Continents change position over time
• Alfred Wegener
• Hypothesis – continental drift
• Earth’s continents were once
joined
• Super Continent called Pangaea
• Evidence
• Fossils
• Climate
• geology
• Fossils
• Mesosaurus – found on the west
coast of Africa and the east
coast of South America
• Climate
• Ice scratches on South America
• Tropical plant fossil in Greenland
• Geology
• Rocks that make up the
continents
• North America lines up with
Scotland
• Theory
• Evidence and explanations lead
to the Theory of Plate Tectonics
• Sea Floor
Evidence
• Sea Floor Spreading
• Age of the Sea Floor
• Trenches
• Sea Floor
Spreading
• Ridges form on the sea floor
• As the plates move apart molten
rock pushes up
• Called spreading centers
• Mid-Atlantic Ridge
• Age of the Sea
floor
• The rocks are older the further
you move away from the
spreading centers
• Ocean Trenches
• Deep canyons on the ocean floor
• Dense crust is sinking into the
asthenosphere
• Size of earth
• Earth isn’t changing size because
at the same rate that new crust is
forming (at spreading centers),
old crust is being destroyed (at
trenches)
• How does it
happen?
• Convection Currents
– Hot molten rock rises
– Cooled rock sinks
– Moves the tectonic plates like a
conveyer belt
Convection Currents
• Hypothesis
becomes theory
• Based on the facts from the
ocean floor the hypothesis of
Continental Drift becomes the
Theory of Plate Tectonics