Lecture'5'Continental'Drift'• Continents'move'relative'to'each'other'• Wegener'was'right'How'does'drift'occur?'The'Ocean'floor'• Prior'to'1950s,'very'little'was'known'about'seaEfloor'bathymetry'(shape'of'the'sea'floor'surface)'• Invention'of'echo'sounding'(sonar)'permitted'mapping'of'the'ocean'floor'• Velocity='distance/time'• Distance=velocity'x'time'Oceanographers'were'surprised'to'discover'that…'• The'deepest'parts'of'the'ocean'occur'near'land'• A'mountain'range'runs'through'every'ocean'basin'• Submarine'volcanoes'form'lines'across'oceans'floors''Sonar'mapping'delineated'bathymetric'features'• Abyssal'plain'(~4E5'below'sea'level)'• Mid'oceans'ridges'(Mors):'submarine'mountain'ranges'(peaks'~2E3km'below'sea'level)'• Trenches'(near'land,'>5km).'• Volcanic'islands'• Seamount,'guyots'• Fractures'zones'Modern'views'of'the'ocean'floor'reveal''• MidEocean'ridges'• Trenches'• Fracture'zones'Global'Sea'floor'topography'Oceanic'Crust'By'1950,'we'had'learned'much'about'oceanic'crust'Oceanic'crust'is'covered'by'sediment'• Thickest'near'the'continents'• Thinnest'(or'absent)'at'the'midEocean'ridge'Oceanic'crust'is'mafic'(basalt'and'gabbro)'• No'granitic'rocks'• No'metamorphic'rocks'High'heat'flow'characterizes'the'midEocean'ridge'Belts'of'concentrated'subsea'earthquakes'were'found'• Parts'of'oceanic'fracture'zones'• Mid'ocean'axes''• Deep'ocean'trenches'The'earthquakes'were'limited'to…'• Parts'of'oceanic'fracture'zones'• Mid'ocean'ridge'axes'• Deep'ocean'trenches'Sea'Floor'Spreading'In'1960,'Harry'Hess'published'his'“essay'in'Geopoetry.”'He'called'his'theory'“seaEfloor'spreading”'• Upwelling'mantle'erupts'at'the'mid'ocean'ridges'• New'crust'moves'away'from'ridges,'gathering'sediment'• At'trenches'the'seaEfloor'dives'back'into'the'mantle'Provided'a'potential'mechanism'for'continental'drift'Magnetic'Anomalies'• Strength'of'Earth’s'magnetic'field'can'be'measured'by'magnetometer'• At'any'given'location,'the'magnetic'field'includes'two'parts:'• One'created'by'the'main'dipole'of'the'Earth'• Another'created'by'the'magnetism'of'near'surface'rock'• Anomaly'–difference'between'the'expected'strength'of'the'Earth’s'main'field'at'a'certain'location'and'the'actual'measured'strength'of'the'magnetic'field'at'that'location''• +'Or'–'magnetic'anomalies'• Towed'magnetometers'measure'ocean'crust'• Magnetic'anomaly'oscillates'perpendicular'to'the'MOR'• Anomalies'(+'And'E)'are'linear'belts'that'parallel' M OR'Magnetic'Reversals'• Layered'lava'flows'reveal'reversals'in'polarit y'• When'reversed,'the'north'magnetic'pole'is'near'the'south'geographic'pole'• Reversals'are'geologically'rapid'• Radioactivity'permits'rock'ageEdating'• A'geomagnetic'reversal'time'scale'has'been'assembled'• Reversals'occur'every'500E700'ka'• 171'are'known'since'the'end'Cretaceous'MagneticEReversal'Chronology'• Reversals'do'not'occur'regularly'• The'lengths'of'different'polarity'chrons'(i.e.'time'intervals'between'reversals)'are'different'Sea'floor'spreading:'Proof'• Polarity'reversals'explain'magnetic'anomalies'• Positive'anomalies'–crust'with'normal'polarity'• Negative'anomalies'–crust'with'reversed'polarity'• Magnetic'anomalies'are'symmetric'across'the'MOR'Magnetic'anomalies'mimic'layered'lava'flows'• Magnetic'“stripes”'form'as'lava'cools'at'a'MOR'• Ocean'crust'spreads'away'from'MOR'• Reversals'are'recorded'with'cooled'lava'Sea'Floor'spreading'is'the'mechanism'of'continental'drift'Sea'Floor'Spreading'• Age'of'oceanic'crust'increases'away'from'the'MOR'• Ages'are'“mirror'images”'across'the'MOR'• Sediments'atop'oceanic'basalts'become'thicker'away'from'the'MOR'• LowerEmost'(oldest)'layers'become'progressively'older'away'from'the'MOR'Sea'Floor'Spreading'supported'by:'• Global'earthquake'belts'• SeaEFloor'topography'oceanic'transform'fault'motion'• Linear'magnetic'anomalies''• Young'age'of'oceanic'crust'•
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