2. Upwelling Magma new crust added felsic and low density “light and fluffy” mafic and high density “dark and dense” Mid-Ocean Ridge Continental Crust Oceanic Crust Subducting Slab old crust destroyed Mantle Aesthenosphere thick and buoyant does not subduct thin and sinks does subduct Tectonic plates “float” on the higher density mantle.

4. The plates “float” on the asthenosphere, which is a thin boundary layer incorporating parts of the uppermost mantle and the base of the crust. The asthenosphere is “mooshy” (can be sheared relatively easily) allowing the plates above the slide along.

5. mantle convection The driving mechanism of plate tectonics is mantle convection. Hot mantle material rises at ridges and cooler mantle material sinks at subduction zones.

6. There are several large tectonic plates and a number of much smaller plates. The Earth’s continents sit on plates composed of both oceanic and continental crust. The huge Pacific plate is composed almost entirely of oceanic crust, and is being subducted around almost its entire western boundary. PacificPacific NorthAmericanNorthAmerican NazcaNazca SouthAmericanSouthAmerican AntarcticAntarctic AfricanAfrican EurasianEurasian Australian-IndianAustralian-Indian PhillipinePhillipine CaribbeanCaribbean PacificPacific Major Tectonic Plates

7. convergent divergent transform The different kinds of boundaries have different properties: Convergent Convergent – old oceanic crust destroyed through subduction Divergent Divergent – new oceanic crust accreted at mid-ocean ridges Transform Transform – crust neither created or destroyed

8. Convergent Boundaries subductiontrenches It is impossible for plates to move toward each other unless crust is “moved out of the way” - usually by subduction and destruction of oceanic crust at trenches.

9. Oceanic-Continental Convergent Boundaries Subduction leads to orogeny http://geo.lsa.umich.edu/~crlb/COURSES/270/ partially melt A sinking slab of oceanic crust will partially melt as it heats up, creating pockets of magma that rise through the crust, forming volcanoes.

10. Convergent Boundaries partially melt A sinking slab of oceanic crust will partially melt as it heats up, creating pockets of magma that rise through the crust, forming volcanoes. http://geo.lsa.umich.edu/~crlb/COURSES/270/ Edge of North American Plate

11. Convergent Boundaries When oceanic crust converges with oceanic crust, the denser plate (usually the oldest and coldest) generally subducts. An arc of volcanic islands forms from upwelling magma. Oceanic-Oceanic http://geo.lsa.umich.edu/~crlb/COURSES/270/

12. Convergent Boundaries http://geo.lsa.umich.edu/~crlb/COURSES/270/ Oceanic-Oceanic

13. Convergent Boundaries Since continental crust is buoyant, it does not subduct easily. While the edge of a plate can be dragged under by the weight of attached oceanic crust, it does not go far. Continental-Continental http://geo.lsa.umich.edu/~crlb/COURSES/270/

14. Divergent Boundaries

15. Mid-ocean ridges are long mountains formed parallel to oceanic rifts. spreading center. The plates on either side of the ridge grow as new crustal material is added at the spreading center. Rifts are characterized by relatively shallow earthquake foci along the length of the rift. http://wwwneic.cr.usgs.gov/ Mid-Ocean Ridges

16. Divergent Boundaries Spreading Centers http://geo.lsa.umich.edu/~crlb/COURSES/270/ upwelling magma causes bulge

17. Divergent Boundaries Spreading Centers upwelling magma causes bulge rift valley forms as plates pull apart http://geo.lsa.umich.edu/~crlb/COURSES/270/

18. Divergent Boundaries Spreading Centers upwelling magma causes bulge rift valley forms as plates pull apart oceanic crust accretes to continental crust as ridge forms http://geo.lsa.umich.edu/~crlb/COURSES/270/

19. Divergent Boundaries Spreading Centers upwelling magma causes bulge rift valley forms as plates pull apart oceanic crust accretes to continental crust as ridge forms continents continue to move apart, opening ocean http://geo.lsa.umich.edu/~crlb/COURSES/270/

20. Transform Boundaries Most transform boundaries are associated with mid-ocean ridges (they form perpendicular to the rifts). This movement resolves stresses caused by different rates of spreading along the divergent boundary.

21. Transform Boundaries

22. San Andreas fault system The San Andreas fault system is part of a system of strike-slip faults caused by the relative motion of the North American and Pacific plates. http://pubs.usgs.gov http://wwwneic.cr.usgs.gov/ http://www.ingv.it/~roma/cultura/ingescuola/terremotopagina/SanAndreas.html

23. Boundary Type ConvergentDivergentTransformVolcanoesyesyesnoMountainsyesyesnoEarthquakesshallow-deepshallowvaries