Ch07 Plate Tectonics Underlies All Earth History Notes.docx

The Earth Through Time Chapter 7—Plate Tectonics Underlies All Earth History CHAPTER OUTLINE FOR TEACHING I. Seismic Waves A. Body Waves 1. Primary (P-waves) 2. Secondary (S-waves) B. Surface Waves 1. Love 2. Rayleigh II. Earth’s Interior A. Main Discontinuities in Earth 1. Mohorovi?i? (30-40 km) 2. Gutenberg (2,900 km) B. Earth’s Core 1. Detected by P- and S-wave shadow zones 2. Average density 10.7 g/cm3 3. Composed mainly of Fe and Ni 4. Radius: 3500 km 5. Inner core (solid) and outer core (liquid) C. Earth’s Mantle 1. Stony composition (4.5 gm/cm3) 2. Peridotite (including olivines and pyroxenes) 3. Upper mantle features a. low velocity zone b. asthenosphere (plastic layer) D. Earth’s Crust 1. Oceanic crust (basaltic; 3.0 gm/cm3) 2. Continental crust (granitic; 2.7 gm/cm3) a. thickest crust (35 km) b. floats according to isostasy III. Crustal Structures A. Faults 1. Normal (gravity) 2. Reverse (including thrust faults) 3. Lateral (strike-slip faults) B. Folds 1. Anticlines and synclines 2. Domes and basins 3. Monoclines IV. Plate Tectonics A. Older Concept of Continental Drift 1. Ancient supercontinents—later fragmented and dispersed a. Gondwanaland (Suess’ idea of the 19th century) b. Pangea (Wegener’s idea of 1915) 2. Evidence cited for drifting a. geologic and geographic fit b. climatic evidence c. fossil distribution (Glossopteris, Mesosaurus) d. rock sequences B. Discoveries about Paleomagnetism (1950s-1960s) 1. Magnetic poles susceptible to switching, N-S 2. Rocks preserve weak remnant magnetism from time of formation a. themal b. depositional 3. Apparent polar-wandering paths C. Basic Concept of Modern Plate Tectonics 1. Integration of ideas: unifying concept a. continental drift b. sea-floor spreading c. paleomagnetic data 2. Mechanism of movement: plates float on asthenosphere 3. Plate boundaries a. divergent (sea-floor spreading, rifting) b. convergent (trenches, folded mountains, subduction zones) c. transform (great lateral faults) 4. Wilson cycles: opening and closing of an ocean basin on time scale of hundreds of millions of years 5. Driving mechanism: competing theories a. mantle convection—induced drag on the lithosphere (thermal plumes) b. “ridge-push” and “slab-pull” 6. Effect of mantle’s thermal plumes a. doming of overlying plate b. three-armed crustal rifts 7. Test of plate tectonics theory a. sea-floor studies of Hess (1960s) b. paleomagnetism of seafloor (Vine, 1963) c. sea-floor ages d. sediment cores from drilling ships e. laser measurements of crustal motion from space f. seismic and gravity evidence g. ages of hot spot volcanoes like the Hawaiian islands 8. Hot spots: island chains and seamounts 9. Exotic terrains (far-traveled microcontinents) Answers to Discussion Questions 1. Drilling from the North Pole to Earth’s center would penetrate oceanic crust, lithosphere, asthenosphere, upper mantle, lower mantle, outer core, and inner core. 2. The three major categories of seismic waves are primary, secondary, and surface. Primary (P) waves travel fastest at 4 to 5 km/sec by pushing particles in directions parallel to direction of propagation. Secondary (S) waves travel 1 to 2 km/sec by moving particles at right angles to the direction of propagation. Surface waves are large-motion waves that travel through the outer crust resembling outward-moving concentric water waves. 3. An S-wave shadow zone indicates that the Earth has a liquid outer core as liquid absorbs S-waves thus creating a shadow zone. 4. A seismic discontinuity is a narrow zone within Earth where there is an abrupt change in velocity or direction of body waves. The Gutenberg and Mohorovi?i? discontinuities mark boundaries between major internal zones (crust-lithosphere and lower mantle-outer core boundaries, respectively). 5. Eroded anticlines and domes tend to expose the oldest rocks of the folded strata near their centers, whereas eroded synclines and basins tend to expose their youngest strata near their centers. 6. The average continental crustal composition is exemplified by granite; the oceanic crust by basalt. Peridotite, an ultramafic rock, exemplifies the mantle. 7. P-waves are the only sort of body waves typically detected at a site 180o from the epicenter of an earthquake. 8. The main types of faults are normal, reverse, and lateral. Compressional forces create reverse faults (including a low-angle variety called thrust fault). Tensional forces tend to create normal faults (including oblique faults with rotational movement). 