Name: _______________________________________________ Date: ___________ Period: ____________
9.1: Continental Drift
I. An Idea Before Its Time
• Better world maps brought an idea: continents seem to fit together like puzzle pieces.
• 1915 – German scientist Alfred Wegener – continental drift hypothesis: continents had once been joined
to form a single supercontinent
▫ He called it Pangaea (“all land”)
▫ Further hypothesized that about 200 million years ago, Pangaea began breaking and drifting apart.
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Evidence #1: The Continental Puzzle
Continents seem to fit like a puzzle
▫ EX: South America and Africa
▫ Opponents argued that erosion continually changes shorelines over time.
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Evidence #2: Matching Fossils
Several fossil organisms have been found on different landmasses.
▫ The organisms could not have crossed vast seas.
▫ Opponents argued for the possibility of land bridges when sea levels were lower, but there would be
evidence on the ocean floor.
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Evidence #3: Rock Types & Structures
Several mountain belts that end at one coastline reappear on a landmass across the ocean with matching
rock types.
▫ EX: The Appalachians end in Newfoundland, pick up in the British Isles and Scandinavia.
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Evidence #4: Ancient Climates
Scratches and grooves left by the movement of glaciers make it appear that some glaciers moved from the
sea onto land.
▫ Pangaea connects these areas.
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At the time of the glaciers, large tropical swamps existed in the Northern Hemisphere.
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Wegener’s model of Pangaea places the Northern Hemisphere continents near the tropics and the onceglaciated landmasses near the South Pole.
II. Rejecting a Hypothesis
• A major criticism was that Wegener couldn’t give a mechanism for how it happened.
▫ He suggested tidal influence of the moon; the force needed would’ve stopped the Earth.
▫ He suggested the continents acted like ice-breaker ships through the ocean floor; no evidence.
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In 1968, extensive data on earthquake activity and Earth’s magnetic field led to a new theory: plate
tectonics
9.2: Plate Tectonics
I. Earth’s Major Plates
• The uppermost mantle and crust make up the lithosphere, a strong, rigid, stony shell.
▫ The lithosphere sits atop the asthenosphere, a weaker, plastic layer of the mantle.
• The lithosphere is divided into plates that continuously move and change shape as a unit.
▫ There are 7 major plates: North American, South American, Pacific, African, Eurasian, AustralianIndian, & Antarctic
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There are 6 intermediate plates: Caribbean, Nazca, Phillipine, Arabian, Cocos, & Scotia
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Several of the plates include a continent and a large area of seafloor.
Lithospheric plates move about 5 cm per year; this is driven by the unequal distribution of heat
within the Earth (creates convection currents).
 Movement of lithospheric plates generates earthquakes, creates volcanoes, and deforms large
masses of rock into mountains.
II. Types of Plate Boundaries
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Divergent Boundaries
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Two plates move apart
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Comes from upwelling of material from the mantle, usually to create new seafloor
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Usually called “rift valleys”
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Convergent Boundaries
Two plates move together
Oceanic lithosphere goes under continental OR two continental plates collide and eventually
merge
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Transform Fault Boundaries
Two plates grind past each other without making or destroying lithosphere
 EX: San Andreas Fault
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Total surface area of Earth doesn’t change because plate shrinkage equals plate growth.
▫ Antarctic plate is getting larger; Philippine plate is shrinking