plate tectonics: sea floor spreading; plate motions!plate tectonics!Earth’s surface composed of!lithospheric plates!•“float or ride” on !top of asthenosphere!• move relative to interior!• concentrate deformation!at their boundaries!Arabia!Africa!Asia!India!• plates can contain both!continental and oceanic !crust and the upper mantle!major plates!from: http://www.uh.edu/~jbutler/physical!plate motion!absolute motion!motion relative to an external reference frame!orange: divergence "blue: convergence "green: transcurrent!plate motion!relative motion!absolute motions of plates leads to their motion! relative to each other!relative vs. absolute motion!think about two trains traveling down the track!absolute motions:!"train from Westford: "70 mph to east (+ direction)!"train from Eastford: "60 mph to west (- direction)!relative motion of two trains with respect to each other:!(70 mph east) - (-60 mph west) = 130 mph toward each other!relative vs. absolute motion!same two trains traveling down the track from Westford!absolute motions:!"train 1 from Westford: "50 mph to east (+ direction)!"train 2 from Westford: "70 mph to east (+ direction)!relative motion of two trains with respect to each other:! (70 mph east) - (50 mph east) = 20 mph!50 mph!70 mph!relative plate motions yield 3 different types of boundaries:!from: http://pubs.usgs.gov/publications/text!Alfred Wegener proposed idea in 1912!German meteorologist, astronomer, !"balloonist, explorer!• detailed older evidence:! continuity of structures, formations,!"fossil flora and fauna!argued for a supercontinent called Pangaea!plate tectonics!has its intellectual roots in continental drift!• brought new information:! widespread Paleozoic glaciation!"in southern continent!• noted fit of continental coastlines!from: http://www.uwgb.edu/dutchs/202ovhds/platetec.htm!supercontinent Pangaea!composed of northern Laurentia and southern Gondwana!break-up of supercontinent Pangaea!motion of continents through time!Plate reconstruction shown here starts at 730 MyBP - Late Proterozoic!continental drift: fossil evidence!Permian-Pennsylvanian!Triassic!Permian!Triassic!Africa, South America, Antarctica, India, Australia! have nearly identical fossils!from: http://pubs.usgs.gov/publications/text/continents.html!continental drift: rock evidence!matching rock types !and fit of continents !(fit not great, but ok)!distribution of Paleozoic glaciers !and striations for glacial flow!continents assembled together!explains striation patterns!important new contribution by Wegener!"(other evidence was well known)!continental drift: evidence of glaciation!glaciation: 2 possible alternatives to explain!1) continents stayed in place and poles wandered!…look at modern distribution of climate zones…!2) poles remained fixed and continents wandered!from: http://volcano.und.nodak.edu:80/vwdocs/vwlessons/plate_tectonics/part5.html!present-day climate!apparent polar wander!Wegener used climate zones to reconstruct where!the South Pole had to be through time!pole seems to move!Wegener believed the!continents moved,!not the poles!continental drift: initially rejected (1920’s)!Wegener proposed that continents “plowed” through oceans!• violated what was known about the strength of rocks! (if you pushed with the force necessary, rocks would break, not move)!• could not explain motion of continents by Earth’s rotation!and/or tidal forces (too small)!continental drift rejected for lack of “driving mechanism”!earth’s magnetic field!and!magnetism of rocks!formed at various!latitudes differ!(remember “bar” magnet)!paleomagnetism: development of field in 1950’s!dip, or inclination, of!magnetic lines of force!change as field!exits South pole and!enters North pole!steeper inclinations!near poles!paleomagnetism: measuring magnetism in rocks of!" " ""any age!measure rocks in !North America!inclinations are consistent!with forming in a!magnetic field!with a pole in Asia!apparent polar wander paths!--apparent movement of poles--!--paths for continents are different--!alternative: pole is (relatively) fixed and continents move!…resulting in “apparent” (large) motion of pole…!paleomagnetism: measurement in rocks of many ages! reveals magnetic north changes through time! -- older rocks have magnetic poles farther away!Europe!North America!reconstruction of continents: using paleomagnetic! data and fit of continental shelves worked well!lining up of rock units!match of glacial!striations!fit of continental shelves!sea floor spreading: key, critical concept!have already discussed many of the components!• reversals of Earth’s magnetic field!• acquisition of magnetism in rocks!• convection in mantle (heat loss from interior)!• presence of mid-ocean ridges!• bathymetry of sea-floor!• composition of oceanic crust!sea floor spreading!diagram shows what Harry Hess proposed in 1950’s!• sea floor moves away from ridge due to mantle convection!• mantle convection reflects rising hot material/sinking cold!• hot material rises under ridge -- high elevation, heat flow,!" " """"basaltic volcanism!sea floor spreading and plate tectonics!bottom diagram is modern understanding!• sea floor spreading from rising hot mantle rock at ridge!• subduction of cooled oceanic crust at trench!sea floor spreading: what happens?!divergent boundary -- plates move apart at ridge!basaltic volcanism occurs at ridge!to generate new oceanic crust!new oceanic crust forms sea floor!and occupies space from spreading!basalts acquire magnetism of!field at time of eruption!older sea floor moves away!from ridge and cools!pattern of normal and!reversed “stripes” of!magnetism is!recorded on sea floor!sea floor spreading: magnetic “stripes”!ships measuring magnetic field discovered “stripes”!magnetic field strength!did not match that!of present field, but was!greater (positive anomaly)!or !lesser (negative anomaly)!in symmetric patterns!centered on ridge axis!corresponded to!normally magnetized !(+ add to modern field)!and!reversely magnetized !(- subtract from modern)!sea floor basalt!sea floor spreading: magnetic “stripes”!sea floor pattern matches that seen in continental rocks! (Vine and Matthews)!can determine age of seafloor!--seafloor is older farther from ridge--!Gilbert Gaussknow age (time) and distance from ridge (length)! …yields spreading velocity…!1 cm/yr spreading rate