9. A gravity anomaly is the difference between the observed value of Earth’s gravity at any point on the Earth and the computed theoretical value. Negative gravity anomalies occur where there is excess low-density rock below the surface, for example at deep-sea trenches and mid-ocean ridges. 10. Folds are bent rock strata in the Earth’s crust. The principal kinds of folds are anticline, dome, syncline, basin, and monocline. 11. Alfred Wegener might have cited the following evidence in support of his ideas on continental motion: geological similarities of India, Africa, and South America; the geographic fit of the continents; similar climates of parts indicated in widely separated areas; evidence that some areas at low latitude had once been at a pole; tropical fossils and rock types occurring today at high latitude; and the distribution of key fossil species that could only be explained by continental linkage in the past. 12. Oceanic volcanoes issue basaltic lavas; continental volcanoes (adjacent to subduction zones) issue basaltic and andesitic lavas. The difference in lavas is due to the origin of the liquid rock, which is partially melted upper mantle in the instance of basalt and partially melted down-going plate in the instance of rhyolite-andesite. 13. The Himalayan Mountains resulted from the collision of the Indo-Australian plate (with India riding on the northern tip) with the southern margin of the Eurasian plate. The San Andreas fault is a sliding boundary created by the lateral passage of the North American plate and the Pacific plate. The Dead Sea and the Red Sea are two areas of a three-arm rift that is splitting the African and Eurasian plates apart. 14. According to plate tectonics, new sea-floor material is continually added at the mid-ocean ridges and destroyed continually at the oceanic trenches. 15. The leading edge of the North American plate is located along the western coast of North America (California to Alaska) and the trailing edge is located along the mid-ocean ridge in the North Atlantic. 16. Paleomagnetism is the weak magnetic character of rock acquired at the time of formation. In igneous rocks, it is acquired when the rock cools below the Curie point, 578o C. In sedimentary rock, it is acquired upon settling of magnetic grains to seafloor. The magnetic inclination and declination can be measured via magnetometer to indicate vectors toward the ancient pole. These vectors would parallel lines of force in Earth’s past field. Validation of plate tectonics has come through confirmation of sea-floor spreading (“magnetic stripes”) and assisting in latitudinal reconstruction of continental positions in past. 17. Ancient rocks are confined to the continents because the lighter continental rocks are not subducted and re-melted during plate tectonics. 18. Exotic terrains will have rock types and fossil contents that are significantly different from directly adjacent areas. In the Appalachians, a terrain which showed southern ocean faunas would be a likely alien terrain. A micro-continent-derived terrain would consist of continental rock, probably granitic rock and sedimentary rock, whereas an island-arc terrain would have a volcanic rock composition mainly. 19. Variations of magnetization of seafloor show parallel, symmetrically arranged “stripes” on the seafloor about ridge axes. Thus, they show continuous spreading of the ridges while Earth’s magnetic field changed over time. 20. The alignment and age distribution of volcanic islands in the Pacific Ocean shows progressive plate movement over the site of a stationary mantle plume’s upwelling of basaltic magmas (a hot spot). 21. a 22. d 23. b CHAPTER ACTIVITIES Student activities for in-depth learning: 1. On Earth, plate tectonics has been a dynamic process for hundreds of millions of years. Take a look at the “plate tectonics movie” on the University of California, Berkeley, Museum of Paleontology web page (http://www.ucmp.berkeley.edu/geology/tectonics.html). Do this by clicking on “animated GIFs” for the last 750 million years. Play the movie. As time passes, the continents seem to move around. Why is this so? What is “pushing” the continents around and why? Why do the continents appear to move around over time? 2. Using the fault-motion animations at http://www.iris.edu/gifs/animations/faults.htm observe the differences between a normal and a reverse fault and the differences and similarities (if any) between a strike-slip and an oblique slip fault. Record your observations on how these pairs of faults are different and how they are similar. What kind of forces: compression or extension would be involved in forming these faults. (Observe the video and see for yourself.) Chapter 7—Plate