GEOLOGYGEOLOGY

Chapter 10 Plate TectonicsChapter 10 Plate Tectonics10.2 The Theory of Plate 10.2 The Theory of Plate

TectonicsTectonics

10.2 The Theory of Plate 10.2 The Theory of Plate Tectonics ObjectivesTectonics Objectives

Summarize the theory of plate Summarize the theory of plate tectonics.tectonics.

Identify and describe the three types Identify and describe the three types of plate boundaries.of plate boundaries.

List and describe three causes of List and describe three causes of plate movement.plate movement.

IntroductionIntroduction The theory of plate tectonics states The theory of plate tectonics states

that the Earth’s crust and rigid upper that the Earth’s crust and rigid upper mantle are broken into enormous mantle are broken into enormous slabs called tectonic plates which slabs called tectonic plates which move in different directions at move in different directions at different rates over Earth’s surface.different rates over Earth’s surface.

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

Plate BoundariesPlate Boundaries Tectonic plates interact at places Tectonic plates interact at places

called plate boundaries.called plate boundaries. There are three types of plate There are three types of plate

boundaries, each with certain boundaries, each with certain geologic characteristics and geologic characteristics and processes associated with it.processes associated with it.• Divergent boundaryDivergent boundary• Convergent boundaryConvergent boundary• Transform boundaryTransform boundary

Plate BoundariesPlate Boundaries Divergent boundaries are places where Divergent boundaries are places where

two tectonic plates are moving apart.two tectonic plates are moving apart. Most divergent boundaries are found on Most divergent boundaries are found on

the seafloor where they form ocean ridges.the seafloor where they form ocean ridges. The boundary itself is found in the rift The boundary itself is found in the rift

which forms along the axis of the ridge.which forms along the axis of the ridge. High heat flow, volcanism, and High heat flow, volcanism, and

earthquakes are associated with divergent earthquakes are associated with divergent boundaries where new ocean crust is boundaries where new ocean crust is being formed.being formed.

Plate BoundariesPlate Boundaries The Atlantic Ocean is spreading at a rate The Atlantic Ocean is spreading at a rate

of about 2 to 3 cm/y.of about 2 to 3 cm/y. Some divergent boundaries can form on Some divergent boundaries can form on

continents.continents. When continental crust begins to separate, When continental crust begins to separate,

the stretched crust forms a long narrow the stretched crust forms a long narrow depression called a rift valley.depression called a rift valley.

Rifting of continents may eventually lead t Rifting of continents may eventually lead t the formation of a new ocean basin.the formation of a new ocean basin.

Plate BoundariesPlate Boundaries

Video LinksVideo Links Divergent Boundary (0:26) - Divergent Boundary (0:26) -

http://youtu.be/t-ctk4KR-KUhttp://youtu.be/t-ctk4KR-KU Convergent Boundary (0:29) - Convergent Boundary (0:29) -

http://youtu.be/ep2_axAA9Mwhttp://youtu.be/ep2_axAA9Mw

Plate BoundariesPlate Boundaries Convergent boundaries are places Convergent boundaries are places

where two tectonic plates are moving where two tectonic plates are moving toward each other.toward each other.

There are three types of convergent There are three types of convergent boundaries.boundaries.• Oceanic-OceanicOceanic-Oceanic• Oceanic-ContinentalOceanic-Continental• Continental-ContinentalContinental-Continental

Plate BoundariesPlate Boundaries In Oceanic-Oceanic collisions, the older, In Oceanic-Oceanic collisions, the older,

slightly more dense oceanic plate is slightly more dense oceanic plate is subducted underneath the younger, subducted underneath the younger, slightly less dense oceanic plate.slightly less dense oceanic plate.• The process of subduction creates deep sea The process of subduction creates deep sea

trenchestrenches• The subducted plate descends into the mantle The subducted plate descends into the mantle

and melts causing the subducted crust to be and melts causing the subducted crust to be recycledrecycled

