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Continental Drift
Evidence: Fossils
Evidence: Rock Sequences
Evidence: Glaciation
Evidence: Glaciation (cont’d)
Evidence: Rock Types/Climates
• glacial till &
striations (scratches on
rock) = polar climate
• sand dunes =
desert climate
• coral reefs =
tropical climate
Evidence: Rock Types/Climates
Evidence: Polar Wander??
“Uh-Oh!”: Problems with
Continental Drift….
…an alternate explanation?
1928: Arthur Holmes suggested fluids rise
toward the crust, spread, and sink again.
1930: Wegner dies, unable to adapt
Holmes’ idea to his theory.
Mantle/Crust Structure
Earth’s Structure
A New Hope: Sea-floor Spreading
Testing the model….
Paleomagnetic reversals….
The seafloor spreading model
is confirmed!
Today’s model:
Boundaries: Converging
Boundaries: Diverging
Boundaries: Transform
So, what causes plates to move?
• Convection currents: drag and move the
lithospheric plates above the asthenosphere
(three sources of heat produce the convection curents):
(1)cooling of Earth’s core
(2)radioactivity within the mantle/crust
(3)cooling of mantle
Convection Cells
Convection Cells
SUMMARY OF P.T.:
• The plate tectonic model suggests:
(1) continents can move across the
surface of the globe
(2) patterns of volcanism can
change and shift across the globe
as plates and their boundaries
evolve and move
(3) new oceans may grow
(4) oceans basins close and are
deformed to produce mountains