Tectonics Underlies All Earth History CHAPTER OVERVIEW This chapter addresses the dynamic, physical processes and internal energy that drives the Earth. These processes are best understood through the study of seismic waves which has led to the current interpretation of the Earth’s internal structures: the core, mantle, and crust. Major crustal structures such as faults, folds, anticlines and synclines are also discussed in order to show some of the associations that extend to plate tectonics. This conceptual view of how continents grow, where mountains come from, and how and why volcanoes and earthquakes occur offers a comprehensive view of a dynamic Earth. These explanations give a clearer approach of applying the principles of structural deformation and regional geology to plate tectonics. LEARNING OBJECTIVES By reading and completing information within this chapter, you should gain an understanding of the following concepts: Explain the differences between the three types of seismic waves and the characteristics of primary (P), secondary (S) and surface waves that are generated during an earthquake. Discuss the formation of the Earth’s three major zones (core, mantle, crust) based on density, composition, and discontinuities. Describe how the wave characteristics validate the composition of the Earth’s major zones. Explain how the following structural features are formed: faults, folds, anticlines, synclines, domes and basins. Describe today’s theory of plate tectonics including an explanation of how the movement of lithospheric plates is driven by convection in the underlying mantle. Discuss how the theory of continental drift contributes to today’s theory of plate tectonics. Discuss paleomagnetism and its use in present day magnetism and remnant magnetism. Discuss some of the concept of seafloor spreading as it relates to plate tectonics. Describe what occurs at plate margins. CHAPTER OUTLINE Earthquake Waves Reveal Earth’s Mysterious Interior Primary Waves or P-Waves (Compressional) Secondary Waves or S-Waves (Shear) Body Waves Surface Waves Earth’s Internal Zones Mohorovicic Discontinuity (Moho) Gutenberg Discontinuity Earth’s Liquid/Solid Core The Mantle Earth’s Two Types of Crust Oceanic Crust (More Dense) Continental Crust (Less Dense) Isostacy Plate Tectonics Theory Ties It All Together Drifting Continents Early Hypotheses Alfred Wegener Evidence for Continental Drift Clues From Global Geography Clues From Paleoclimatology Clues From Fossils Clues From Rock Sequence Paleomagnetism: Ancient Magnetism Locked Into Rocks How Is Earth’s Magnetic Field Recorded in Rocks? Earth’s Wandering Magnetic Poles Today’s Plate Tectonics Theory Seafloor Spreading (Divergent Boundaries) Transform Boundaries Convergent Boundaries What Happens at Plate Margins? Continental-Crust Convergence Oceanic-Oceanic Crust Convergence Continental-Oceanic Crust Convergence Wilson Cycles: Closings and Openings of Oceanic Basins What Drives Plate Tectonics? Convection Cells in the Mantle Ridge-Push and Slab-Pull Model Thermal Plumes Verifying Plate Tectonics Theory Further Paleomagnetic Evidence Determining Seafloor Age Calculating Rates of Seafloor Spreading Oceanic Sediment Evidence Satellite Evidence Seismic Evidence Gravity Evidence Thermal Plumes, Hotspots, and Hawaii Exotic Terranes Exotic Terranes—Continental Crust Exotic Terranes—Oceanic Crust Exotic Terranes and Earth History Broken, Squeezed, or Stretched Rocks Produce Geologic Structures Faults Folds Key Terms (pages given in parentheses) accretionary prism (192): The contorted and metamorphosed body of rock compressed onto the margin of a continent. anticline (206): A geologic structure in which strata are bent into an upfold or arch. The oldest rocks are at the center and the youngest are on the flanks. apparent polar wandering path (183): Lines on a map connecting ancient pole positions relative to a specific continent for various times during the geologic past. asthenosphere (184): A zone between 50 and 250 kilometers below the surface of the Earth where shock waves of earthquakes travel at much reduced speeds, perhaps because of less rigidity. May be a zone where convective flow of material occurs. basin (211): A depressed area that serves as a catchment area for sediments (basin of deposition). A structural basin is an area in which strata slope inward toward a central location. Has an elliptical to roughly circular outcrop pattern in which beds dip from all sides toward the center of the structure. blue schist (192): A distinctive kind of metamorphic rock containing blue amphiboles. Formed at high pressure, but relatively low