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THE EARTH THROUGH TIME TENTH EDITION H A R O L D L. L E V I N © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 1 CHAPTER 7 Plate Tectonics Underlies All Earth History © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 2 EARTHQUAKES Earthquake = The rapid release of energy by the sudden movement of the Earth. Much of the energy is released in the form of seismic waves. Scientist use seismic waves to investigate the interior of the Earth. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 3 SEISMIC WAVES Focus (or hypocenter) = the place within the Earth where the rock breaks, producing an earthquake. Epicenter = the point on the ground surface directly above the focus. Energy moving outward from the focus of an earthquake travels in the form of seismic waves. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 4 TYPES OF SEISMIC WAVES Body waves—Seismic waves that travel through the interior of the Earth a. P-waves b. S-waves Surface waves—Seismic waves that travel along the interface between the surface of the crust and the atmosphere. a. Love waves b. Rayleigh waves © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 5 TYPES OF SEISMIC WAVES Body waves a. b. P-waves Primary, pressure, push-pull Fastest seismic wave (6 km/sec in crust; 8 km/sec in uppermost mantle) Travel through solids & liquids S-waves Secondary, shear, side-to-side Slower (3.5 km/sec in crust; 5 km/sec in upper mantle km/sec) Travel through solids only © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. FIGURE 7-2 P- and S-type seismic waves. 6 TYPES OF SEISMIC WAVES Surface wave types Love waves—Shear motion Rayleigh waves—Orbital motion (similar to ocean waves) Surface wave characteristics Slowest Typically localized near the epicenter Causes damage to structures during an earthquake © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 7 SEISMOGRAPHS Earthquakes are recorded on an instrument called a seismograph. The record of the earthquake produced by the seismograph is called a seismogram. FIGURE 7-1 Typical seismograph record. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 8 DETERMINING THE EARTH'S INTERNAL STRUCTURE The Earth is a layered body. The layered structure is determined from studies of how seismic waves behave as they pass through the Earth. P- and S-wave travel times depend on properties of rock materials through which they pass. Differences in travel times correspond to differences in rock properties. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 9 DETERMINING THE EARTH'S INTERNAL STRUCTURE Seismic wave velocity depends on the density and elasticity of rock. Seismic waves travel faster in denser rock. Speed of seismic waves increases with depth (pressure and density increase downward). © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 10 DETERMINING THE EARTH'S INTERNAL STRUCTURE Boundaries between the layers are called discontinuities (produced by abrupt changes in seismic wave velocities typically linked to changes in rock properties). Mohorovičić discontinuity (Moho) between crust and mantle Gutenberg discontinuity between mantle and core © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 11 DETERMINING THE EARTH'S INTERNAL STRUCTURE Curved wave paths indicate gradual increases in density and seismic wave velocity with depth. Refraction (bending of waves) occurs at discontinuities between layers. FIGURE 7-6 Seismic waves refract (bend) as they travel through Earth. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 12 S-WAVE SHADOW ZONE Place where no S-waves are received by seismograph. Extends across the globe on side opposite from the epicenter. S-waves cannot travel through the molten (liquid) outer core. Larger than the P-wave shadow zone. FIGURE 7-6 Seismic waves refract (bend) as they travel through Earth. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 13 P-WAVE SHADOW ZONE Place where no Pwaves are received by seismographs. Makes a ring around the globe. Smaller than the S-wave shadow zone. FIGURE 7-6 Seismic waves refract (bend) as they travel through Earth. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 14 THE EARTH'S INTERNAL STRUCTURE Crust Mantle Outer core Inner core FIGURE 7-5 What’s inside Earth. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 15 CRUST Continental Crust—A heterogeneous mixtures of rocks that approximates the composition of granite. Oceanic Crust—A relatively homogeneous rock of basaltic composition. FIGURE 7-10 Generalized crosssection showing Mohorovičić discontinuity. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 16 CONTINENTAL CRUST Rock composition mixture that approximate Granite composition Averages about 35 km thick; 60 km in mountain ranges about 2.7 g/cm3. