Plate TectonicsChapter 20Early Evidence (Wegener)• The geometric fit of the continents.• The similarity in rock age groups betweenadjoining regions.• The similarity in Paleozoic fossils betweenadjoining areas.• The distribution of Paleozoic glaciation inS.America, S.Africa, Australia, and India.Thegeometric fitof thecontinents.Similarity of PaleozoicFossils in adjoining regions.Early Objections• Mantle is solid (Transmits S-waves).• How can continents move and remain intact?• What is the driving force?Polar Wandering• Paleomagnetics is the study of rock magnetism.• Magnetic minerals in rocks record the direction ofthe Earth’s magnetic field at the time they cool.• Each continent gives a different path of the pole.• Rocks from the same continent gave the samepath.Polar WanderingMagnetic Anomalies• Magnetic minerals such as magnetite (Fe3O4)record Earth’s field.• They also perturb the field by a small amount.• Perturbations are called magnetic anomalies.• Anomalies can be mapped using magnetometersdragged behind aircraft or ships.Magnetic Anomalies• Ship-born magnetometers recorded patterns ofmagnetic anomalies in the North Atlantic (1965-7)• Patterns form stripes parallel to Mid-AtlanticRidge.• Stripes are symmetric about the Ridge.• Stripes are due to magnetic minerals in the basaltrecording the magnetic field at the time of theiremplacement.Magnetic Anomalies• Anomalies were first solid evidence of sea-floor spreading.• Here was a credible hypothesis thatdemanded testing.Test the Hypothesis• Rocks in adjoining parts of Africa and SouthAmerica were age-dated using new radio-isotopicmethods: they matched perfectly.• Rocks were dredged from the sea floor: theyshowed ages symmetrically increasing away fromthe ridge. There were no old (>250MY) rocks.• The fit of continents was revised using the edge ofthe continental slope rather than the coastline: thematch was nearly perfect.Rocks were dredged from the sea floor:They showed ages symmetricallyincreasing away from the ridge.There were no old rocks (>250MY).• Seismic evidence suggested that crust wasbeing subducted (returned to the mantle) atconvergent boundaries to balance crustproduction at the ridges.• Hess proposed a plausible mechanism forthe driving force that moved continents:thermal convection in the solid mantle.Test the HypothesisTheory of Plate Tectonics• There appear to be 13 major plates that cover theglobe.• The plates can contain oceanic, or continentalcrust or both.• New oceanic crust is created at the mid-oceanridge (divergent boundary).• Old oceanic crust is consumed (subducted) atconvergent plate boundaries.Deep Earthquakes are only below deep oceantrenches. Velocity structure suggests subduction.Red = slow = hot green = fast = coldThere appear to be 13 major plates thatThere appear to be 13 major plates thatThere appear to be 13 major plates thatThere appear to be 13 major plates thatcover the globecover the globecover the globecover the globe.Theory of Plate Tectonics• Continental crust resists subduction.• Continent-continent convergent boundaries formmajor mountains.• Ocean-ocean and ocean-continent convergentboundaries form subduction zones marked bydeep ocean trenches and Benioff Zones (deepearthquake zones extending to 670km).Plate Boundaries• Divergent (Ocean - Ocean) Mid Atlantic Ridge:Ocean ridge volcanism• Convergent– (Ocean - Ocean) Tonga Trench: Island arcvolcanism– (Ocean - Continent) West Coast of South America:Andesitic to silicic volcanism– (Continent - Continent) Alps, Himalayas: Little orno volcanism• Transform (San Andreas) Little or no volcanismDivergent and TransformBoundariesPassive and Active Continental MarginsPassive and Active Continental MarginsPassive and Active Continental MarginsPassive and Active Continental
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