Drifting(Continents(and(Spreading(Sea s(Proposed(by(German(meteorologist(Alfred(Wegener((• The(origins(of(continents(and(oceans(published(in(1915(• Wegener(hypothesized(a(former(supercontinent(Pangaea(• Idea(was(based(on(abundant(of(evidence(Continental(Drift(Wegener’s(idea(was(debated(and(ridiculed(• Most(scientists(didn’t(believe(him(• Lack(of(a(mechanism(for(drift(–a(major(criticism(Wegener(died(in(1930(at(the(age(of(40(Lacking(an(advocate,(the(drift(hypothesis(fa ded(His(idea(was(revived(in(the(1950’s(Continental(Drift(• Sea(floor(spreading(• Subduction(• Plate(tectonics(What(is(plate(tectonics?(• Plate(tectonics(is(the(theory(that(the(Earth’s(surface(is(covered(with(a(small(number(of(internally(rigid(lithospheric(plates(• The(plates(move(about(on(the(Earth’s(surface,(and(the(vast(majority(of(earthquakes,(volcanoes(and(tectonic(activity(occurs(where(they(interact(What(you(should(know((continental(drift)(Continental(drift(is(the(idea(that(the(continents(have(moved(Pangaea((Know(Alfred(Wegener’s(theory(• Based(on(distributions(of:(fossils,(coastlines,(old(mountains(belts,(and(glacial(and(tropical(rock(units(• Problems(with(mechanisms((Before(continental(drift(What(were(scientists(thinking?(• The(oceans(and(the(continents(were(permanently(fixed.(• There(were(no(changes(in(continents(back(in(time,(Evidence(of(drastic(changes(interpreted(as…(• Shrinkage(effects(• Rebound(from(thick(sediment(loading(• The(result(of(“upheaval”(Continental(“fit”(• In(1915,(Alfred(Wegener(published(comprehensive(book(on(continental(drift(• The(continents(seem(to(fit(together(• The(fit(could(not(be(coincidental(Alfred(Wegener(Correctly(theorized(that:(• The(continents(moved(• 200(Ma(they(formed(the(super(continent(Pangaea(Proposed(incorrectly(that(the(continents(“plowed(through(basaltic(ocean(basins”(Wegener’s(line(of(geologic(evidence(fro(continental(drift(• Fit(of(the(continents((• Locations(of(past(glaciations(• Location(of(past(tropical(regions(• Distribution(of(fossils(• Continuity(of(geologic(units(an(mountain(belts(between(continents((Glacial(evidence(• Glaciers(form(mostly(at(high(latitudes(• Past(locations(of(glaciers(past(locations(of(continents(• At(several(times(during(earth’s(history,(glaciers(covered(large(areas(of(continents(ice(ages(• Permian(glacial(till(is(found(on(5(continents(• The(tills(in(Africa(and(India(are(now(near(equator(• A(cooler(earth?(No,(tropical(plants(also(preserved(Paleoclimatic(Evidence((• Placing(Pangaea(over(the(Permian(South(Pole(• He(correctly(predicted…(• Tropical(coals(• Tropical(reefs(• Subtropical(deserts(• Subtropical(eavporites((Fossil(Evidence(Identical(fossils(found(on(widely(separated(land.(• Mesosaurus(–(a(freshwater(reptile((• Glossopteris(–subpolar(plant(with(heavy(seeds(• Lystrosaurus(–A(land(dwelling(reptile(• Cynognathus(–A(land(dwelling(reptile(These(organisms(could(not(have(crossed(an(ocean(Matching(geology(• Geologic(phenomena(match(across(the(Atlantic((• Geologic(structures(• Rock(types((• Rock(ages(Geologic(phenomenon(match(across(the(Atlantic(Mountain(belts(• Appalachians(• Caledonides(Criticisms(of(drift(Why(want(continental(drift(accepted?(• Wegener(suggested(that(continents(plowed(through(the(ocean(crust,(much(like(ice(breakers(cut(through(ice(• He(invoked(rotational((centrifugal)(forces( (• These(ideas(were(easily(discredited;(so(was(his(theory(No(mechanism(that(worked(• Geophysicists(were(particularly(critical(because(they(rightly(showed(that(the(proposed(mechanisms(could(not(function((• Cambridge(Professor(Sir(Harold(Jeffreys(proved(that(Wegener’s(mechanism(was(impossible((continents(plowing(through(basalt(like(an(ice(breaker)(After(Wegener,(but(before(plate(tectonics(Arthur(Holmes(• British(geologist(• Did(pioneering(geochronologic(work(on(the(age( of( the(Earth(billions(of(years(old((• In(the(1930’s(and(1940’s(argued(for(the(existence(of(convection(• (Cells(in(the(Earth’s(mantle(as(a(mechanism(of(continental(drift(• Holmes(got(the(mechanism(of(plate(tectonics(basically(correct,(but(lacked(hard(data(to(back(it(up(!Paleomagnetism!• *(In(1950’s,(scientists(discovered(that(rocks(can(become(magnetized(in(the(direction(of(the(Earth’s(magnetic(field(at(the(time(they(are(formed.(• *Remnant(magnetism(can(be(preserved(for(millions(of(years(What(you(should(know((paleomagnetism)(• the(Earth(has(a(dipole(magnetic(field(• Remnant(magnetism(preserved(in(rocks((• Curie(point((• The(difference(between(inclination(and(declination(and(what(they(can(tell(us(• Apparent(polar(wander(curves(• Why(magnetic(reversals(are(important(to(plate(tectonics(The(Earth’s(magnetic(field(*Dipole(magnetic(field(*During(normal!polarity(a(compass(needle(points(north(((Inclination(of(the(Earth’s(magnetic(field(changes(from(0(deg.(at(the(equator(to(90(deg.(at(the(magnetic(pole(Magnetic!declination!c The(angle(between(magnetic(north((the(direction(the(north(end(of(a(compass(needle(points)(and(true(north.(c (c Curie!Point!c Temperature(above(which(a(material(loses(all(magnetic(information(c *Magnetite(has(a(curie(point(of(≈(580°(C(c *Magnetite(is(important(because(it(forms(a(strong(magnetic(field(Can!the!study!of!paleomagnetism!detect!changes!in!the!position!of!the!Earth’s!magnetic!poles,!relative!to!the!continents,!over!geologic!time?!What!is!actually!measured!in!paleomagnetism!?!Inclination:!Angle(of(the(remnant(magnetic(vector(with(respect(to(horizontal((0c90(deg.)(Declination:!Angle(of(the(remnant(magnetic(vector(with(respect(to(geographic(North((0c360(deg.)(Use(of(Paleomagnetism(c Allows(the(latitude((NcS)(at(which(a(rock(formed(to(be(calculated(based(on(inclination(c Allows(determination(of(the(location(of(the(magnetic(pole((90clatitude(in(the(direction(of(the(declination)(Continental(Drift(c In(early(1950’s(measurements(of(the(paleomagnetism(in(a(collection(of(rock(samples(from(Britain(implied(that(the(position(of(Earth’s(magnetic(poles(had(indeed(changed,(relative(c Have(Earth’s(magnetic(poles(been(moving(relative(to(Britain(OR(c Has(Britain(been(moving(or(drifting(relative(to(Earth’s(magnetic(poles?(c Each(continent(has(a(different(polarcwander(path(c Earth’s(magnetic(poles(do(not(move(respect(to(fixed(continents(c
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