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 17 OCEANIC CRUST Basaltic composition 5–12 km thick About 3.0 g/cm3 Has layered structure consisting of: Thin layer of sediment covers basaltic igneous rock (about 200 m thick) Pillow basalts: basalts that erupted under water (about 2 km thick) Gabbro: coarse grained equivalent of basalt; cooled slowly (about 6 km thick) © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 18 LITHOSPHERE Lithosphere = outermost 100 km of Earth. Consists of the crust plus the outermost part of the mantle. This layer tends to behave in a ridged manner. Divided into tectonic or lithospheric plates that cover surface of Earth FIGURE 7-5 What’s inside Earth. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 19 ASTHENOSPHERE Asthenosphere = low velocity zone (seismic wave velocity decreases) below the lithosphere. Rocks are at or near melting point. Magmas generated here. Solid that flows (rheid); plastic behavior. Slip surface for plate motion above. FIGURE 7-5 What’s inside Earth. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 20 MANTLE Silica based composed (“rocky”) rich in iron and magnesium based mineral. Peridotite (Mg Fe silicates, olivine) Kimberlite (contains diamonds) Eclogite 2885 km thick Average density = 4.5 g/cm3 Solid that flows; plastic behavior. Not uniform. Several concentric layers with differing properties. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 21 ISOSTASY (CRUST-MANTLE INTERACTION) Buoyancy and floating of the Earth's crust on the mantle. Denser oceanic crust floats lower, forming ocean basins. Less dense continental crust floats higher, forming continents. As erosion removes part of the crust, it rises isostatically to a new level. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 22 CORE Outer core Molten Fe (85%) with some Ni. May contain lighter elements such as Si, S, C, or O. 2250 km thick Liquid. S-waves do not pass through outer core. Inner core Solid Fe (85%) with some Ni 1220 km radius (slightly larger than the Moon) Solid © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 23 CORE AND MAGNETIC FIELD Convection in liquid outer core plus spin of solid inner core generates Earth's magnetic field. Magnetic field is also evidence for a dominantly iron core. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 24 CRUSTAL STRUCTURES—FAULTS A fault is a crack in the Earth's crust along which movement has occurred. Types of faults: Dip-slip faults: movement is vertical Normal faults Reverse faults and thrust faults Strike-slip faults or lateral faults: movement is horizontal. Oblique-slip faults: both vertical and horizontal movement © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 25 FAULTS FIGURE 7-57 Types of faults. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 26 CRUSTAL STRUCTURES—FOLDS During mountain building or compressional stress, rocks may deform plastically to produce folds. Types of folds Anticline Syncline Monocline Dome Basin A. B. C. D. E. FIGURE 7-63 Types of folds. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 27 ANTICLINE © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. R. R. Mudge/USGS 28 SYNCLINE © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. USGS 29 PLATE TECTONICS Plate Tectonic theory was proposed in mid-twentieth century. It is a unifying theory showing how a large number of diverse, seemingly-unrelated geologic facts are interrelated. The theory was the linkage to two ideas: Continental Drift and Sea Floor Spreading. Plate Tectonic theory involves a number of large plates plus numerous small plates composed of crust and upper mantle (Lithosphere) that move slowly, change size, and shape. The Earth’s surface is a dynamic surface. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 30 THE DATA BEHIND PLATE TECTONICS Geophysical data collected after World War II provided foundation for scientific breakthrough: Echo sounding for sea floor mapping discovered patterns of midocean ridges and deep sea trenches. Magnetometers charted the Earth's magnetic field over large areas of the sea floor. Global network of seismometers (established to monitor atomic explosions) provided information on worldwide earthquake patterns. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 31 EVIDENCE IN SUPPORT OF THE THEORY OF PLATE TECTONICS 1. Shape of the coastlines: the jigsaw puzzle fit of Africa and South America. FIGURE 7-15 Fit of the continents about 200 million years ago. