TODAY: Plate Tectonics (Part I) • Intro: Chemical and physical structure of Earth 1) The plate tectonic system 2) A theory is born 3) Early evidence for continental drift Read Chapter 3! Solid Earth Solid Earth z Rest of the planet Crust Mantle Core Lithosphere and asthenosphere Lithosphere: Crust and uppermost mantle. Broken into 12 plates. Brittle, rigid. 1-100km Asthenosphere: Lower portion of upper mantle, down to 660km Soft, deformable, small amount of melting. 3-18 From Press, Siever: Understanding Earth, 4th edition Sources of Energy for the Earth System 1-Sun drives Earth’s external engine and is responsible for our climate and weather (2) 3-Earth’s internal engine is powered by heat and radioactivity (4) 5-Heat radiating from Earth balances solar and internal heat 6-Meteors add mass from the cosmos From Press, Siever: Understanding Earth, 4th edition 1)1) TheThe plateplate tectonictectonic systemsystem Fig. 3.10 1. The lithosphere is the outer rigid shell of earth, about 100km thick. It is broken into a dozen large plates that move on the plastic asthenosphere (lower part of the upper mantle) 2. The lithosphere is created at mid-ocean spreading centers and subducts when converging with another plate. 3. The driving force of this movement are convection currents in the asthenosphere. 1)1) TheThe plateplate tectonictectonic systemsystem Convection PLATEPLATE1)1) TheThe plateplate1)1) OverviewOverview tectonictectonic systemsystem TECTONICSTECTONICS Compare with Fig. 3-14 PLATEPLATE TECTONICSTECTONICS 2)2) AA theorytheory isis bornborn 1915 Alfred Wegener published hypothesis of continental drift (‘The origins of continents and oceans’) He hypothesized: existence of single “super-continent” Pangaea (“pan - G - uh”) ~ 200 million years ago Pangaea broke into smaller pieces, & “drifted” to present positions P A I S A R U 200 million years ago A L N GONDWANALAND A E A PLATEPLATE 2)2) AA theorytheory isis bornborn TECTONICSTECTONICS 1924 Wegeners book translated to English & met with hostile criticism Main objection: no way to explain continental drift. 3. Early evidence for continental drift JigsawJigsaw puzzlepuzzle ofof thethe continentscontinents FossilFossil recordrecord RockRock typestypes andand structuralstructural similaritiessimilarities PaleoclimatePaleoclimate 3. Early evidence for continental drift JigsawJigsaw puzzlepuzzle ofof thethe continentscontinents The fit of all continents around the Atlantic is almost perfect. The best fit is achieved when taking the 1000 m depth line, the true edge of continents. (1960’s by Sir Edward Bullard) Compare with 3-3 3. Early evidence for continental drift FossilFossil evidenceevidence Several fossil organisms have been found in common on different continents 3. Early evidence for continental drift FossilFossil evidenceevidence 3. Early evidence for continental drift FossilFossil evidenceevidence Several fossil organisms have been found in common on different continents How can the same species evolve on widely separated continents??? PLATE TECTONICS 3. Early evidence for continental drift RockRock evidenceevidence Mountain belts on one continent match up with another. Similar rock structure and age: Appalachians (eastern US) British Isles, Scandanavia 3. Early evidence for continental drift RockRock evidenceevidence Mountain belts on one continent match up with another. PLATE TECTONICS 3. Early evidence for continental drift PaleoPaleoclimateclimate evidenceevidence (ancient) Ice sheets covered big areas of southern hemisphere ~ 220-300 million years ago PLATE TECTONICS 3. Early evidence for continental drift PaleoclimatePaleoclimate evidenceevidence Ice sheets covered big areas of southern hemisphere ~ 220-300 million years ago Glacial striations Plate Tectonics 1)The plate tectonic system 2)A theory is born 3)Early evidence for continental drift 4)Continental drift and paleomagnetism PLATE TECTONICS 4)4) ContinentalContinental driftdrift andand paleomagnetismpaleomagnetism Wegener’s idea died until 1950’s. Renewed interest from rock magnetism Paleomagnetism: ancient magnetic field of Earth recorded and frozen into rocks How? Need to know about 2 things: - Earth’s magnetic field - Magnetism in rocks PLATE TECTONICS 4)4) ContinentalContinental driftdrift andand paleomagnetismpaleomagnetism Rock magnetism -Certain minerals are magnetic (e.g., magnetite, iron) -They loose magnetization when heated above Curie point (580oC for iron) -When cooled below Curie pt, magnetic grain aligns w/ Earth’s magnetic field PLATE TECTONICS 4)4) ContinentalContinental driftdrift andand paleomagnetismpaleomagnetism Rock magnetism -Certain minerals are magnetic (e.g., magnetite, iron) -They loose magnetization when heated above Curie point (580oC for iron) -When cooled below Curie pt, magnetic grain aligns w/ Earth’s magnetic field Fig. 3-7 PLATE TECTONICS 4)4) ContinentalContinental driftdrift andand paleomagnetismpaleomagnetism Geomagnetic reversals Earth’s magnetic field “reverses” recorded in lava flows Fig 3-9 4)4) ContinentalContinental driftdrift andand paleomagnetismpaleomagnetism From dating reversals in the lava record in different continental lava flows, a magnetic time line can be established. There is a reversal about every 500,000 years (short reversal ‘events’ may interrupt the longer magnetic ‘epochs’)..