Upwelling Magma new crust added

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

2. Tectonic plates “float” on the higher density mantle.

3. 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.

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

5. Major Tectonic Plates 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. Eurasian North American Phillipine Caribbean Pacific Pacific African South American Nazca Australian-Indian Antarctic

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

7. Convergent Boundaries 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.

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

9. Convergent Boundaries Edge of North American Plate http://geo.lsa.umich.edu/~crlb/COURSES/270/ 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 Oceanic-Oceanic http://geo.lsa.umich.edu/~crlb/COURSES/270/ 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.

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

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

13. Divergent Boundaries

14. Divergent Boundaries Mid-Ocean Ridges Mid-ocean ridges are long mountains formed parallel to oceanic rifts. 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/

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

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

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

18. Divergent Boundaries Spreading Centers http://geo.lsa.umich.edu/~crlb/COURSES/270/ • 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

19. 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.

20. Transform Boundaries

21. Transform Boundaries 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

22. Boundary Type Convergent Divergent Transform Volcanoes yes yes no Mountains yes yes no Earthquakes shallow-deep shallow varies