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Continental Drift
• Hypothesis proposed by Alfred Wegener in 1915
• The idea that Earth’s continents were once
joined in a single landmass and gradually drifted
• This was not believed until the mid-1900’s
• Wegener found many things that he claimed
proved his hypothesis
Evidence for Continental Drift
• Fossils
• Ancient reptile found in both South America
and Western Africa - Mesosaurus
Evidence for Continental Drift
Evidence for Continental Drift
• Geology
• Rocks in Brazil matched perfectly to rocks in
• Limestone in Appalachian Mts. (N.A.) same as
Scottish Highlands
Evidence for Continental Drift
• Climate
• The same glacial till of the same age is found
in southern Africa, South America, India and
• Large coal deposits were formed from tropical
swamps in both N. America and Europe at the
same time
Super Continent
• Wegener named his super continent
• Pangaea
• From the Greek word meaning “all lands”
• Was a continent that reached from pole to
• Began to split apart 200 million years ago
New Evidence
• In the 1960’s scientists really started studying the sea
• Found underwater mountain ranges
• Called mid-ocean ridges
• Found in every ocean
• Seemed to circle the Earth like the seams of a baseball
• Sea-floor Spreading
• Where the ridges form
• Cracks in the crust where molten rock rises, cools, and
forms new crust
New Evidence
• Age of the Sea Floor
• Rock near the cracks are younger then rocks
further away
• Ocean Trenches
• Where the oceanic crust sinks back into the
• Scientists put this new evidence together with
Wegener's hypotheses to create the Theory of
Plate Tectonics
Theory of Plate Tectonics
• States that Earth’s lithosphere is broken into
huge plates that move over the surface of the
• Driving force of this movement is the convection
current in the Asthenosphere
• Most major earthquakes, volcanoes, and
mountain ranges are where two plates meet.
What causes Plate Movement
• Convection Current
• Motion that transfers heat energy in a
• As molten rock heats up it becomes less
dense, as it rises it starts to cool
becoming more dense and sinking again
Convection Currents Create
• Ridge Push
• Happens at mid-ocean ridges
• Pushes plates apart
• Creates new sea floor
• Slab Pull
• Happens at Ocean Trenches
• The denser plate sinks
• Melts back into the Asthenosphere
Hot Spots
• Area where hot molten rock rises to the surface in
plumes (columns) from the mantle
• Usually found in the middle of tectonic plates but
can be near boundaries
• Hawaiian Islands
• Can tell the direction and speed of plate
• Yellowstone National Park
• Sometimes found at divergent boundaries
• Iceland
• Azores
• Galapagos Islands
Magnetic Reversal
• Recorded in the sea floor
• Magnetic minerals in igneous rock lines up
according to the direction of the poles
• Last one was about 760,000 yrs ago
• Flipped 170 times over the last 100 million
• The prediction for it happening again is
1,500 – 2,000 yrs from now