temperatures. These conditions are characteristic of subduction zones where the relatively cool oceanic plate plunges rapidly into deep zones of high pressure. body waves (170): Term used to describe waves that are able to penetrate deep into the interior or body of the planet. Body waves travel faster in rocks of greater elasticity, and their speeds therefore increase steadily as they move downward into more elastic zones of the Earth’s interior and then decrease as they begin to make their ascent toward the Earth’s surface. Primary and secondary waves are considered body waves. continental crust (176): That portion of the Earth’s crust which lies beneath the Earth’s continents. Thickness averages 35 kilometers. It is thicker and less dense than oceanic crust. The continents “float” higher on the denser mantle than the adjacent oceanic crustal segments. convection cell (194): As mantel material heats, it expands consequently becoming less dense and slowly rises. This displaces cooler material which sinks. convergent plate boundary (188): Develop when two plates move toward one another and collide. Characterized by a high frequency of earthquakes and are thought to be the zones along which folded mountain ranges or deep-sea trenches may develop. dip (184): The angle of inclination of the tilted layer also measured from the horizontal plane. discontinuity (seismic) (172): Boundaries where seismic waves experience an abrupt change in velocity or direction. divergent plate boundary (185): Develop when two plates move away from each other. May manifest themselves as mid-oceanic ridges complete with tensional (pull-apart) geologic structures. The rending of the crust is accompanied by earthquakes and enormous outpourings of volcanic materials that are piled high to produce the ridges itself. dome (211): An upfold in rocks having the general configuration of an inverted bowl. Strata in a dome dip outward and downward in all directions away from a central area. exotic terrane (204): Small patches of the crust that may become incorporated into the crumpled margin of larger continent. fault (205): A fracture in the Earth’s crust along which rocks on one side have been displaced relative to rocks on the other side. fold (206): Bends in rock strata that are evidence of crustal movement. Metamorphic action due to the compressional force. Tend to occur like a series of petrified wave crests and troughs. footwall (205): In normal faults, the mass of rock that lies below the shear plane. It appears to move upward relative to the opposite side or hanging wall. In reverse faults, the footwall appears to move downward relative to the hanging wall. Glossopteris flora (181): An assemblage of fossil plants found in rocks of Late Paleozoic and early Triassic age in South Africa, India, Australia, and South America. The flora takes its name from the seed fern Glossopteris. Gondwana (178): The great Permo-Carboniferous continent of the southern hemisphere, which comprised the assembled present continents of South Africa, India, Australia, Africa-Arabia, and Antarctica. gravity anomaly (201): The difference between the observed value of gravity at any point on the Earth and the calculated theoretic value. Gutenberg discontinuity (185): The boundary separating the mantle of the Earth from the core below. The Gutenberg discontinuity lies about 2900 kilometers below the surface. guyot (189): Submerged mountains with flat, rather than conical, summits. hanging wall (205): In normal faults, the mass of rock that lies above the shear plane. It appears to move downward relative to the opposite side or footwall. In reverse faults, the hanging wall has moved up relative to the footwall. hotspot (202): Are formed when the upwelling of mantel rock of lava works its way to the surface to erupt as a volcano on the seafloor. As the seafloor moves over the volcano, a series of islands are formed, i.e., Hawaiian Islands. isostasy (176): The condition of balance that exists among segments of the Earth’s crust as they come into flotational equilibrium with denser mantle material. lateral fault (strike-slip fault) (205): A fault in which the movement is largely horizontal and in the direction of the trend of the fault plane. Sometimes called a strike-slip fault. Laurasia (178): A hypothetical supercontinent composed of what is now Europe, Asia, Greenland, and North America. lithosphere (184): The outer shell of the Earth, lying above the asthenosphere and comprising the crust and upper mantle. mantle (170): A thick, homogeneous layer surrounding the core composed of several concentric layers. Believed to have a stony, rather