Plate Tectonics

1. Plate Tectonics Chapter 3 (p 61-82)

2. A New Understanding of Earth • Earth has a geologically active surface • How do internal layering and heat contribute to mountain building, the arrangement of continents, the nature of the seafloors, and the wealth of seemingly randomly distributed geological features found everywhere? • Patterns?

3. The Age Debate • Know to be 4.6 billion years old • Late 1700s most scientists believed that earth was about 6,000 years old • Based on interpretation of the Bible • Uniformitarianism – earth processes happening today are identical to those in the past • Going on for a very long time based on current rates

4. The Age Debate • Catastrophism – biblical catastrophic events shaped the young earth • Flood • Explained mountain-top fossils • Natural selection • Time was needed for many species to exist Age-of-earth arguments led to discoveries during the 1800s  ancient earth http://images.google.com/imgres?imgurl=http://www.cnn.com/2000/NATURE/09/13/great.flood.finds.ap/beam.ap.jpg&imgrefurl=http://www.cnn.com/2000/NATURE/09/13/great.flood.finds.ap/index.html&h=168&w=220&sz=12&hl=en&start=15&tbnid=bA5UiDTi_m_xCM:&tbnh=82&tbnw=107&prev=/images%3Fq%3Dgreat%2Bflood%26gbv%3D2%26svnum%3D10%26hl%3Den

5. A Puzzling Fit • Fit of South Atlantic continents • Evidence for single large landmass • Fossil evidence • Fit of continental shelves • All by chance? • No mechanism for continental movement suggested

6. Continental Drift • 1912 – Alfred Wegener • Pangaea (pan = all, gaea = earth) • Single supercontinent • Pieces broke apart 200 million years ago • Still moving today • Evidence • Fit of shorelines • Fossils of tropical plants in Antarctica

7. Seismic Events

8. Seafloor Spreading • 1960 - Mid-Atlantic ridge suggested as origin of new seafloor (spreading center) • Explains “fit” of continents • Mechanism for movement – convection currents in mantle • Then ridges should be hot  they are • New crust should become more dense over time  it does • Crust furthest from ridge should be oldest  it is

9. Where does the old crust go? • Subduction zones • Crust plunges into the mantle in the Pacific • Creates a balanced system

10. Plate Tectonics • 1965 - John Tuzo Wilson suggests that there are 12 plates that make up Earth’s lithosphere (crust) • These plates float on the asthenosphere • Moved by hot mantle becoming less dense  rising • Lifts and cracks the crust = plate edges • Avg. movement ~ 2 in per year

12. Plate Tectonics • Plate movement caused by two forces: • Plates form and slide off the raised ridges of the spreading centers • Plates are pulled downward into the mantle by their cool, dense leading edges • This theory explains many previously unanswered questions

13. Major Plates

14. Plate Boundaries

15. Divergent Plate Boundary • Line along which 2 plates are moving apart • Mid-Atlantic Ridge • Cooling magma creates new crust

16. Convergent Plate Boundary • Areas of violent geologic activity where plates are pushed together • Oceanic crust is destroyed

17. Convergent Plate Boundary • What happens when 2 convergent plates are of equal density?

18. Transform Plate Boundary • Plates moving laterally past one another • Necessary since Earth is a sphere • Does not create or destroy crust • Creates earthquakes

19. Transform Plate Boundary