Plate Tectonics Physical Geology 13 Chapter 19 Tim Horner CSUS Geology Department Plate Tectonics Basic idea of plate tectonics Earth s surface is composed of a few large thick plates that move slowly and change in size Intense geologic activity is concentrated at plate boundaries where plates move away toward or past each other Combination of continental drift and seafloor spreading hypotheses in late 1960s Evidence for Plate Tectonics Fit of the continents Similarity of rock sequences Location of volcanos Location of deep earthquakes Paleomagnetism apparent polar wandering seafloor spreading Early Case for Continental Drift Puzzle piece fit of coastlines of Africa and South America has long been known In early 1900s Alfred Wegener noted South America Africa India Antarctica and Australia have almost identical late Paleozoic rocks and fossils Glossopteris plant Lystrosaurus and Cynognathus animals fossils found on all five continents Mesosaurus reptile fossils found in Brazil and South Africa only Early Case for Continental Drift Wegener reassembled continents into the supercontinent Pangaea Pangea initially separated into Laurasia and Gondwanaland Laurasia northern supercontinent containing North America and Asia excluding India Gondwanaland southern supercontinent containing South America Africa India Antarctica and Australia Late Paleozoic glaciation patterns on southern continents best explained by their reconstruction into Gondwanaland Coal beds of North America and Europe support reconstruction into Laurasia Early Case for Continental Drift Reconstructed paleoclimate belts suggested polar wandering potential evidence for Continental Drift Continental Drift hypothesis initially rejected Wegener could not come up with viable driving force continents should not be able to plow through sea floor rocks while crumpling themselves but not the sea floor Paleomagnetism and Continental Drift Revived Studies of rock magnetism allowed determination of magnetic pole locations close to geographic poles through time Paleomagnetism uses mineral magnetic alignment direction and dip angle to determine the direction and distance to the magnetic pole when rocks formed Steeper dip angles indicate rocks formed closer to the magnetic poles Rocks with increasing age point to pole locations increasingly far from present magnetic pole positions Paleomagnetism and Continental Drift Revived Apparent polar wander curves for different continents suggest real movement relative to one another Reconstruction of supercontinents using paleomagnetic information fits Africa and South America like puzzle pieces Improved fit results in rock units and glacial ice flow directions precisely matching up across continent margins Seafloor Spreading In 1962 Harry Hess proposed seafloor spreading Seafloor moves away from the midoceanic ridge due to mantle convection Convection is circulation driven by rising hot material and or sinking cooler material Hot mantle rock rises under mid oceanic ridge Ridge elevation high heat flow and abundant basaltic volcanism are evidence of this Seafloor Spreading Seafloor rocks and mantle rocks beneath them cool and become more dense with distance from mid oceanic ridge When sufficiently cool and dense these rocks may sink back into the mantle at subduction zones Downward plunge of cold rocks gives rise to oceanic trenches Overall young age for sea floor rocks everywhere 200 million years is explained by this model Plates and Plate Motion Tectonic plates are composed of relatively rigid lithosphere Lithospheric thickness and age of seafloor increase with distance from mid oceanic ridge Plates float upon ductile asthenosphere Plates interact at their boundaries which are classified by relative plate motion Plates move apart at divergent boundaries together at convergent boundaries and slide past one another at transform boundaries the Evidence of Plate Motion Marine magnetic anomalies bands of stronger and weaker than average magnetic field strength Parallel mid oceanic ridges Field strength related to basalts magnetized with same and opposite polarities as current magnetic field Symmetric bar code anomaly pattern reflects plate motion away from ridge coupled with magnetic field reversals Matches pattern of reversals seen in continental rocks Vine and Matthews Evidence of Plate Motion Seafloor age increases with distance from mid oceanic ridge Rate of plate motion equals distance from ridge divided by age of rocks Symmetric age pattern reflects plate motion away from ridge Evidence of Plate Motion Mid oceanic ridges are offset along fracture zones Fracture zone segment between offset ridge crests is a transform fault Relative motion along fault is result of seafloor spreading from adjacent ridges Plate motion can be measured using satellites radar lasers and global positioning systems Measurements accurate to within 1 cm Motion rates closely match those predicted using seafloor magnetic anomalies Divergent Plate Boundaries At divergent plate boundaries plates move away from each other Can occur in the middle of the ocean within a continent Divergent motion eventually creates a ocean basin or new Marked by rifting basaltic volcanism and eventual ridge uplift During rifting crust is stretched and thinned Graben valleys mark rift zones Volcanism common as magma rises through thinner crust along normal faults Ridge uplift by thermal expansion of hot rock Transform Plate Boundaries At transform plate boundaries plates slide horizontally past one another Marked by transform faults Transform faults may connect Two offset segments of mid oceanic ridge A mid oceanic ridge and a trench Two trenches Transform offsets of mid oceanic ridges allow series of straight line segments to approximate curved boundaries required by spheroidal Earth Convergent Plate Boundaries At convergent plate boundaries plates move toward one another Nature of boundary depends on plates involved oceanic vs continental Ocean ocean plate convergence Marked by ocean trench Benioff zone and volcanic island arc Ocean continent plate convergence Marked by ocean trench Benioff zone volcanic arc and mountain belt Continent Continent plate convergence Marked by mountain belts and thrust faults What Causes Plate Motions Causes of plate motion are not yet fully understood but any proposed mechanism must explain why Mid oceanic ridges are hot and elevated while trenches are cold and deep Ridge crests have tensional cracks The