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 32 EVIDENCE IN SUPPORT OF THE THEORY OF PLATE TECTONICS 2. • • • • • Paleoclimatic evidence: Ancient climatic zones match up when continents are moved back to their past positions. Glacial tillites Glacial striations Coal deposits Carbonate deposits Evaporite deposits © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 33 EVIDENCE IN SUPPORT OF THE THEORY OF PLATE TECTONICS 3. Fossil evidence implies once-continuous land connections between now-separated areas Mesosaurus © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. Glossopteris 34 EVIDENCE IN SUPPORT OF THE THEORY OF PLATE TECTONICS 4. Distribution of present-day organisms indicates that they evolved in genetic isolation on separated continents (such as Australian marsupials). © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 35 EVIDENCE IN SUPPORT OF THE THEORY OF PLATE TECTONICS 5. Geologic similarities between South America, Africa, and India Same stratigraphic sequence (same sequence of layered rocks of same ages in each place) Mountain belts and geologic structures (trends of folded and faulted rocks line up) Precambrian basement rocks are similar in Gabon (Africa) and Brazil. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 36 EVIDENCE IN SUPPORT OF THE THEORY OF PLATE TECTONICS 6. 7. 8. Geologic similarities between Appalachian Mountains and Caledonian Mountains in British Isles and Scandinavia. Rift Valleys of East Africa indicate a continent breaking up. Evidence for subsidence in oceans Guyots: flat-topped sea mounts (erosion when at or above sea level). Chains of volcanic islands that are older away from site of current volcanic activity: Hawaiian Islands and Emperor Sea Mounts (also subsiding as they go away from site of current volcanic activity). 6. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 37 EVIDENCE IN SUPPORT OF THE THEORY OF PLATE TECTONICS 9. Mid-ocean ridges are sites of sea floor spreading. They have the following characteristics: High heat flow. Seismic wave velocity decreases at the ridges, due to high temperatures. A valley is present along the center of ridge. Volcanoes are present along the ridge. Earthquakes occur along the ridge. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 38 EVIDENCE IN SUPPORT OF THE THEORY OF PLATE TECTONICS 10. Paleomagnetism and Polar Wandering Curves. The Earth's magnetic field behaves as if there were a bar magnet in the center of the Earth FIGURE 7-20 Dipole model of Earth’s magnetic field. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 39 PALEOMAGNETISM AND POLAR WANDERING CURVES As lava cools on the surface of the Earth, tiny crystals of magnetite form. When the lava cools to a certain temperature, known as the Curie point, the crystals become magnetized and aligned with Earth's magnetic field. The orientation of the magnetite crystals records the orientation of the Earth's magnetic field at that time. As tiny magnetite grains are deposited as sediment, they become aligned with Earth's magnetic field. The grains become locked into place when the sediment becomes cemented. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 40 PALEOMAGNETISM AND POLAR WANDERING CURVES The orientation of Earth's magnetic field is described by inclination and declination. Inclination = the angle of the magnetic field with respect to the horizontal (or the dip of the magnetic field). Inclination = 90o at poles Inclination = 0o at the equator Declination = the angle between where a compass needle points (magnetic north) and the true geographic north pole (axis of the Earth). © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 41 APPARENT POLAR WANDERING Paleomagnetic data confirm that the continents have moved continuously. When ancient magnetic pole positions are plotted on maps, we can see that they were in different places, relative to a continent, at different times in the past. This is called apparent polar wandering. The poles have not moved. The continents have moved. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 42 APPARENT POLAR WANDERING Different polar wandering paths are seen in rocks of different continents. Put continents back together (like they were in the past) and the polar wandering curves match up. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. FIGURE 7-23 Highly mobile locations of Earth’s north magnetic pole during the past 43 half-billion years. THE LITHOSPHERE IS DIVIDED INTO PLATES (ABOUT 7 LARGE PLATES AND 20 SMALLER ONES) FIGURE 7-25 Earth’s major tectonic plates © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 44 LITHOSPHERE & ASTHENOSPHERE Lithosphere = rigid, brittle crust plus uppermost mantle. Asthenosphere = partially molten part of upper mantle, below lithosphere. Rigid lithospheric plates "float" on flowing asthenosphere. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 45 LITHOSPHERE Two types of crust are present in the upper part of the lithosphere: 1. Oceanic crust: thin, dense, basaltic 2. Continental crust: thick, low density, granitic FIGURE 7-26 Earth’s lithosphere. