GEOLOGY Chapter 10 Plate Tectonics 10.2 The Theory of Plate Tectonics

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

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. 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. There are a dozen or so major plates and several smaller plates.

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

Plate Boundaries Divergent boundaries are places where two tectonic plates are moving apart. Divergent boundaries are places where two tectonic plates are moving apart. Most divergent boundaries are found on the seafloor where they form ocean ridges. 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. 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. High heat flow, volcanism, and earthquakes are associated with divergent boundaries where new ocean crust is being formed.

Plate Boundaries The Atlantic Ocean is spreading at a rate of about 2 to 3 cm/y. The Atlantic Ocean is spreading at a rate of about 2 to 3 cm/y. Some divergent boundaries can form on continents. 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. 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. Rifting of continents may eventually lead t the formation of a new ocean basin.

Plate Boundaries

Video Links Divergent Boundary (0:26) - Divergent Boundary (0:26) Convergent Boundary (0:29) - Convergent Boundary (0:29) -

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

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

Plate Boundaries In Oceanic-Continental collisions, the denser oceanic plate is subducted under the continental plate. 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)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 collisionsMountain ranges with many volcanoes, like the Andes of South America, result form Oceanic- Continental collisions

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. 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.Mountain ranges are formed this way such as the Appalachians and the much more recent Himalayas.

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

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

Convection Currents

Mantle Convection Convection currents in the mantle are thought to be the driving mechanism of plate movements. 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. 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. The resulting convection currents can be thousands of kilometers across but flow at rates of only a few centimeters per year.

Mantle Convection

The rising part of a convection current spreads out as it reaches the upper mantle and causes both upward and lateral forces. 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. 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. 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. The downward part of a convection current occurs where a sinking force pulls tectonic plates downward at convergent boundaries.

Mantle Convection As ocean ridges form, forces in the mantle cause the asthenosphere to rise. 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. 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. 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. The horizontal flow of a convection current could also create drag on the lithosphere and contribute to plate motion.

Mantle Convection

Mantle convection in a sinking region of the mantle may also serve to suck an oceanic plate downward into a subduction zone. 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. 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. Even though scientists agree that convection currents in the mantle are related to plate movement, their exact role still requires further study.

Mantle Convection

References Tectonic Plates - ates Tectonic Plates - ates ates ates Growing Ocean Basin - ml Growing Ocean Basin - ml ml ml Divergent Boundary (and Magnetism) - ml Divergent Boundary (and Magnetism) - ml ml ml Plate Boundaries htm Plate Boundaries htm 2.htm 2.htm

References Convergent Boundaries 2 – ml Convergent Boundaries 2 – ml ml ml Transform Boundary - boyce/3z03/San_Andreas Transform Boundary - boyce/3z03/San_Andreas boyce/3z03/San_Andreas boyce/3z03/San_Andreas Three Types of Plate Boundaries - projects.com/science_fair_projects_encyclop edia/Plate_tectonics Three Types of Plate Boundaries - projects.com/science_fair_projects_encyclop edia/Plate_tectonics projects.com/science_fair_projects_encyclop edia/Plate_tectonics projects.com/science_fair_projects_encyclop edia/Plate_tectonics

References Convection Currents in a Room – Convection Currents in a Room – Convection Currents in a Pot – hydraulics/module1/learningmaterial/section 1p1bis.htm Convection Currents in a Pot – hydraulics/module1/learningmaterial/section 1p1bis.htm hydraulics/module1/learningmaterial/section 1p1bis.htm hydraulics/module1/learningmaterial/section 1p1bis.htm Mantle Convection 1 – Mantle Convection 1 – Mantle Convection 2 – ography3b/NEWlectures/lecture02.htm Mantle Convection 2 – ography3b/NEWlectures/lecture02.htm ography3b/NEWlectures/lecture02.htm ography3b/NEWlectures/lecture02.htm

References Mantle Convection and Slab Pull - ns/geography3b/misc Mantle Convection and Slab Pull - ns/geography3b/misc ns/geography3b/misc ns/geography3b/misc