• Plate tectonics – Earth’s surface composed thick plates that move
• Intense geologic activity is concentrated at plate boundaries
• Combination of continental drift and seafloor spreading hypotheses proposed in late 1960s
Review: Three Types of Plate Boundaries
Transform Convergent Divergent (strike-slip) (subduction) (spreading)
But how do we know that plates move at all ?
Early Case for Continental Drift
• Puzzle-piece fit of coastlines of Africa and South America has long been known
•In early 1900s, Alfred Wegner noted South America, Africa, India, Antarctica, and Australia have almost identical rocks and fossils
Early Case for Continental Drift
– Glossopteris (plant), Lystrosaurus and Cynognathus (animals) fossils found on all five continents
– Mesosaurus (reptile) fossils found in Brazil and South Africa only
Glaciers
• Most of the Earth’s ice is found in Antarctic continental glacier. • Where are some other continental glaciers ?
• Glacial striations on a rock from stones grinding at the base of a heavy ice sheet leave these shiny linear marks on the bedrock below.
Glacial Characteristics
• Glaciers flow downhill as a solid mass that creates channels, and walls made of ground up rock debris known as a merraine.
Erosional Landscapes
• Erosional landforms produced by valley glaciers include: – U-shaped valleys
– Hanging valleys
• Smaller tributary glacial valleys left stranded above more quickly eroded central valleys
Early Case for Continental Drift
• Wegner reassembled continents into the supercontinent Pangaea
• Late Paleozoic glaciation patterns on southern continents best explained by their reconstruction into (Pangaea) Gondwanaland
Early Case for Continental Drift
• Coal beds of North America and Europe indicate Laurasia super continent
• Continental Drift hypothesis initially rejected
– Wegener could not come up with viable driving force
– continents should not be able to “plow through” sea floor rocks
The Earth’s Magnetic Field Can Give Us Clues
Paleomagnetism and Continental Drift Revived • Studies of rock magnetism allowed
determination of magnetic pole locations (close to geographic poles)
• Paleomagnetism uses mineral magnetic alignment and dip angle to determine the distance to the magnetic pole when rocks formed
– Steeper dip angles indicate rocks formed closer to the magnetic poles
• Rocks with increasing age point to pole locations increasingly far from present magnetic pole positions
Paleomagnetism and Continental Drift Revived
• Apparent polar wander curves for different continents suggested plate movement !
• Wegner was right!
* Earthquakes don’t occur randomly – but in patterns
* Friction and motion at plate boundaries seem to produce earthquakes
Early Case for Continental Drift
• Puzzle-piece fit of coastlines of Africa and South America has long been known
Plate Tectonics and the Scientific Method
How does an idea become a theory ?
* When Wegner first suggest the idea of Plate tectonic what was it ? a) theory b) hypothesis c) data d) proof
* What data was later acquired to test this ?
* When did plate tectonics become an accepted theory ? (What discovery proved the original predictions ?)
The Scientific Method
Step 1: A question is asked
Step 2: Gather data
Step 3: Use data to propose a hypothesis (Has this hypothesis been tested yet ? NO!
Step 4: Make a prediction (based on hypothesis)
Step 5: Test prediction (with new data)
Step 6: Hypothesis becomes a theory ! Yeah! (if predictions and tests are