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Chapter 8 The Theory of Plate Tectonics 1 Objectives Given information about Plate Tectonics you will be able to describe: • • • • The history & development of the theory Supporting evidence Plate tectonic processes Phenomena explained by tectonic theory 2 Guiding Questions • What is the evidence for Gondwanaland? • How does paleomagnetism demonstrate that continents have moved in the course of time? • How was paleomagnetism used to show that lithosphere forms at mid-ocean ridges and then migrates away? • How did features of the seafloor engender the concept of plate tectonics? • Why do faulting and volcanism occur along oceanic plate margins? • What causes lithospheric plates to move? 3 • How can geologists measure rates of plate movement? 4 Plate Tectonics • Tectonics – Movement of Earth’s crust • Plate tectonics – Movement of discrete segments of Earth’s crust in relation to one another 5 Continental Drift • Idea that continents move horizontally over Earth’s surface – Alfred Wegener – Alexander Logie du Toit Wegener 6 du Toit Continental Drift • Early scientists recognized relationship between fossils on continents separated by sea – Proposed land bridges 7 Continental Drift • Glossopteris flora present only in southern hemisphere continents 8 Continental Drift • Early recognition that Mid-Atlantic Ridge was site of landmass rupture and Atlantic Ocean formation 9 Continental Drift • Wegener’s Evidence – Continents fit together • Pangaea – Geologic similarities – Floral and faunal similarities 10 Continental Drift • du Toit’s evidence – Expanded Wegener’s ideas – Proposed Pangaea split into Laurasia & Gondwanaland – Mesosaurus fossils • Found on Gondwana continents • Freshwater – Could not swim across Atlantic 11 Continental Drift • Geologic Similarities – Brazil and South Africa have nearly identical geologic sequences • Similar in Antarctica and India – Glacial sediments – Coal 12 Continental Drift • Glaciers – Orientation of glacial markings on all continents suggests they were linked – Must reconfigure continents back to Pangaea for this to make sense 13 Additional Evidence • Lystrosaurus – Heavyset herbivore – Pig-like tetrapod – Found in Antarctica • 1969 14 Paleomagnetism • Magnetization of ancient rocks at the time of their formation • Declination – Angle that a compass needle makes with the line running to the geographic north pole • Rocks lock in this orientation at formation 15 Paleomagnetism • Apparent Polar Wander – First studies indicated poles had moved – Instead, plates had moved – North American and European paths met 16 Rise of Plate Tectonics • Harry Hess, 1962 – Geopoetry • Continents didn’t plow through seafloor • Entire crust moved – Crust must be created and destroyed • Sedimentary cover too thin for four billion years of accumulation – Driven by convective 17 cells Rise of Plate Tectonics • Ridges – Site of crustal formation – Hot rising mantle material rises to top of lithosphere, cools – Ocean crust is formed – Bends away from center to form ridge 18 Rise of Plate Tectonics • Guyots – Had identified flattopped seamounts in Pacific – Realized they were volcanoes that had been eroded by waves at sea level – Postulated as crust moved away from ridge it cools and sinks 19 Rise of Plate Tectonics • Crust is destroyed at subduction zones 20 Test of Plate Tectonics • Paleomagnetism – Vine and Matthews, 1963 – Measured magnetization of rocks across the Indian Ocean central ridge • Found normal and reversed “stripes” • Mirror image 21 Processes at Plate Boundaries • Normal faults – Extensional motion • Thrust faults – Compressional motion • Strike-slip faults – San Andreas fault 22 Processes at Plate Boundaries • Mid-Ocean Ridges (MOR) – Graben • Valley bounded by normal faults along which a central block has slipped downward – Pillow basalt 23 Processes at Plate Boundaries • Transform faults – Offset MORs – Enormous strikeslip faults – Seismically active 24 Processes at Plate Boundaries • Subduction – Descending slab undergoes dehydration which causes partial melting of the overlying mantle – Molten material is less dense, rises • Common around Pacific – Ring of Fire • Location of most of the world’s trenches 25 Processes at Plate Boundaries • Associated with: – Volcanoes • Island arc – Deep-focus earthquakes • > 300 km depth – Forearc basin • Zone of intensely deformed rocks in belt between island arc and deep-sea trench 26 Processes at Plate Boundaries • Forearc Basin – Deep-ocean dark muds and graywackes with ocean crust mixed in • Mélange – Chaotic deformed mixture of rocks • Accretionary Wedge – Body of rock that accumulates as plate is subducted 27 Plate Motion • Why plates move: – Drag on the base of the plate – “Slab Pull” – Elevation at ridge pushes plate ahead of it – “Ridge Push”, “Gravity Sliding” – Plate is pulled into subduction zone by preceding parent plate – “Slab Pull” – Broken plate segments create additional forces 28 Plate Motion • Eight large plates • Several small plates • Not all plates move at the same rate – Relative motion 29 Plate Motion • Absolute plate motion – Establish using fixed point – Hot spot • Small geographic area where heating and igneous activity occur within the crust • Yellowstone 30 • Hawaii Plate Motion • Hawaiian hot spot – Thermal plume creates volcano – Plate moves away from plume – Stranded volcano cools, leaves a chain – Chain indicates direction and rate 31 Plate Motion • GPS – Global Positioning System – Earth-orbiting satellites identify motion • Transmitter on satellite • Ground-based receiver • Average rate – 5 cm/year 32 33 Summary • Continental drift –proposed by Wegener, added to by du Toit • Plate Tectonic theory developed by late 1960’s • Evidence = fossils, rock types, shapes of continents, paleomagnetic data, gps • Driven by convection – heat in earth’s interior • 3 plate motions – convergent, divergent, transform • Phenomena explained on earth = mountains, trenches, ore deposits, volcanoes, earthquakes, tsunamis, etc. 34