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 46 TYPES OF PLATE BOUNDARIES Divergent—The plates move apart from one another. New crust is generated between the diverging plates. Convergent—The plates move toward one another and collide. Crust is destroyed as one plate is pushed beneath another. Transform—The plates slide horizontally past each other. Crust is neither produced nor destroyed. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 47 DIVERGENT PLATE BOUNDARIES Plates move apart from one another Tensional stress Rifting occurs Normal faults Igneous intrusions, commonly basalt, forming new ocean crust © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 48 SEAFLOOR SPREADING AT DIVERGENT PLATE BOUNDARY FIGURE 7-28 Seafloor spreading marks a divergent boundary between two tectonic plates. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 49 CONVERGENT PLATE BOUNDARIES A. B. Continental collision Subduction FIGURE 7-32 Convergence: two types of convergent plate boundaries. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 50 CONTINENTAL COLLISION Continental collisions form mountain belts with: Folded sedimentary rocks Faulting Metamorphism FIGURE 7-32 Igneous intrusions Slabs of continental crust may override one another Suture zone = zone of convergence between two continental plates © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 51 SUBDUCTION An oceanic plate is pushed beneath another plate, forming a deep-sea trench. Rocks and sediments of downward-moving plate are subducted into the mantle and heated. Partial melting occurs in the mantle. Molten rock rises to form: Volcanic island arcs Intrusive igneous rocks © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 52 OCEAN-TO-OCEAN SUBDUCTION An oceanic plate is subducted beneath another oceanic plate, forming a deep-sea trench, with an associated basaltic volcanic island arc. FIGURE 7-32 © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 53 OCEAN-TO-CONTINENT SUBDUCTION An oceanic plate is subducted beneath a continental plate, forming a trench adjacent to a continent, and volcanic mountains along the edge of the continent. FIGURE 7-31 The juncture of North American and Pacific plates. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 54 OCEAN-TO-CONTINENT SUBDUCTION ZONE INCLUDES 1. 2. Accretionary prism or accretionary wedge— Highly contorted and metamorphosed sediments that are scraped off the descending plate and accreted onto the continental margin. Mélange—A complexly folded jumble of deformed and transported rocks. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 55 OCEAN-TO-CONTINENT SUBDUCTION ZONE INCLUDES 3. 4. Ophiolite suite—Piece of descending oceanic plate that was scraped off and incorporated into the accretionary wedge. Contains: Deep-sea sediments Submarine basalts (pillow lavas) Metamorphosed mantle rocks (serpentinized peridotite) Blueschists—metamorphic minerals (glaucophane and lawsonite) indicating high pressures but low temperatures. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 56 TRANSFORM PLATE BOUNDARIES Plates slide past one another (Shear stress) Transform faults link/offset mid-ocean ridges and convergent boundaries A natural consequence of horizontal spreading of seafloor on a curved globe Example: San Andreas Fault © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 57 TYPES OF TRANSFORM FAULTS FIGURE 7-30 Three types of transform faults. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 58 PLATE BOUNDARIES Red = Midoceanic ridges Black = Transform faults Blue = Deep-sea trenches FIGURE 7-27 Midoceanic ridges (red) and trenches (blue). © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 59 WILSON CYCLES Plate tectonic model for opening and closing of an ocean basin over time. 1. Opening of new ocean basin at divergent plate boundary 2. Seafloor spreading continues and subduction begins 3. Final stage of continental collision © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 60 WILSON CYCLES 1. Opening of a new ocean basin at a divergent plate boundary. Sedimentary deposits include: a. Quartz sandstones b. Shallow-water platform carbonates c. Deeper water shales with chert © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 61 WILSON CYCLES 2. Expansion of ocean basin as seafloor spreading continues and subduction begins. Sedimentary deposits include: a. b. c. Graywacke Turbidites Volcanic rocks Also mélange, thrust faults, and ophiolite sequences near the subduction zone. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 62 WILSON CYCLES 3. Final stage of continental collision. Sedimentary deposits include: Conglomerates Red sandstones Shales Deposited in alluvial fans, rivers, and deltas as older seafloor sediments are uplifted to form mountains, and eroded. