosphere?
 Outer part of
the Earth, 100-200
km thick
 Includes the
crust and part of
the upper mantle
 Behaves rigidly
 Forms plates
Tectonic plates
Large plates (6+1):
North American,
South American,
Eurasian, African,
Australian,
Antarctic
Pacific
Small plates (2 + 6):
Arabian, Indian
Contain continental
lithosphere
Purely oceanic
Scotia, Caribbean,
Nazca, Cocos, Juan
de Fuca, Philippine
Sea
J. Tuzo Wilson
• Proposed hotspot
hypothesis in 1963, in
support of Hess’
seafloor spreading
model
• Had difficulty
getting this work
published!
J. Tuzo Wilson
• Proposed transform plate
boundary hypothesis in
1965 to explain linear
fracture zones in oceans
• Realized that motion on an
oceanic transform fault is
opposite to apparent offset
of ridges
The Wilson Cycle
1. Continental
breakup (rifting)
2. Opening of ocean
basin
3. Subduction zones
form
4. Ocean basin
closes
5. Continents
collide and form
high mountains
6. Crust thins by
erosion
Types of plate boundaries