Chapter 2 The Way the Earth Works: Plate Tectonics Essentials of Geology, 4th edition by Stephen Marshak © 2013 W.W. Norton & Company PowerPoint slides prepared by Rick Oches, Professor of Geology & Environmental Sciences, Bentley University, Waltham, Massachusetts 1 Chapter 2 The Way The Earth Works: Plate Tectonics 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 movement 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 units 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 to move the immense mass of a continent?” 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. " Continental drift, sea-floor spreading, and subduction. " Earth’s lithosphere is broken into ~20 plates that interact. 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. Paleoclimatic Evidence ! Placing Pangaea over the Late Paleozoic South Pole: ! Wegener predicted rocks defining Pangea climate belts. " Tropical coals " Tropical reefs " Subtropical deserts " Subtropical evaporites 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. Matching Geologic Units ! Distinctive rock assemblages and mountain belts match across the Atlantic. 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. 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). 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. 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 " Longitude 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: " Latitude 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 continental drift. 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 field. Polar Wandering ! Layered basalts record magnetic changes over time. ! Inclination and declination indicate change in position. 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 continent Polar Wandering ! Each continent had a separate polar wandering path. Now understood to represent that: " The location of the magnetic pole is fixed. " The continents themselves have moved. ! These curves align when continents are reassembled. Sea-Floor Bathymetry ! Before World War II, we knew little about the sea floor. ! Echo-sounding (sonar) allowed rapid sea-floor mapping. ! Sea-floor maps created by ships crossing the oceans. The Ocean Floor ! Oceanographers were surprised to discover that: " A mid-ocean mountain range runs through every ocean. " Deep-ocean trenches occur near volcanic island chains. " Submarine volcanoes poke up from the ocean floor. " Huge fracture zones segment the mid-ocean ridge. ! These observations are all explained by plate tectonics. The Ocean Floor ! Sonar mapping delineated bathymetric features. " Mid-ocean ridges " Deep-ocean trenches " Volcanic islands " Seamounts " Fracture zones The Ocean Floor ! Today’s view of the ocean floor reveals the location of: " Mid-ocean ridges " Deep-ocean trenches " Oceanic fracture zones The Oceanic Crust ! By 1950, we had learned much about oceanic crust. ! Oceanic crust is covered by sediment. " Thickest near the continents " Thinnest (or absent) at the mid-ocean ridge ! Oceanic crust consists primarily of basalt. " Lacks variety of continental rock types " No metamorphic rocks ! Heat flow