than metallic, composition. Oxygen and silicon probably predominate and are accompanied by iron and magnesium as the most abundant metallic ions. Probably composed of peridotite, an iron- and magnesium-rich rock. mélange (192): A body of intricately folded, faulted, and severely metamorphosed rocks, examples of which can be seen in the Franciscan rocks of California. microplate (204): A type of exotic terrane that was once a large fragment of oceanic crust but were scraped off in the subduction area and made part of the adjacent continental mass. microcontinent (203): Term used to designate bits of continental crust that are surrounded by oceanic crust. They are recognized by their granitic composition, by the velocity with which compressional seismic waves traverse them, by their general elevation above the oceanic crust, and by their comparatively quiet seismic nature. Mohorovi?i? discontinuity (172): A plane that marks the boundary separating the crust of the Earth from the underlying mantle. The “moho,” as it is sometimes called, is at a depth of about 70 kilometers below the surface of the continents and 6 to 14 km below the floor of the ocean. moncline (207): A simple bend or flexure in otherwise horizontal or uniformly dipping rock layers. normal fault (205): A fault in which the hanging wall appears to have moved downward relative to the footwall; normally occurring in areas of crustal tension (forces that tend to stretch the crust). oceanic crust (176): That part of the crust which lies beneath the ocean floors. Approximately 5 to 12 kilometers thick. Consists of three layers: the upper surface is a thin layer of unconsolidated sediment that rests on the irregular surface of the igneous basement layer; the second layer consists of basalts that have been extruded under water; the nature of the deepest layer is not clear. ophiolite suite (192): Splinters of the oceanic plate that were scraped off the upper part of the descending plant and inserted into the crushed forward edge of the continent. Ophiolites mark the zone of contact between colliding continental and oceanic plates. paleomagnetism (182): The Earth’s magnetic field and magnetic properties in the geologic past. Studies of paleomagnetism are helpful in determining position of continents and magnetic poles. Pangea (178): In Alfred Wegener’s theory of continental drift, the supercontinent that included all present major continental masses. Panthalassa (178): The great universal ocean that surrounded the supercontinent Pangea prior to its breakup. passive continental margin (trailing edge) (185): The void created at divergent plate boundaries by the separating plates is filled with molten rock which rises from below the lithosphere and solidifies in the fissure. New crust is added to the trailing edge of each separating plate as it moves slowly away from the mid-oceanic ridge. Trailing edges are the actual edges of the plates as they move apart. plate tectonics (177): The theory that explains the tectonic behavior of the crust of the Earth in terms of several moving plates that are formed by volcanic activity at oceanic ridges and destroyed along great ocean trenches. primary seismic wave (P-wave) (170): Seismic waves that are propagated through solid rock as a train of compressions and dilations. Direction of vibration is parallel to direction of propagation. Are able to pass through solids, liquids, and gases. reverse fault (205): A fault formed by compression in which the hanging wall appears to move up relative to the footwall. ridge-push, slab-pull model (195): Spreading centers, as mid-ocean ridges, stand high on the ocean floor. Their elevation above adjacent regions of the ocean floor results in a tendency for the ridge material to slide down slope, thereby transmitting a push to the tectonic plate. seafloor spreading (185): The process by which new seafloor crust is produced along mid-oceanic ridges (divergent zones) and slowly conveyed away from the ridges. secondary seismic wave (S-wave) (170): A seismic wave in which the direction of vibration of wave energy is at right angles to the direction the wave travels. seismic wave (170): A term used for elastic waves that are produced by earthquakes or explosions that permit scientists to determine the location, thickness, and properties of the Earth’s interior. seismogram (170): The record made by a seismograph that would record an earthquake or explosion. seismograph (170): An instrument used to record all three types of waves generated by the Earth. shadow zone (172): Area in which seismic waves from earthquakes do not appear. The outer core is a barrier to