1Plate Tectonics• Introduction• Predecessor of plate tectonics: Continental drift hypothesis• What is the theory of plate tectonics? • What is a plate? • How do we find their edges? •How many plates are there?1•How many plates are there?• Why is plate tectonics important?• Types of relative plate movements (and related stresses)• Types of plate boundaries• Plate boundaries and earthquake depths• Measuring plate motions• Why do plates move?Continental DriftIn 1912, Alfred Lothar Wegener (1880-1930), a German meteorologist, published the idea of continental drift. Wegener proposed that all of the continents were once joined together as one supercontinent that he called Pangea. He also compiled a great deal of evidence to support his hypothesis, including the distribution of fossils and mountain ranges.2This Dynamic Earth - USGSFossil evidence for once joined continents3This Dynamic Earth - USGSLater studies have allowed the positions of the continents at different times in Earth history to be determined, and have documented the breakup of Pangea.4http://media.pearsoncmg.com/bc/bc_0media_geo/active_art/hdew_2e.html?PlateMoTimeThis Dynamic Earth - USGS2Plate TectonicsToday, we know that it isn’t just the continents that have moved. Rather, the Earth’s surface is broken into pieces of continental and/or ocean crust called plates. Plate tectonics deals with the nature of these plates, what happens at their boundaries, how and why they move, etc.5Fig 20.3 - Understanding EarthPlate tectonics also explains the locations of earthquake faults, volcanoes, and major mountain ranges. Photos from: This Dynamic Earth (USGS) and Understanding Earth 6Active continental margins (plate boundary)vs.Passivecontinental margins (no plate boundary)The western margin of North America is an active area with earthquakes (yellow) and volcanic activity (red). In contrast, the eastern margin of North America is quiet with no active volcanoes and few earthquakes. The eastern margin is passive. We now know that the active zones coincide with the boundaries between plates (blue).7http://media.pearsoncmg.com/bc/bc_0media_geo/active_art/hdew_2e.html?PlateBoundUSGSZones of progressively deeper earthquakes revealed the existence of subduction. This led to an understanding of how ocean crust is recycled at ocean trenches.8Geology 2nded. -Chernicoff3The discovery of extensive ocean ridges and young ages for the ocean floor (in the 1940s and 1950s) provided another important clue.9Fig 17.27 -Understanding EarthWhat is a plate? – The brittle outer part of the earth (crust and uppermost mantle) is called the lithosphere. The lithosphere is broken into plates that move on the asthenosphere, a part of the mantle which is plastic (able to flow).10This Dynamic Earth - USGSExtensionCompressionReverse and thrust faults11Types of relative plate movements and boundariesand related stresses and fault typesShearNormal faultsStretching and thinningStrike-slip faultsShearingFolding12This Dynamic Earth - USGS4The mid-ocean ridge (shown in red) winds its way between the continents much like the seam on a baseballThis Dynamic Earth - USGS13This Dynamic Earth - USGSFig 1.15 - Understanding Earth14Mid-Atlantic Ridge at IcelandContinental Rifting15East African RiftThis Dynamic Earth - USGSEast African RiftGulf of AdenAfricaSOUTH16http://media.pearsoncmg.com/bc/bc_0media_geo/active_art/hdew_2e.html?DivergBoundContinental rifting: The Afar TrianglePhoto from Space Shuttle:STS061-079-024 - NASARed SeaNext: 20-Plate Divergence.mov(Red Sea rift)5Fig 20.3 - Understanding EarthGeology 2nded. - Chernicoff (originally from Bullard, 1969, The Origin of the Continents: Scientific American)17Transform Boundaries18This Dynamic Earth - USGShttp://media.pearsoncmg.com/bc/bc_0media_geo/active_art/hdew_2e.html?TranFaultTransform plate boundarySan Andreas fault, CaliforniaFig 1.17 - Understanding Earth19Ocean-ocean convergent boundary This Dynamic Earth - USGS20621Japan, an example of an oceanic island arcPhoto from Space Shuttle: STS059-218-044 - NASAOcean-continent convergent boundary This Dynamic Earth - USGS22http://www.npr.org/templates/story/story.php?storyId=124361777(Charles Darwin witnessed 1895 Chilean quake, deduced that the Andes required millions of years to build)Intrusive igneous rocks formed along an ancient volcanic arc: Part of the Sierra-Nevada batholith in Yosemite National Park23Next: 20-Ocean-Continent.mov (Andean subduction zone)Continent-continent convergent boundary This Dynamic Earth - USGS247Formation of the highest mountains on earth by continent-continent collisionThis Dynamic Earth - USGS25Next: 20-Continent-Continent.mov(India-Asia collision zone)PT Summary: http://media.pearsoncmg.com/bc/bc_0media_geo/active_art/hdew_2e.html?MotionatPlate12All types of plate boundaries can produce shallow earthquakes. The deepest earthquakes (and some of the largest earthquakes) occur in subduction zones.Plate boundaries and earthquake depths26Fig. 10-16 Chernicoff - GeologyMeasuring plate motions using hot spots27dia.pearsoncmg.com/bc/bc_0media_geo/actidew_2e.html?HotSpot12Map of part of the Pacific ocean floor showing the 6,000-km-long Hawaiian Ridge-Emperor Seamounts chain – This volcanic trail of the Hawaiian “hotspot” could be related to a mantle plume This Dynamic Earth - USGSHotspots and plate movements28Geology 2nded. - ChernicoffHot spots and the opening of the south Atlantic Oceanhttp://earth.leeds.ac.uk/dynamicearth/plates_move/hotspots/index.htm8Ages of the ocean crust Fig 20.11 - Understanding Earth29 30A GPS ground receiver -- here set up on the flank of Augustine Volcano (Cook Inlet, Alaska) -- recording the signals sent by four or more of the orbiting GPS satellites. Artist's conception of a Global Positioning System satellite in orbit also shown.Source: USGSMajor plate boundaries and their rates of movementFig 20.12 - Understanding Earth31 32Projected future positions of the continents - 100 million years from now: If current plate movements continue unchanged, part of California will become an island off the coast of Washington, the Mediterranean Sea may close, and eastern Africa may become an island.Geology 2nded. - Chernicoff9The general scientific consensus is that plate movements are related to convection within the Earth’s mantle, although the direction and speed of a moving plate doesn’tPlate Tectonics:Why do plates move?33speed of a moving plate doesn t always equal