• Island arcs are often associated with Oceanic-Island arcs are often associated with Oceanic-Oceanic collisions such as the Aleutian IslandsOceanic collisions such as the Aleutian Islands

Plate BoundariesPlate Boundaries In Oceanic-Continental collisions, the In Oceanic-Continental collisions, the

denser oceanic plate is subducted under denser oceanic plate is subducted under the continental plate.the continental plate.• The subduction at this type of boundary also The subduction at this type of boundary also

forms a trench and series of volcanoes (except forms a trench and series of volcanoes (except that the volcanoes occur along the edge of the that the volcanoes occur along the edge of the continental plate)continental plate)

• Mountain ranges with many volcanoes, like the Mountain ranges with many volcanoes, like the Andes of South America, result form Oceanic-Andes of South America, result form Oceanic-Continental collisionsContinental collisions

Plate BoundariesPlate Boundaries In Continental-Continental collisions, In Continental-Continental collisions,

the ocean basin between two the ocean basin between two continental masses has been entirely continental masses has been entirely subducted and the thicker, less subducted and the thicker, less dense continental masses crumple dense continental masses crumple and are uplifted as they are forced and are uplifted as they are forced together.together.• Mountain ranges are formed this way Mountain ranges are formed this way

such as the Appalachians and the much such as the Appalachians and the much more recent Himalayas.more recent Himalayas.

Plate BoundariesPlate Boundaries Transform boundaries are places where Transform boundaries are places where

two plates slide laterally past each other.two plates slide laterally past each other.• At transform boundaries, crust is not At transform boundaries, crust is not

subducted, it is only deformed or fractured.subducted, it is only deformed or fractured.• Transform boundaries are characterized by Transform boundaries are characterized by

long faults and shallow earthquakes.long faults and shallow earthquakes.• Most transform boundaries often offset Most transform boundaries often offset

sections of ocean ridges.sections of ocean ridges.• The San Andreas Fault one of the best known The San Andreas Fault one of the best known

exceptions as it is not on an ocean ridge.exceptions as it is not on an ocean ridge.

Mantle ConvectionMantle Convection Convection is the transfer of thermal energy by Convection is the transfer of thermal energy by

the movement of heated matter.the movement of heated matter. The heating of matter causes it to expand and The heating of matter causes it to expand and

decrease in density.decrease in density. The warmed matter then rises as a result of The warmed matter then rises as a result of

buoyancy.buoyancy. The cooler part of the matter sinks as a result of The cooler part of the matter sinks as a result of

gravity.gravity. The resulting up and down flow produces the The resulting up and down flow produces the

pattern of motion that we call a convection pattern of motion that we call a convection current.current.

Convection CurrentsConvection Currents

Mantle ConvectionMantle Convection Convection currents in the mantle are Convection currents in the mantle are

thought to be the driving mechanism of thought to be the driving mechanism of plate movements.plate movements.

Hot mantle material is less dense than Hot mantle material is less dense than cool mantle material, so the hot material is cool mantle material, so the hot material is slowly forced toward the crust and the slowly forced toward the crust and the cooler material sinks back toward the cooler material sinks back toward the core.core.

The resulting convection currents can be The resulting convection currents can be thousands of kilometers across but flow at thousands of kilometers across but flow at rates of only a few centimeters per year.rates of only a few centimeters per year.

Mantle ConvectionMantle Convection

Mantle ConvectionMantle Convection The rising part of a convection current spreads The rising part of a convection current spreads

out as it reaches the upper mantle and causes out as it reaches the upper mantle and causes both upward and lateral forces.both upward and lateral forces.

These forces lift and split the lithosphere at These forces lift and split the lithosphere at divergent plate boundaries.divergent plate boundaries.

As the plates separate, material rising from the As the plates separate, material rising from the mantle supplies the magma that hardens to form mantle supplies the magma that hardens to form the new ocean crust.the new ocean crust.

The downward part of a convection current The downward part of a convection current occurs where a sinking force pulls tectonic plates occurs where a sinking force pulls tectonic plates downward at convergent boundaries.downward at convergent boundaries.

