2 In this Chapter3 Alfred Wegener4 Plate Tectonics5 Plate Tectonics6 Glacial Evidence7 Paleoclimatic Evidence8 Fossil Evidence9 Matching Geologic Units10 Criticisms of Wegener’s Ideas11 Earth’s Magnetic Field12 Magnetic Poles13 The Earth’s Magnetic Field14 The Earth’s Magnetic Field15 Paleomagnetism16 Paleomagnetism17 Polar Wandering18 Apparent Polar Wandering19 Polar Wandering20 Sea-Floor Bathymetry21 The Ocean Floor22 The Ocean Floor23 The Ocean Floor24 The Oceanic Crust25 The Oceanic Crust26 Sea-Floor Spreading27 Sea-Floor Spreading28 Evidence of Sea-Floor Spreading29 Evidence of Sea-Floor Spreading30 Magnetic Reversals31 Magnetic Reversals32 Sea-Floor Spreading33 Sea-Floor Spreading34 Plate Tectonics35 Lithosphere36 Two Types of Lithosphere37 Plate Boundaries39 Plate Boundaries40 Plate Boundaries: Three Types41 Plate Boundaries: Three Types42 Plate Boundaries: Three Types43 Divergent Boundaries44 Divergent Boundaries45 Divergent Boundaries46 Mid-Ocean Ridges47 Mid-Ocean Ridges48 Ocean Crustal Age49 Oceanic Lithosphere50 Convergent Boundaries51 Subduction52 Convergent Boundaries53 Subduction Features54 Transform Boundaries55 Plate Collision56 Driving Mechanisms57 Plate Velocities58 Plate Velocities59 The Dynamic PlanetEssentials of Geology, 4th editionby Stephen Marshak© 2013 W.W. Norton & CompanyChapter 2The Way the Earth Works: Plate TectonicsPowerPoint slides prepared by Rick Oches, Professor of Geology & Environmental Sciences, Bentley University, Waltham, Massachusetts11Chapter 2: The Way The Earth Works: Plate Tectonics2 In this ChapterWhat were Wegener’s observations?Paleomagnetism: the key proof of continental driftObservations that led Harry Hess to sea-floor spreadingThree kinds of plate boundariesPlate interactions and rates of plate movement3 Alfred WegenerGerman meteorologist and polar explorer.Wrote The Origins of the Continents and Oceans in 1915. He hypothesized a former supercontinent, Pangaea. He suggested that land masses slowly move (continental drift).These were based on strong evidence.“Fit” of the continentsGlacial deposits far from polar regionsPaleoclimatic beltsDistribution of fossilsMatching geologic units4 Plate TectonicsWegener’s idea was the basis of a scientific revolution.Earth continually changes.Continents move, split apart, and recombine.Ocean basins open and close.His hypothesis was met with strong resistance: “What force could possibly be great enough tomove the immense mass of a continent?”Essentials of Geology, 4th editionby Stephen Marshak© 2013 W.W. Norton & CompanyChapter 2The Way the Earth Works: Plate Tectonics5 Plate TectonicsThe scientific revolution began in 1960.Harry Hess (Princeton) proposed sea-floor spreading.As continents drift apart, new ocean floor forms between.Continents converge when ocean floor sinks into the interior.By 1968, a complete model had been developed. Continentaldrift, sea-floor spreading, and subduction. Earth’s lithosphere is broken into ~20 plates that interact.6 Glacial EvidenceEvidence of Late Paleozoic glaciers found on five continents.Some of this evidence is now far from the poles.These glaciers could not be explained unless the continents had moved.7 Paleoclimatic EvidencePlacing Pangaea over the Late Paleozoic South Pole:Wegener predicted rocks defining Pangea climate belts.Tropical coals Tropical reefs Subtropical deserts Subtropical evaporites8 Fossil EvidenceIdentical fossils found on widely separated land. Lystrosaurus—A nonswimming, land-dwelling reptile. Cynognathus—A nonswimming, land-dwelling mammal-like reptile.These organisms could not have crossed an oceanPangaea explains the distribution.9 Matching Geologic UnitsDistinctive rock assemblages and mountain belts match across the Atlantic.10 Criticisms of Wegener’s IdeasWegener had multiple lines of strong evidence.Yet, his idea was debated, ridiculed, and ignored. WHY? He couldn’t explain how or why continents moved. Wegener died in 1930 on a Greenland expedition. Over the next three decades, new research, new technology, and new evidence from the oceans revived his hypothesis.11 Earth’s Magnetic FieldFlow in the liquid outer core creates the magnetic field.It is similar to the field produced by a bar magnet.The magnetic pole is tilted ~11.5° from the axis of rotation (geographic north).12 Magnetic PolesThe magnetic pole intersects Earth’s surface just like the geographic pole does.Magnetic N pole and magnetic S pole both exist.Magnetic poles are located near geographic poles.13 The Earth’s Magnetic FieldGeographic and magnetic poles are not the same.A compass points to magnetic N, not geographic N.The difference between geographic N and magnetic N is called declination. It depends on:Absolute position of the two polesGeographic northMagnetic northLongitude14 The Earth’s Magnetic FieldCurved field lines cause a magnetic needle to tilt.Angle between magnetic field line and surface of the Earth is called inclination. It depends on:Latitude15 PaleomagnetismRock magnetism can be measured in the laboratory.The study of fossil magnetism is called paleomagnetism.Iron (Fe) minerals in rock preserve information about the magnetic field at the time the rocks formed.Declination and inclination preserved in rocks often vary from present latitude / longitude.Instruments used in paleomagnetism record changes in position.These data are used to trace continentaldrift.16 PaleomagnetismIron minerals archive the magnetic signal at formation.Cooled magmaLow Temp—permanent magnetizationThermal energy of atoms slows.Dipoles align with Earth’s magnetic field.Magnetic dipoles become frozen in alignment with field17 Polar WanderingLayered basalts record magnetic changes over time.Inclination and declination indicate change in position.18 Apparent Polar WanderingPolar wandering paths were initially misinterpreted:Not the signature of a wandering pole on a fixed continentThe signature of a fixed pole on a wandering continent19 Polar WanderingEach continent had a separate polar wandering path. Now understood to represent that:The location of the magnetic pole is fixed.The