1. Evolution of the Earth and Our Restless Planet: Chapter 33
Plate Tectonics
Evolution of the Earth and Our Restless Planet: Chapter 33

2. Review: Interior

3. 33.1 The Theory of Continental Drift
Scientists in the early 20th century believed
Oceans and continents were fixed in place
The surface is a skin spread over a cooling interior
Cooling caused contraction of the skin= wrinkles and bumps aka mountains and valleys

4. Alfred Wagner Thought Differently!
He believed:
The continents are slowly moving
All the continents has once been attached in a supercontinent and this was called Pangaea “all land”
Pangaea split and the continents moved apart
The boundary of each continent is not at the shoreline but the continental shelf

5. Alfred Wegener cont.
No one believed Wagner…..at least not yet!

6. Scientific Revolution
Discovery of the Earth’s Magnetic Field gave a window into the past
Paleomagnetism (magnetism from the geologic past…shows 3 major things)
The position of the magnetic north versus the geographic north pole
Direction to the magnetic pole at the time the rock was formed
Magnetic latitude of the rock’s location at the time it was formed
The movement of the magnetic pole over time(500 million years) suggests that either the pole moves around or the continents move

7. The Earths North and South pole have flipped many times
These leaves magnetic ‘stripes’ in rock containing iron minerals

8. Seafloor Spreading: H. H. Hess
1950’s had detailed mapping of the ocean floor
Figured out
Some really deep parts are next to the continents and some shallower parts are out in the middle due to the mountains under the water
Why?

9. Finally Wagner's’ Ideas became the: Theory of Plate Tectonics
The earth’s outer shell, the lithosphere, is divided into 8 large plates and a number of smaller ones
Most earthquakes, volcanoes, and mountains occur along the borders

10. All Based on Motion
It turns out that the upper section of the mantle is stuck to the underside side of the crust to form what we call tectonic plates

11. Evidence of Tectonic Plates
Our first evidence of tectonic motion is based on similar fossils and
rock types on opposing sides of the ocean

12. Tectonic Plates Today plate boundaries are determined by examining
the location of volcanoes and earthquakes.
Volcanoes result from the friction (heat) of the plates
motion.
Earthquakes occur where plate rub against one another

13. Tectonic Plates
Volcanoes

14. Tectonic Plates

15. Why do the Plates Move?
No single idea explains everything but we can identify several forces that contribute to the movement of the plates.
Slab pull
The sinking of the cooled dense oceanic plates pulls on the rest of the plate
Ridge rises
The material deposited on the top of the ridge slides downs from the rise pushing on the plate
Convection
Movement within the mantle could be part of the driving force behind the motion of the plates.

16. Why do the Plates Move?

17. Why are they spreading? Why is the Atlantic still getting wider
The plates are pulled apart by convection currents in the mantle below
Caused by heat released from natural radioactive processes
At the mid Atlantic ridge molten rock from below rises up to fill the gap with new basaltic rock

18. Plate Boundaries Convergent – plates move toward one another
Divergent – plates move away from each other
Transform-Fault – plate moves sideways from each other

19. Plate Boundaries

20. Convergent Plates

21. Convergent Plates Ocean-Continental Convergence
Ocean-Ocean Convergence
Continental-Continental Convergence

22. Divergent Plates
Spreading centers on the ocean floor and on land (create rifts)
These create the beginning of a new ocean basin

23. Transform Plates

24. Transform Plates
San Andreas
Fault

25. Mid-Plate Hotspots
Hawaii

26. Pangaea

27. Pangaea

28. Pangaea
The break up
of Pangaea

29. Where are we going? We appear to be headed for another
super continent as North America,
South America, Asia and Australia converge in the
ever shrinking Pacific Ocean