secondary waves and causes a shadow zone on the side of the Earth opposite an earthquake. This shadow zone occurs at 105 degrees from the earthquake focus. The primary wave shadow zone extends from about 105 to 140 degrees from the earthquake focus. These shadow zones are the basis for the theory of a liquid outer core. spreading center (185): An area where two plates would separate as in divergent plate boundaries, or along mid-oceanic boundaries. strike (206): The compass direction of the line produced by intersection of an inclined stadium (or other feature such as fault plane) with a horizontal plane. strike-slip fault (205): A lateral fault in which the main mode of movement is laterally rather than up or down. subduction zone (188): An inclined planar zone, defined by high frequency of earthquakes, that is thought to locate the descending leading edge of a moving oceanic plate. surface seismic wave (170): Seismic or earthquake waves that move only about the surface of the Earth. suture zone (189): The zone of convergence between two plates, recognized by the severity of folding, faulting, and intrusive activity. syncline (206): A geologic structure in which strata are bent into a downfold. The youngest beds are in the center and the oldest rocks are on the flanks. Tethys Sea (189): A sea which existed for extensive periods of geologic time between the northern and southern continents of the Eastern Hemisphere. thermal plume (194): A “hot spot” in the upper mantle believed to exist where a huge column of upwelling magma lies in a fixed position under the lithosphere. Thermal plumes are thought to cause volcanism in the overlying lithosphere. thrust fault (205): A low-angle reverse fault, with inclination of fault plane generally less than 45 degrees. Caused by compressional forces. trailing edge (185): The newly formed edge of an oceanic plate that is nearest the mid-ocean ridge where the oceanic plate originates. transform fault (185): A strike-slip fault bounded at each end by an area of crustal spreading that tends to be more or less perpendicular to the trace of the fault. transform plate boundary (187): A plate boundary where two plates move sideways past each other. Movement is compared to a strike-slip movement, i.e. California’s San Andreas Fault. Wadati-Benioff seismic zone (201): An inclined zone along which frequent earthquake activity occurs and that marks the location of the plunging, forward edge of the lithospheric plate during subduction. Wilson Cycle (192): The opening of a new ocean basin along divergent zones, the expansion of the basin as seafloor spreading continues, and the ultimate closure of the basin as plates converge. CHAPTER 7 Plate Tectonics Underlies All Earth History 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. 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. 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 TYPES OF SEISMIC WAVES Body waves a.P-waves Primary, pressure, push-pull Fastest seismic wave (6 km/sec in crust; 8 km/sec in uppermost mantle) Travel through solids & liquids b.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 6 FIGURE 7-2 P- and S-type seismic waves. 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 SEISMOGRAPHS ?Earthquakes are recorded on an instrument called a seismograph. ?The record of the earthquake produced by the seismograph is called a seismogram. 8 FIGURE 7-1 Typical seismograph record. 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. 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). 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). ?Mohorovi?i? discontinuity (Moho) between crust and mantle ?Gutenberg discontinuity between mantle and core 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. 12 FIGURE 7-6 Seismic waves refract (bend) as they travel through Earth. 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. 13 FIGURE 7-6 Seismic waves refract (bend) as they travel through Earth. P-WAVE SHADOW ZONE Place where no P-waves are received by seismographs. Makes a ring around the globe. Smaller than the S-wave shadow zone. 14 FIGURE 7-6 Seismic waves refract (bend) as they travel through Earth. THE EARTH'S INTERNAL STRUCTURE ?Crust ?Mantle ?Outer core ?Inner core 15 FIGURE 7-5 What’s inside Earth. CRUST ?Continental Crust—A heterogeneous mixtures of rocks that approximates the composition of granite. ?Oceanic Crust—A relatively homogeneous rock of basaltic composition. 16 FIGURE 7-10 Generalized crosssection showing Mohorovi?i? discontinuity. ?Rock composition mixture that approximate Granite composition ?Averages about 35 km thick; 60 km in mountain ranges ?about 2.7 g/cm3. CONTINENTAL CRUST 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) 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 19 FIGURE 7-5 What’s inside Earth. 