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 63 WHAT FORCES DRIVE PLATE TECTONICS? The tectonic plates are moving, but with varying rates and directions. What hypotheses have been proposed to explain the plate motion? Convection Cells in the Mantle Ridge-Push and Slab-Pull Model Thermal Plumes © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 64 CONVECTION CELLS IN THE MANTLE Large-scale thermal convection cells in the mantle may move tectonic plates. Convection cells transfer heat in a circular pattern. Hot material rises; cool material sinks. Mantle heat probably results from radioactive decay. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 65 RIDGE-PUSH MODEL FIGURE 7-40 The ridge-push/slab-pull mechanism for plate movement. Crust forms at mid-ocean ridge spreading center where it is hot and thermally expanded. Crust tends to slide off the thermal bulge, pushing the rest of the oceanic plate ahead of it. This is called ridge-push.. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 66 SLAB-PULL MODEL FIGURE 7-40 The ridge-push/slab-pull mechanism for plate movement. Near subduction zones, oceanic crust is cold and dense (typically denser than the asthenosphere below it), and tends to sink into the mantle, pulling the rest of the oceanic plate behind it. This is referred to as slab-pull. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 67 THERMAL PLUMES Thermal plumes are concentrated areas of heat rising from near the core-mantle boundary. Hot spots are present on the Earth's surface above a thermal. The lithosphere expands and domes upward, above a thermal plume. The uplifted area splits into three radiating fractures and the three plates move outward away from the hot spot. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 68 THERMAL PLUMES A triple junction over a thermal plume. Afar Triangle. FIGURE 7-39 Rising plumes of hot mantle may severely rift the crust, often at 120 angles. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 69 THERMAL PLUMES Thermal plumes do not all produce triple junctions. Hot spots are present across the globe. If the lava from the thermal plume makes its way to the surface, volcanic activity may result. As a tectonic plate moves over a hot spot, a chain of volcanoes is formed. FIGURE 7-52 Major worldwide hotspots. Red dots are hot spot locations. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 70 PALEOMAGNETIC EVIDENCE Magnetic reversals (magnetic north switches with magnetic south) have occurred relatively frequently through geologic time. Magnetization in older rocks has different orientations (as determined by magnetometer towed by a ship). © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 71 PALEOMAGNETIC EVIDENCE Normal (+) and reversed (-) magnetization of the seafloor about the mid-ocean ridge. Note the symmetry on either side of the ridge. FIGURE 7-41 Magnetic field of seafloor near Iceland. FIGURE 7-42 Normal (+) and reversed (-) magnetizations of the seafloor. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. Magnetic stripes on the sea floor are symmetrical about the mid-ocean ridges. 72 MAGNETIC REVERSAL TIME SCALE Reversals in sea floor basalts match the reversal time scale determined from rocks exposed on land. Continental basalts were dated radiometrically and correlated with the oceanic basalts. Using this method, magnetic reversals on the sea floor were dated. FIGURE 7-43 Reversals of Earth’s magnetic field during the past 70 million years. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 73 RATES OF SEAFLOOR SPREADING The velocity of plate movement varies around the world. Plates with large continents tend to move more slowly (up to 2 cm per year). Oceanic plates move more rapidly (averaging 6–9 cm per year). Ocean basins & sea floor are young compared to continental crust Only a thin layer of sediment covers the sea floor basalt. Sea floor rocks date to less than 200 million years (most less than 150 million years). © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 74 MEASUREMENT OF PLATE TECTONICS FROM SPACE Lasers Man-made satellites in orbit around Earth—Global Positioning System By measuring distances between specific points on adjacent tectonic plates over time, rates of plate movement can be determined. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 75 SEISMIC EVIDENCE FOR PLATE TECTONICS Inclined zones of earthquake foci dip at about a 45o angle into the mantle, near a deep-sea trench. Benioff Zones, (or Wadati-Benioff Zones). The zone of earthquake foci marks the movement of the subducting plate as it slides into the mantle. The Benioff Zone provides evidence for subduction where one plate is sliding beneath another, causing earthquakes. