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What happens at tectonic plate boundaries? Essential Questions What are the 3 types of plate boundaries? What happens at each of these types of boundaries? What are some examples of these plate boundaries? The Plate Tectonic System Is energized by Earth’s internal Heat Composed of: Lithosphere Asthenosphere Deep mantle This supplies the energy that melts rocks, moves continents, and ligts up mountains Three types of plate boundary Divergent Convergent Transform World in Motion The Endless Voyage Series http://learning.aliant.net/Player/ALC_ Player.asp?ProgID=INT_ENDVOY06 Complete the self test after the video 27mins Divergent Boundaries Spreading ridges As plates move apart new material is erupted to fill the gap Types of Divergent Boundaries Oceanic Plate Separation Rifting and spreading along a narrow zone have created the Mid-Atlantic Rige, a mid-ocean mountain chain where volcanoes and earthquakes are concentrated Continental Plate Separation In East Africa, rifting and spreading have created parallel valleys in a zone with volcanoes and earthquakes: Great Rift Valley Age of Oceanic Crust Courtesy of www.ngdc.noaa.gov The Seafloor as a Magnetic Tape Recorder Magnetometers Developed during WWII to detect subs Towed behind research vessels to measure the magnetic field of the sea floor Discovered that the intensity of the magnetic field alternated between high and low values Termed magnetic anomalies Magnetic Anomalies Almost perfectly symmetrical with respect to the crest of the md-ocean ridge Are evidence that Earth’s magnetic field does not remain constant Magnetic reversals occur, flipping the orientation of the North and South pole When iron rich lavas cool, they become slightly but permanently magnetized in the direction of Earth’s magnetic field This is thermoremanent magnetization About half of all rocks studied have been found to be magnetized in a direction opposite to the Earth’s present magnetic field Each cycle lasts about 1/2million years b Iceland: An example of continental rifting Iceland has a divergent plate boundary running through its middle Convergent Boundaries There are three styles of convergent plate boundaries Continent-continent collision Continent-oceanic crust collision Ocean-ocean collision Continent-Continent Collision When two continental plates collide The crust crumples and thickens, creating high mountains and a wide plateau Forms mountains, e.g. European Alps, Himalayas Himalayas Continent-Oceanic Crust Collision Called SUBDUCTION When oceanic crust meets continental crust The oceanic plate is subducted, and a volcanic belt of mountains is formed at the continental plate margin Ex: Peru-Chile Trench Subduction Oceanic lithosphere subducts underneath the continental lithosphere Oceanic lithosphere heats and dehydrates as it subsides The melt rises forming volcanism E.g. The Andes Ocean-Ocean Plate Collision When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone. The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench. The worlds deepest parts of the ocean are found along trenches. E.g. The Mariana Trench is 11 km deep! Transform Boundaries Where plates slide past each other Above: View of the San Andreas transform fault Types of Transform-Fault Boundaries Mid-Ocean Ridge Transform Fault Spreading centers are offset by mid-ocean ridge transform faults, where the two oceanic plates slide horizontally past each other Continental Transform Fault Where a continental plate slide past another plate Ex the San Andreas Fault – where the Pacific plate slides past the North American plate