GEOLOGY

1. GEOLOGY Chapter 10 Plate Tectonics 10.2 The Theory of Plate Tectonics

2. 10.2 The Theory of Plate Tectonics Objectives • Summarize the theory of plate tectonics. • Identify and describe the three types of plate boundaries. • List and describe three causes of plate movement.

3. Introduction • The theory of plate tectonics states that the Earth’s crust and rigid upper mantle are broken into enormous slabs called tectonic plates which move in different directions at different rates over Earth’s surface. • There are a dozen or so major plates and several smaller plates.

5. Plate Boundaries • Tectonic plates interact at places called plate boundaries. • There are three types of plate boundaries, each with certain geologic characteristics and processes associated with it. • Divergent boundary • Convergent boundary • Transform boundary

6. Plate Boundaries • Divergent boundaries are places where two tectonic plates are moving apart. • Most divergent boundaries are found on the seafloor where they form ocean ridges. • The boundary itself is found in the rift which forms along the axis of the ridge. • High heat flow, volcanism, and earthquakes are associated with divergent boundaries where new ocean crust is being formed.

7. Plate Boundaries • The Atlantic Ocean is spreading at a rate of about 2 to 3 cm/y. • Some divergent boundaries can form on continents. • When continental crust begins to separate, the stretched crust forms a long narrow depression called a rift valley. • Rifting of continents may eventually lead t the formation of a new ocean basin.

10. Plate Boundaries

11. Video Links • Divergent Boundary (0:26) - http://youtu.be/t-ctk4KR-KU • Convergent Boundary (0:29) - http://youtu.be/ep2_axAA9Mw

12. Plate Boundaries • Convergent boundaries are places where two tectonic plates are moving toward each other. • There are three types of convergent boundaries. • Oceanic-Oceanic • Oceanic-Continental • Continental-Continental

13. Plate Boundaries • In Oceanic-Oceanic collisions, the older, slightly more dense oceanic plate is subducted underneath the younger, slightly less dense oceanic plate. • The process of subduction creates deep sea trenches • The subducted plate descends into the mantle and melts causing the subducted crust to be recycled • Island arcs are often associated with Oceanic-Oceanic collisions such as the Aleutian Islands

14. Plate Boundaries • In Oceanic-Continental collisions, the denser oceanic plate is subducted under the continental plate. • The subduction at this type of boundary also forms a trench and series of volcanoes (except that the volcanoes occur along the edge of the continental plate) • Mountain ranges with many volcanoes, like the Andes of South America, result form Oceanic-Continental collisions

15. Plate Boundaries • In Continental-Continental collisions, the ocean basin between two continental masses has been entirely subducted and the thicker, less dense continental masses crumple and are uplifted as they are forced together. • Mountain ranges are formed this way such as the Appalachians and the much more recent Himalayas.

17. Plate Boundaries • Transform boundaries are places where two plates slide laterally past each other. • At transform boundaries, crust is not subducted, it is only deformed or fractured. • Transform boundaries are characterized by long faults and shallow earthquakes. • Most transform boundaries often offset sections of ocean ridges. • The San Andreas Fault one of the best known exceptions as it is not on an ocean ridge.

20. Mantle Convection • Convection is the transfer of thermal energy by the movement of heated matter. • The heating of matter causes it to expand and decrease in density. • The warmed matter then rises as a result of buoyancy. • The cooler part of the matter sinks as a result of gravity. • The resulting up and down flow produces the pattern of motion that we call a convection current.

21. Convection Currents

22. Mantle Convection • Convection currents in the mantle are thought to be the driving mechanism of plate movements. • Hot mantle material is less dense than cool mantle material, so the hot material is slowly forced toward the crust and the cooler material sinks back toward the core. • The resulting convection currents can be thousands of kilometers across but flow at rates of only a few centimeters per year.

23. Mantle Convection

24. Mantle Convection • The rising part of a convection current spreads out as it reaches the upper mantle and causes both upward and lateral forces. • These forces lift and split the lithosphere at divergent plate boundaries. • As the plates separate, material rising from the mantle supplies the magma that hardens to form the new ocean crust. • The downward part of a convection current occurs where a sinking force pulls tectonic plates downward at convergent boundaries.

25. Mantle Convection • As ocean ridges form, forces in the mantle cause the asthenosphere to rise. • The weight of this massive ridge that is lifted is thought to push the oceanic plate toward the trench formed at the opposing end of the plate at the subduction zone. • This process is known as ridge push. • The horizontal flow of a convection current could also create drag on the lithosphere and contribute to plate motion.

26. Mantle Convection

27. Mantle Convection • Mantle convection in a sinking region of the mantle may also serve to suck an oceanic plate downward into a subduction zone. • Slab pull is the process in which the weight of a subducting plate helps to pull the trailing lithosphere into the subduction zone. • Even though scientists agree that convection currents in the mantle are related to plate movement, their exact role still requires further study.

28. Mantle Convection

29. References • Tectonic Plates - http://www.stchas.edu/faculty/ewilson/Photos/Plates • Growing Ocean Basin - http://www.calstatela.edu/faculty/acolvil/plates.html • Divergent Boundary (and Magnetism) - http://www.calstatela.edu/faculty/acolvil/plates.html • Plate Boundaries - http://ces.iisc.ernet.in/hpg/envis/Remote/introfile2.htm

30. References • Convergent Boundaries 2 – http://www.calstatela.edu/faculty/acolvil/plates.html • Transform Boundary - http://www.science.mcmaster.ca/geo/faculty/boyce/3z03/San_Andreas • Three Types of Plate Boundaries - http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Plate_tectonics

31. References • Convection Currents in a Room – http://www.smartsol.org/page4.html • Convection Currents in a Pot – http://www.iaea.org/ns/nusafe/tutors/thermalhydraulics/module1/learningmaterial/section1p1bis.htm • Mantle Convection 1 – http://rst.gsfc.nasa.gov/Sect2/Sect2_1b.html • Mantle Convection 2 – http://piru.alexandria.ucsb.edu/collections/geography3b/NEWlectures/lecture02.htm

32. References • Mantle Convection and Slab Pull - http://piru.alexandria.ucsb.edu/collections/geography3b/misc