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 76 GRAVITY EVIDENCE A gravity anomaly is the difference between the calculated theoretical value of gravity and the actual measured gravity at a location. Strong negative gravity anomalies occur where there is a large amount of low-density rock beneath the surface. Strong negative gravity anomalies associated with deep sea trenches indicate the location of less dense oceanic crust rocks being subducted into the denser mantle. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 77 GRAVITY EVIDENCE FIGURE 7-50 Gravity variation over a deep-sea trench. Negative gravity anomaly associated with a deep sea trench. Sediments and lower density rocks are subducted into an area that would otherwise be filled with denser rocks. As a result, the force of gravity over the subduction zone is weaker than normal. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 78 THERMAL PLUMES, HOT SPOTS, AND HAWAII Volcanoes develop over hot spots or thermal plumes. As the plate moves across the hot spot (appears to be stationary), a chain of volcanoes forms. The youngest volcano is over the hot spot. The volcanoes become older away from the site of volcanic activity. Chains of volcanic islands and underwater sea mounts extend for thousands of km in the Pacific Ocean as well as other oceans. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 79 THERMAL PLUMES, HOT SPOTS, AND HAWAII FIGURE 7-51 The Hawaiian Island chain. A new volcano, Lo'ihi, is forming above the hot spot, SE of the island of Hawaii. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 80 EXOTIC TERRAINS Small pieces of continental crust surrounded by oceanic crust are called microcontinents. Examples: Greenland, Madagascar, Crete, New Zealand, New Guinea. Microcontinents are moved by seafloor spreading, and may eventually arrive at a subduction zone. They are too low in density and too buoyant to be subducted into the mantle, so they collide with (and become incorporated into the margin of) a larger continent as an exotic terrain. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 81 EXOTIC TERRAINS Exotic terrains are present along the margins of every continent. They are fault-bounded areas with different structure, age, fossils, and rock type, compared with the surrounding rocks. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 82 EXOTIC TERRAINS Green terrains probably originated as parts of other continents. Pink terrains may be displaced parts of North America. The terrains are composed of Paleozoic or older rocks accreted during Mesozoic and Cenozoic. FIGURE 7-55 The western margin of North America is a jumble of exotic terranes. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 83 IMAGE CREDITS • FIGURE 7-2 P- and S-type seismic waves. Source: Harold Levin. • FIGURE 7-1 Typical seismograph record. Source: Harold Levin. • FIGURE 7-6 Seismic waves refract (bend) as they travel through Earth. Source: Harold Levin. • FIGURE 7-5 What’s inside Earth. Source: Harold Levin. • FIGURE 7-10 Generalized cross-section showing Mohorovičić discontinuity. Source: Harold Levin. • FIGURE 7-57 Types of faults. Source: Harold Levin. • FIGURE 7-63 Types of folds. Source: Harold Levin. • FIGURE 7-15 Fit of the continents about 200 million years ago. Source: Thomas Brucker for John Wiley & Sons, Inc. • FIGURE 7-20 Dipole model of Earth’s magnetic field. Source: Harold Levin. • FIGURE 7-23 Highly mobile locations of Earth’s north magnetic pole during the past half-billion years. Source: Harold Levin. • FIGURE 7-25 Earth’s major tectonic plates. Source: Harold Levin. • FIGURE 7-26 Earth’s lithosphere. Source: Harold Levin. • FIGURE 7-32 Convergence: two types of convergent plate boundaries. Source: Harold Levin. • FIGURE 7-31 The juncture of North American and Pacific plates. Source: Harold Levin. • FIGURE 7-30 Three types of transform faults. Source: Harold Levin. •FIGURE 7-27 Midoceanic ridges (red) and trenches (blue). Source: Harold Levin. • FIGURE 7-40 The ridge-push/slab-pull mechanism for plate movement. Source: Harold Levin. • FIGURE 7-39 Rising plumes of hot mantle may severely rift the crust, often at 120 angles. Source: Harold Levin. • FIGURE 7-52 Major worldwide hotspots. Source: Harold Levin. •FIGURE 7-41 Magnetic field of seafloor near Iceland. Source: Harold Levin. • FIGURE 7-42 Normal (+) and reversed ( -- ) magnetizations of the seafloor. Source: Harold Levin. • FIGURE 7-43 Reversals of Earth’s magnetic field during the past 70 million years. Source: After Heirtzler, J., 1968, Jour. Geophysical Res., 73:2119–2136. Modified by permission of American Geophysical Union. •FIGURE 7-50 Gravity variation over a deep-sea trench. Source: Harold Levin. • FIGURE 7-51 The Hawaiian Island chain. Source: Harold Levin. • FIGURE 7-55 The western margin of North America is a jumble of exotic terranes. Source: After Ben-Avraham, Z., 1981, American Scientist, 69:228. Modified by permission of American Geophysical Union. © 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED. 84