Drifting Continents and Spreading Seas 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 faded 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 Based on distributions of fossils coastlines old mountains belts and glacial Know Alfred Wegener s theory 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 The angle between magnetic north the direction the north end of a compass needle points and true north Curie Point Temperature above which a material loses all magnetic information Magnetite has a curie point of 580 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 0 90 deg Declination Angle of the remnant magnetic vector with respect to geographic North 0 360 deg Allows the latitude N S at which a rock formed to be calculated based on Allows determination of the location of the magnetic pole 90 latitude in the Use of Paleomagnetism inclination direction of the declination Continental Drift 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 Have Earth s magnetic poles been moving relative to Britain OR Has Britain been moving or drifting relative to Earth s magnetic poles Each continent has a different polar wander path Earth s magnetic poles do not move respect to fixed continents Continents move relative to each other while Earth s magnetic poles stay roughly fixed
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