Natural Disasters

1. Natural Disasters Plate Tectonics & Physical Hazards Chile Earthquake Chile Tsunami

2. Continental Drift • Geographic fit of continents • Fossils • Mountains • Glaciation • Researchers noted geographic fit of continents • e.g. Africa and S. America • Atlantic formed by separation of Africa from S. America • Seuss, 1885, proposed super continent by studying fossils, rocks, mountains • Wegener and Taylor, early 1900’s, proposed continental drift and Pangaea • Evidence supporting the idea that the continents had drifted.

3. Continental Drift – Geographic Fit • Continents seem to fit like pieces of a puzzle

4. Continental Drift - Fossils • Similar distribution of fossils such as Mesosaurus

5. Continental Drift – Mountain Ranges • Mountain ranges match across oceans

6. Continental Drift - Glaciation • Grooves left by glaciers indicate location of glaciers and direction of movement • Past glaciation indicates position of paleocontinents

7. Problem with Continental Drift • Alfred Wegener • Presented hypothesis to other professionals • Did not provide plausible mechanism to explain how continents drifted

8. New Theory Developed – Seafloor Spreading • Harry Hess suggested new seafloor is created at mid-ocean ridges and destroyed in deep ocean trenches

9. Seafloor Spreading • Continental drift reexamined in 1960’s with new information • Supporting evidence for seafloor spreading • Earthquakes • Volcanos • Age of Seafloor • Paleomagnetism

10. Evidence for Seafloor SpreadingWorld Seismicity • Earthquake distribution matches plate boundaries

11. Evidence for Seafloor SpreadingVolcanism • Volcanoes match some plate boundaries; some are hot spots

12. Evidence for Seafloor Spreading • Age of Seafloor • Youngest sea floor is at mid-ocean ridge • Oldest sea floor away from mid-ocean ridge

13. Seafloor Spreading - Paleomagnetism • Earth has a magnetic field • When rocks cool at the Earth’s surface, they record Earth’s magnetic field • Earth’s magnetic field reverses approx. every 600,000 years • As seafloor spreads, normal and reverse polarities are preserved in the rock record

14. Seafloor Spreading – Age of Seafloor • Young rocks (red) found near mid-ocean ridges (MOR) • Away from MOR, age of seafloor gets progressively older (blue)

15. What Drives Plate Motion? • Convection Currents • Air heats up, expands, and rises • As air moves away, it cools, contracts, and sinks

16. Mechanism for Seafloor Spreading • Convection Currents • As heat rises, it moves away at spreading centers pulling plates apart • Plates slide over asthenosphere • The upper mantle then cools and becomes more dense – sinking at ocean trenches

17. Plate Tectonics Theory • John Tuzo Wilson combined ideas of Continental Drift and Seafloor Spreading into “Plate Tectonics”

18. Earth’s Structure • Earth’s internal structure • Chemical Composition • Physical Property

19. Earth’s Structure • Chemical Composition Layers divided by its composition • Crust • Continental crust • Granite (2.8 g/cm3) • Oceanic Crust • Basalt (3.0 g/cm3) • Mantle • Composed of Fe & Mg • Core • Composed of Fe & Ni • Two parts • Outer core • Inner core

20. Earth’s Structure • Physical Properties Layers divided into rigid, plastic, or liquid • Lithosphere • Rigid • Asthenosphere • Plastic • Mesosphere • Rigid • Outer Core • Liquid Layer • Inner Core • Solid Layer

21. Lithospheric Plates • Comprise approx. 12 large plates and 12 smaller plates • Lithospheric plates are rigid layers that flow over a partially molten (plastic) asthenosphere

22. Plate Boundaries • Where plate boundaries meet . . . • Divergent • Convergent • Transform

23. Divergent Plate Boundary • Boundaries where plates pull apart • New crust is being formed

24. Divergent Examples • East Africa Rift Zone • Mid-Atlantic Ocean Ridge

25. Convergent Plate Boundary • Plates move toward each other (collide) • Crust is being destroyed • Three types: • Ocean-continent • Ocean-ocean • Continent-continent

26. Ocean-Continent Collision • Produce volcanic arc system

27. Ocean-Continent Examples • Cascade Ranges – Northern California, Oregon, and Washington (left) • Form as oceanic crust collides with continental crust • Mount St. Helens, Washington (right)

28. Ocean-Ocean Collision • Form a chain of island arc volcanoes

29. Ocean-Ocean Examples • Japanese Islands • Form along a trench as two oceanic plates collide • Mount Fuji (right) active volcano in Japan

30. Continental-Continental Collision • Continental crust have same densities • Neither plates sink (subduct) • Form high mountain chains

31. Continental-Continental Examples • Himalayas – home of Mount Everest (highest mountain in the world)

32. Transform Plate Boundaries • Plates slide past one another • Crust is neither created nor destroyed

33. Convergent Boundary Examples • San Andreas Fault • Runs almost the entire length of California

34. Hot Spot Volcanism • Hot spot volcanoes around the world • Site of mantle plumes from deep within the core-mantle boundary • Lithospheric plates slide over hot spots leaving a chain of dormant (not active) volcanoes

35. Hot Spot Examples • Formation of a volcanic island chain as oceanic plate moves over a stationary hot spot • The age of the islands increases toward the left • New islands will continue to form over the hot spot • Loihi will be the next island in the chain