1. Plate Tectonics: Overview
Earth’s crust: broken into about 12 major plates
Plates move several cm each year
Asthenosphere drives the plate
Slow moving layer
Convection currents
Plates can be either
Oceanic: thinner but MORE dense (sink)
Continental: thicker but LESS dense (rise)

2. Table of Contents: Types of Plate Boundaries
Convergent boundaries : Compression forces push plates together
Divergent boundaries: Tension forces move plates apart
Transform Plate boundaries: Shearing forces cause plates to slide against each other (opposite directions or same different rates)

3. Convergent Boundary Type 1
Continental—oceanic: denser oceanic crust subducts under continental crust.
Oceanic plate melts as it is driven down, less dense material punches through continental plate to create volcanoes and/or mountains
Forms a subduction zone and oceanic trench
Earthquakes occur as plate is subducted.
Ex: Andes Mtns and Peru-Chile trench
animation of the processes at a destructive plate boundary

4. Convergent Boundary Type 2
Oceanic—oceanic: one subducts creating a VERY deep trench and an island arc
Ex. Kuril Islands and Marianas Trench: plunges deeper into the Earth's interior (~11,000 m) than Mt Everest, the world's tallest mountain, rises above sea level (~8,854 m).

5. Island arcs (archipelagos) are created along plate boundaries!
Hot spots (like Hawaii) are located within the margins of center, at plate boundaries (everywhere) magma pushes to surface
Do NOT confuse the
creation of islands at
convergent boundaries with
islands created at hot spots!
Hotspot animation

6. Convergent Boundary Type 3
Continental—Continental: Both resist subduction
Land buckles/folds upward to create fold mountains
No volcanic activity but major earthquakes often occur due to collision. Ex: Himalayas: Indo-Australian and Eurasian Plate The Making of: the Himalayas

7. Divergent Plate boundaries
Processes: Hot magma rises to fill the 'gap' creating new crust and mountain ranges to form the mid-ocean ridge.
Both earthquakes and volcanoes occur at this type of boundary.
Rising magma can build up above ocean surface to create a volcanic island (for example Surtsey, Iceland).
(ex. North American and Eurasian Plate form the Mid-Atlantic Ridge) animation

9. Transform Plate boundaries
Movement: two plates move past each other crust is neither created or destroyed Processes:
Shearing forces build btw. the rock
Pressure builds b/c of friction between the plates, slipping releases energy = earthquake. Example: San Andreas Fault (N. American + Pacific Plates)

10. Divergent Convergent Transform
Continental crust
 rift valley
Oceanic crust  mid-ocean ridge
2 continental plates  mountain range
Plates move against each other
Stress builds up
Stress is released
earthquake
2 oceanic plates or
Oceanic/continental
subduction

11. Lets put it all together…

13. Review Name the type of plate boundaries
Name the plates: continental or oceanic
Name resulting features or earthquakes

14. Wegener:Early Observations
German Scientist Alfred Wegener (1912)
proposed the idea of moving continents
scientific hypothesis based on fossil evidence, rock & mineral formations

15. Evidence from Rock Formations
Similar rock types on opposite sides of the Atlantic Ocean.
Appalachian Mountains in the US identical to rock in Greenland and Europe.

16. Fossil Evidence: Reptiles
Lystrosaurus:
Early Triassic Period (250 mya)
Land Animal
Antarctica, India, S Africa
Cynognathus:
Late Triassic Period
“Mammal-like reptile”
S Africa, South America, China, Antarctica

17. Evidence from Fossils

18. Climate Evidence: Ferns & Coal
Certain climates support diff fern types
Land connections were proved by same fossils on different continents
Coal in Northern US originated in an equatorial climate but drifted North over millions of years

19. Why wasn’t it accepted? Problems with his theory:
Couldn’t explain how large land masses were pushed such great distances
Didn’t explain how continents moved through the solid crust

20. Accepted Today because:
Know that asthenosphere has convection currents
Currents push magma which then pushes plates
Plates move at different rates
Not all pieces of plate move at same rate or same direction as the plate in general!

22. Seafloor Spreading

23. Age of Rock Rock NEAR ocean ridge= YOUNGER
Rock FAR from ocean ridge= OLDER
Oceanic rocks are relatively young compared to continental rocks—>3.8 billion years old but oceanic is subduction zones
Thickness increases with distance from ridge

24. Magnetism & Seafloor
Earth’s magnetic field generated by flow of molten iron in outer core (Fe)compass
Flow changes magnetic reversal compass point S
Oceanic crust basaltic rock-contains large amts of Fe-bearing minerals  MAGNETIC!
Ocean floor shows record of past reversals using magnetometers under water “strips” along seafloor
“strips” are mirror images on either side of ridge symmetrical!! WOW!

25. When iron cools it is magnetized by the magnetic field of the earth.
So the magnetic field of the sea floor at any time is aligned with the Earth's magnetic field.
But the Earth's magnetic field reverses every several thousand years and so when that happens there is new material on the sea floor which is magnetically orientated differently than the older material.
This discovery led to the understanding that we can study rock samples far from ridge and understand what core was like/magnetic polarity millions of years ago.

26. Magnetic Seafloor Symmetry

27. Isochron
Isochron imaginary line on map showing points that have same age same time)

28. Seafloor Spreading New oceanic crust
ocean ridges
deep-sea trenches
Magma forced to crust along ridge b/c hotter and less dense than mantle
Fills gap of spreading ridge
Magma solidifies ocean floor
Continents like passengers ride along while ocean crust slowly moves