1. Plate Tectonics Theory
What is the Theory of Continental Drift?
What is Plate Tectonics Theory?
What causes the plates to move?
What is the evidence for Plate Tectonics Theory?
What was the missing piece to Alfred Wegener’s theory of Continental Drift?

2. Continental Drift Alfred Wegener: German meteorologist and explorer
The Origin of Continents and Oceans, 1915
Introduced the Theory of Continental Drift
Proposed the supercontinent Pangaea
Pangaea: “all lands”
Very controversial at the time
Couldn’t explain the mechanism that moved the continents

3. Pangaea: the Supercontinent
mya: Paleozoic and Mesozoic Eras
Included all the major continents
200 mya: Early Mesozoic (Triassic)
Atlantic Ocean began forming

4. Pangaea: the Supercontinent
145 mya: Middle Mesozoic (Jurassic)
India moves northward
Atlantic expands
65 mya: Late Mesozoic (Cretaceous)
Australia separates from Antarctica
Present Day: Continents continue to move

5. Wegener's Evidence Shape of continents
Better maps let people see the “puzzle” pieces
People noticed the coastlines of South America and Africa

6. Wegener's Evidence Fossil Correlation Mesosaurus: freshwater reptile
Found in South America and Africa
Cynognathus: land reptile
Lystrosaurus: land reptile
Found in Africa, India, & Antarctica
Glossopteris: fern
Found in South America, Africa, India, Antarctica, & Australia

7. Fossil Correlation

8. Wegener's Evidence Rock Types and Structures Mountain Chains
When the continents are put together, mountain ranges match up
EX: Appalachian Mts., British Isles, Caledonian Mts.

9. Wegener's Evidence Paleoclimate Glacial Deposits and Formations
Layers of glacial till
Glacial grooves and scratches on bedrock
Areas now near the equator, were once at the south pole and covered by ice

10. Wegener's Evidence Paleoclimate Coal Deposits
Areas now in the Northern hemisphere were once at the equator
Lush tropical swamps eventually formed the coal beds of the eastern U.S., Europe, and Siberia

11. Wegener's Evidence Paleoclimate Sedimentary Rocks
Sedimentary rocks form in specific environments
Shale: calm water; swamps, lakes, etc.
Limestone: warm shallow seas
Sandstone: running water; rivers, beaches
or dry, windy environments like deserts
Sedimentary rocks contain fossils that give clues about past climate

12. Where did Wegener go wrong?
How did the continents move?
Tidal force of the Moon? WRONG ☹
Centrifugal force of Earth’s rotation? WRONG ☹
Believed continents “plowed” through the oceanic crust like an icebreaker through ice
Couldn’t explain the “how” so other scientists rejected it
His theory was before its time!

13. Plate Tectonics Theory
A New Theory
New technologies and discoveries led to a reconsideration of Wegener’s theory
Lithosphere
The Earth’s crust and upper mantle are a rigid layer
Plates: pieces of the lithosphere
Continental and Oceanic
Asthenosphere
The weaker layer underlying the lithosphere
Behaves like a fluid over a very long time

14. Plate Tectonics Theory: The Evidence
Paleomagnetism
Earth’s magnetic field switches polarity
Recorded in igneous rocks

15. Plate Tectonics Theory: The Evidence
Earthquake and Volcano Distribution
Earthquakes and volcanoes coincide with plate boundaries

16. Plate Tectonics Theory: The Evidence
Ocean Drilling
Deep Sea Drilling Project
Age of ocean floor increases as you move away from mid-ocean ridges
Ocean basins are much younger than continents
Oceanic crust: none older than ~ 180 million years old
Continents: 4.0 Billion years old

17. Plate Tectonics: The Plates
7 major plates and many smaller plates
Plates move relative to one another
About 5 cm/year on average

18. Driving Forces of Plate Movement
Mantle Convection
Uneven distribution of heat in the Earth’s interior causes thermal convection within the mantle.
Warm mantle rises
Mantle Plumes
Cold mantle sinks
Computer model of mantle convection

19. Driving Forces of Plate Movement
Slab Pull and Ridge Push
Mantle convection pulls oceanic crust down into the asthenosphere
Rising convection currents at mid ocean ridges push plates apart