Week 21: Plate Tectonics
Plate Tectonic Theory is relatively new. It was developed by synthesizing the ideas of
Continental Drift and Sea Floor Spreading.
I.
Continental Drift
a. Proposed by Alfred Wegener in 1912
b. Wegener compiled the evidence to suggest all the landmasses were once
connected as a supercontinent he called PANGEA.
i. Continents seem to fit like pieces of a jigsaw puzzle
ii. Fossil evidence: Mesosaurus fossils found on S. America and
Africa.
iii. Matching Mountain Ranges: Appalachians of N. America match
the Caledonians of Scandinavia
iv. Glacial deposits in locations that now have a tropical climate –
Striations on bedrock in central Africa.
v. Tropical deposits in locations that now have a polar climate –
Coal and oil in Alaska.
c. Problem with Wegener’s theory was he couldn’t explain what could cause
the continents to drift (NO MOTIVE). No known force at the time could
be strong enough to move entire landmasses.
II.
Sea Floor Spreading – Toward the end of WWII a lot of information about
the sea floor was being gathered by submarines.
a. Sonar was used to determine the depths of the seafloor.
b. Large underwater volcanic mountain ranges were discovered  MID
OCEAN RIDGES.
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c. Harry Hess a geologist with the US Navy was able to collect and date
rocks from the bottom of the Atlantic on either side of the Mid Atlantic
Ridge. The youngest rocks were nearest the ridge and as distance from
the ridge increased so did the age of the bedrock. The age of the sea
floor was symmetrical on either side of the ridge.
d. Paleomagnetism (ancient magnetism) also suggested Sea Floor Spreading.
Bands of normal and reversed polarity rocks were also symmetrical on
either side of the ridge.
e. The driving force behind the spread is CONVECTION CELLS IN THE
EARTH’S MANTLE (asthenosphere).
i. In the 1930’s seismic wave analysis led to the distinction between
the Earth’s Inner and Outer cores. The interior was known to be
hot. And the heat of the Earth’s interior causes the Mantle material
to convect, dragging the lithospheric plates along.
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f. Sea Floor Spreading provided a possible mechanism for Wegener’s theory
of Continental Drift.
III.
Plate Tectonic Theory – Combining Continental Drift and Sea Floor
Spreading theories led to the current Plate Tectonic Theory
a. If new ocean floor is being created at the Mid-Ocean Ridges as Harry
Hess suggested, and the Earth isn’t getting any larger, then somewhere in
the world old ocean crust must be getting destroyed  SUBDUCTION.
b. The mid-ocean ridges are Divergent boundaries ( ).
c. Convergent boundaries ( ) could result in subduction as long as
oceanic (mafic & dense) crust is involved.
d. Transform boundaries exist where the plates are moving past each other
(not colliding or separating, just slipping by one another). The San
Andreas Fault in California is a great example here in the US.
e. Hot Spots are locations of rising magma that is independent of the mantle
convection cells. Yellowstone and Hawaii are Hot Spots.
Know all the features associated with each type of boundary. Study the
chart below.
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Transform Plate Boundary
Divergent Plate Boundary
San Andreas Fault – Shallow EQ’s
Mid-Atlantic Ridge – New oceanic crust is formed as mafic
magma rises due to convection currents in the mantle.
Continent & Continent
Because both plates are felsic (low
density) the plates rise up in an
orogeny instead of subduction.
Earthquakes are moderate and
shallow depth.
Convergent Plate Boundary
Continent & Ocean
The oceanic (mafic) plate is more
dense then the continental (felsic)
plate, so the oceanic plate subducts
beneath the continent. A trench
forms. Deep, shallow & moderate
EQ occur. Volcanoes also are
present.
Ocean & Ocean
Since both plates are mafic, the cooler
(older) plate is the one that subducts
under the other. Volcanoes form here
too. Chains of volcanic islands
created by this kind of subduction are
called Island Arcs. EQ’s are of
moderate and deep depths.
Hot spots
The Hot Spot doesn’t change location.
The plume of rising magma is
independent of the convection of the
mantle. The lithospheric plate moves over
the hot spot carrying volcanic islands
away from the plume. Over time the
volcanoes disconnect from the plume and
become extinct. The islands farther from
the hot spot are older and smaller. If they
erode below the sea surface they are
called sea mounts.
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f. The pattern of where earthquakes occur and their depths along with
the locations of volcanoes led to identifying all the plate boundaries.
g. Putting everything together gives us a complex and dynamic crust, with
regions of separation creating new ocean floor, regions of destruction were
old ocean floor returns back down into the mantle, zones of frequent
earthquake activity where plates slide past each other, and other areas
away from the boundaries called passive margins.
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