Plate Tectonics

1. Plate Tectonics • The theory --the surface of the Earth is broken into large plates. The size and position of these plates change over time. The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes, and mountain building. • Plate tectonics is a combination of two earlier ideas, continental drift and sea-floor spreading. Continental drift is the movement of continents over the Earth's surface and in their change in position relative to each other. Sea-floor spreading is the creation of new oceanic crust at mid-ocean ridges and movement of the crust away from the mid-ocean ridges. http://volcano.und.edu/vwdocs/vwlessons/plate_tectonics/introduction.html

2. Earth Structure

3. Mexico quake-- magnitude at 7.0, and epicenter was in the western Pacific state of Michoacan. Its depth was about 40 miles • Indonesia-- earthquake off coast of Indonesia Wednesday, April 11 with magnitude of 8.6, sparking tsunami warnings

4. Lithosphere • Consists of continental, oceanic and upper part of mantle • Continents composed of granite-type rock, quartz and feldspar minerals, density+2.8g/cm3 • Oceanic crust formed of basalt; basalt rich in iron/magnesium minerals, density+3.0 g/cm3 • Lithosphere is rigid layer of crust and mantle overlying partially-molten asthenosphere

5. Continental Drift Evidence • 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.

6. Continental Drift Geographic Fit • Continents seem to fit together like pieces of a puzzle

7. Continental DriftFossils • Similar distribution of fossils such as the Mesosaurus

8. Continental DriftMountains • Mountain ranges match across oceans

9. Continental DriftGlaciation • Glacial ages and climate evidence

10. Objectionsto theContinental Drift Model • Wegener did not provide a plausible mechanism to explain how the continents could have drifted.

11. Seafloor Spreading • Continental drift reexamined in 1960’s with new information • New theory developed – Seafloor spreading • Supporting evidence for seafloor spreading • World seismicity • Volcanism • Age of seafloor • Paleomagnetism • Heat flow • Theory combining continental drift and seafloor spreading termed “Plate Tectonics”

12. Seafloor Spreading • New sea floor created at the mid-ocean ridge and destroyed in deep ocean trenches

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

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

15. Evidence for Seafloor SpreadingAge of Seafloor • Youngest sea floor is at mid-ocean ridge • Oldest sea floor away from mid-ocean ridge

16. Evidence for Seafloor SpreadingPaleomagnetism • When rocks cool at the Earth’s surface, they record Earth’s magnetic field (normal or reverse polarity) • Earth has a magnetic field - Probably caused by rotation of solid inner core in liquid outer core (both mostly Fe)

17. Evidence for Seafloor SpreadingPaleomagnetism • Paleomagnetic studies indicate alternating stripes of normal and reverse polarity at the mid-ocean ridge.

18. Seafloor SpreadingHeat Flow • In 1960, convection currents were proposed as driving force to move continents

19. Theory of Plate Tectonics • John Tuzo Wilson combined ideas of continental drift and seafloor spreading into “Plate Tectonics”

20. Principles of Plate Tectonics • Earth’s outermost layer composed of thin rigid plates moving horizontally • Plates interact with each other along their edges (plate boundaries) • Plate boundaries have a high degree of tectonic activity • mountain building • earthquakes • volcanoes

21. Plate BoundariesThree types

22. Plate BoundariesDivergent • Plates move away from each other • New crust is being formed

23. Divergent Plate BoundariesExamples Mid-Atlantic Ocean Ridge East African Rift

24. Plate BoundariesConvergent Three Types: • Ocean-continent • Ocean-ocean • Plates are moving toward each other • Crust is being destroyed • Continent-continent

25. Convergent Plate BoundariesExamples Mount Fuji, Japan Mount Lassen, California Andes, South America

26. Plate BoundariesTransform xxxxxxxxxxxxxxxxxxx • Plates slide past one another • Crust is neither created nor destroyed

27. Transform Plate BoundariesExamples Calexico, California San Andreas Fault Carrizo Plains, Central California

28. Stop Here

29. Origin of the Earth • Meteors and Asteroids bombarded the Earth • Gravitational compression • Density Stratified planet

30. Earth’s Interior • Crust • Outermost layer • Continental – granite-type, density=2.8 g/cm • Oceanic – basalt, density=3.0 g/cm • Rigid • Core • dense • Nickel and Iron • Inner core – solid • Outer core – liquid • Mantle • Less dense than core • Iron and Magnesium silicates • Partially molten

31. Granite Basalt Evidence of Internal Structure • Density • calculate density of Earth • Speculate on probable compositions • Meteorites • Use composition and age to determine composition and age of Earth • Seismic waves • Travel times and direction give indication of internal structure of Earth

32. Types of Seismic Waves • P waves • Primary, pressure, push and pull • Travel fastest of the seismic waves (average 6 km/sec) • Travels through solids and liquids • S waves • Secondary, shaking, side-to-side • Slower (average 4 km/sec) • Travel through solids only

33. Seismic Waves Through Earth