1. PLATE TECTONICS

2. Alfred Wegener German scientist
1912 hypothesized that the continents were once joined together in a single supercontinent, which then broke into pieces that moved apart.
Called the supercontinent Pangaea = “all land”
Hypothesized that continents moved in a process called Continental Drift.
ucmp.berkeley.edu

3. Evidence for Continental Drift
The shapes of North and South America fit together with Europe and Africa.

4. Evidence for Continental Drift
Cynognathus
Fossils of land-based plants and animals were discovered on continents separated by oceans.
Glossopteris

5. suu.edu

6. Evidence for Continental Drift
Mountain ranges, rock layers and glacial markers also matched up across continents

8. Problems with Continental Drift
Unable to explain how the continents plowed through the sea floor
Unable to explain what force could move continents

9. New Evidence
Mid 1900s scientists found a chain of underwater mountains which they called the mid-ocean ridge.
platetectonics.com

10. Mid-ocean Ridge Extends into all of Earth’s oceans
Forms the world’s longest mountain chain

11. Mid-ocean Ridge
Found a deep valley running the length of the ridge’s crest
Discovered that the rocks of the ocean floor were youngest near the ridge

12. Harry Hess American geologist
Proposed the theory of sea-floor spreading in 1960

13. Sea-floor Spreading
Sea-floor spreading = the process by which new oceanic crust is created at mid-ocean ridges as older crust moves away.

14. Sea-floor Spreading Magma is pulled upward at mid ocean ridge
Crust on either side of the ridge is pushed apart.
Magma from the mantle solidifies to form new oceanic crust.

16. Sea-floor spreading creates new oceanic crust at mid-ocean ridges
Sea-floor spreading creates new oceanic crust at mid-ocean ridges. Subduction destroys old oceanic crust at subduction zones. Ocean floor is renewed about every 200 million years.
geosci.usyd.edu.au

17. Subduction

18. geosci.usyd.edu.au

19. Evidence of Sea-floor Spreading
Magnetic “stripes”
Age of rocks

20. Magnetic “stripes”
The Earth’s magnetic field has reversed itself several times.
Stripes form when Earth’s magnetic field caused rock crystals to line up in a certain way before the rock solidified.
Samples of rocks on either side of the mid-ocean ridge had identical parallel magnetic “stripes”.

21. Magnetic Stripes

22. Age of Rocks
Scientists used radioactive dating to determine the ages of rocks from the ocean floor.
Rocks closer to the mid-ocean ridge were younger, rocks further from the ridge were older.

23. Theory of Plate Tectonics
Evidence of sea-floor spreading provided a way for the pieces of crust to move.
Theory states that Earth’s plates are constantly moving at different rates and in different directions.
What force is powerful enough to move the plates?...convection currents

24. Convection Currents Hot rock rises at mid-ocean ridges
As it cools, it spreads out horizontally as ocean lithosphere
Sinks back into the mantle at subduction zones
Plate motions are the visible part of mantle convection

25. Where does the heat come from?
The heat that drives the convection in the mantle comes from two sources:
Earth was very hot when it formed, some heat comes from the gradual cooling of the interior
Decay of radioactive isotopes that are in the mantle and the crust

26. Plate Boundaries Three types of plate boundaries Divergent Convergent
Transform

27. Divergent Boundary Plates move away from each other
Mid-ocean ridge is a divergent boundary
New rock is formed

28. Convergent Boundary Plates come together or collide
Oceanic vs. continental crust = oceanic crust subducts under continental crust
Continental vs. continental = crust buckles, folds creating mountains

29. Transform Boundary Plates slide past each other in opposite directions
Rock is neither created nor destroyed at transform boundaries

30. Mountain Building Where do most mountains form? Along plate boundaries
Divergent or convergent boundaries