Mantle ConvectionMantle Convection As ocean ridges form, forces in the mantle As ocean ridges form, forces in the mantle

cause the asthenosphere to rise.cause the asthenosphere to rise. The weight of this massive ridge that is The weight of this massive ridge that is

lifted is thought to push the oceanic plate lifted is thought to push the oceanic plate toward the trench formed at the opposing toward the trench formed at the opposing end of the plate at the subduction zone.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 The horizontal flow of a convection current

could also create drag on the lithosphere could also create drag on the lithosphere and contribute to plate motion.and contribute to plate motion.

Mantle ConvectionMantle Convection

Mantle ConvectionMantle Convection Mantle convection in a sinking region of the Mantle convection in a sinking region of the

mantle may also serve to suck an oceanic mantle may also serve to suck an oceanic plate downward into a subduction zone.plate downward into a subduction zone.

Slab pull is the process in which the weight Slab pull is the process in which the weight of a subducting plate helps to pull the of a subducting plate helps to pull the trailing lithosphere into the subduction zone.trailing lithosphere into the subduction zone.

Even though scientists agree that convection Even though scientists agree that convection currents in the mantle are related to plate currents in the mantle are related to plate movement, their exact role still requires movement, their exact role still requires further study.further study.

Mantle ConvectionMantle Convection

ReferencesReferences Tectonic Plates - Tectonic Plates - http://http://

www.stchas.edu/faculty/ewilson/Photos/Plateswww.stchas.edu/faculty/ewilson/Photos/Plates Growing Ocean Basin - Growing Ocean Basin - http://http://

www.calstatela.edu/faculty/acolvil/plates.htmlwww.calstatela.edu/faculty/acolvil/plates.html Divergent Boundary (and Magnetism) - Divergent Boundary (and Magnetism) - http://http://

www.calstatela.edu/faculty/acolvil/plates.htmlwww.calstatela.edu/faculty/acolvil/plates.html Plate Boundaries - Plate Boundaries -

http://ces.iisc.ernet.in/hpg/envis/Remote/introfile2.hthttp://ces.iisc.ernet.in/hpg/envis/Remote/introfile2.htmm

ReferencesReferences Convergent Boundaries 2 – Convergent Boundaries 2 – http://http://

www.calstatela.edu/faculty/acolvil/plates.htmlwww.calstatela.edu/faculty/acolvil/plates.html Transform Boundary - Transform Boundary -

http://www.science.mcmaster.ca/geo/faculty/boyhttp://www.science.mcmaster.ca/geo/faculty/boyce/3z03/San_Andreasce/3z03/San_Andreas

Three Types of Plate Boundaries - Three Types of Plate Boundaries - http://www.all-science-fair-projects.com/science_http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Plate_tectonicsfair_projects_encyclopedia/Plate_tectonics

ReferencesReferences Convection Currents in a Room – Convection Currents in a Room –

http://www.smartsol.org/page4.htmlhttp://www.smartsol.org/page4.html Convection Currents in a Pot – Convection Currents in a Pot – http://www.iaea.org/ns/nusafe/tutors/thermalhttp://www.iaea.org/ns/nusafe/tutors/thermalhydraulics/module1/learningmaterial/hydraulics/module1/learningmaterial/section1p1bis.htmsection1p1bis.htm

Mantle Convection 1 – Mantle Convection 1 – http://rst.gsfc.nasa.gov/Sect2/Sect2_1b.htmlhttp://rst.gsfc.nasa.gov/Sect2/Sect2_1b.html

Mantle Convection 2 – Mantle Convection 2 – http://piru.alexandria.ucsb.edu/collections/geogrhttp://piru.alexandria.ucsb.edu/collections/geography3b/NEWlectures/lecture02.htmaphy3b/NEWlectures/lecture02.htm

ReferencesReferences Mantle Convection and Slab Pull - Mantle Convection and Slab Pull - http://piru.alexandria.ucsb.edu/collectiohttp://piru.alexandria.ucsb.edu/collections/geography3b/miscns/geography3b/misc