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. 20 FIGURE 7-5 What’s inside Earth. MANTLE ?Silica based composed (“rocky”) rich in iron and magnesium based mineral. ?Peridotite (Mg Fe silicates, olivine) ?Kimberlite (contains diamonds) ?????????? ?2885 km thick ?Average density = 4.5 g/cm3 ?Solid that flows; plastic behavior. ?Not uniform. Several concentric layers with differing properties. 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. 22 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. ???????????? ?Solid Fe (85%) with some Ni ?1220 km radius (slightly larger than the Moon) ?Solid 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. 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 25 FAULTS 26 FIGURE 7-57 Types of faults. CRUSTAL STRUCTURES—FOLDS ?During mountain building or compressional stress, rocks may deform plastically to produce folds. ?Types of folds A.Anticline B.Syncline C.Monocline D.Dome E.Basin 27 FIGURE 7-63 Types of folds. ANTICLINE 28 R. R. Mudge/USGS SYNCLINE 29 USGS 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. 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. 31 EVIDENCE IN SUPPORT OF THE THEORY OF PLATE TECTONICS 1.Shape of the coastlines: the jigsaw puzzle fit of Africa and South America. 32 FIGURE 7-15 Fit of the continents about 200 million years ago. 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 33 EVIDENCE IN SUPPORT OF THE THEORY OF PLATE TECTONICS 3.Fossil evidence implies once-continuous land connections between now-separated areas Mesosaurus 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). 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. 36 EVIDENCE IN SUPPORT OF THE THEORY OF PLATE TECTONICS 6.Geologic similarities between Appalachian Mountains and Caledonian Mountains in British Isles and Scandinavia. 7.Rift Valleys of East Africa indicate a continent breaking up. 8.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. 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. 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 39 FIGURE 7-20 Dipole model of Earth’s magnetic field. 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. 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). 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. 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. 43 FIGURE 7-23 Highly mobile locations of Earth’s north magnetic pole during the past half-billion years. THE LITHOSPHERE IS DIVIDED INTO PLATES (ABOUT 7 LARGE PLATES AND 20 SMALLER ONES) 44 FIGURE 7-25 Earth’s major tectonic plates 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. 45 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 LITHOSPHERE 46 FIGURE 7-26 Earth’s lithosphere. 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. 47 DIVERGENT PLATE BOUNDARIES ?Plates move apart from one another ?Tensional stress ?Rifting occurs ?Normal faults ?Igneous intrusions, commonly basalt, forming new ocean crust 48 SEAFLOOR SPREADING AT DIVERGENT PLATE BOUNDARY 49 FIGURE 7-28 Seafloor spreading marks a divergent boundary between two tectonic plates. CONVERGENT PLATE BOUNDARIES A.Continental collision B.Subduction 50 FIGURE 7-32 Convergence: two types of convergent plate boundaries. CONTINENTAL COLLISION ?Continental collisions form mountain belts with: ?Folded sedimentary rocks ?Faulting ?Metamorphism ?Igneous intrusions ?Slabs of continental crust may override one another ?Suture zone = zone of convergence between two continental plates 51 FIGURE 7-32 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 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. 53 FIGURE 7-32 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. 54 FIGURE 7-31 The juncture of North American and Pacific plates. OCEAN-TO-CONTINENT SUBDUCTION ZONE INCLUDES 1.Accretionary prism or accretionary wedge—Highly contorted and metamorphosed sediments that are scraped off the descending plate and accreted onto the continental margin. 2.Mélange—A complexly folded jumble of deformed and transported rocks. 55 OCEAN-TO-CONTINENT SUBDUCTION ZONE INCLUDES 3.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) 4.Blueschists—metamorphic minerals (glaucophane and lawsonite) indicating high pressures but low temperatures. 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 57 TYPES OF TRANSFORM FAULTS 58 FIGURE 7-30 Three types of transform faults. PLATE BOUNDARIES Red = Midoceanic ridges Blue = Deep-sea trenches Black = Transform faults 59 FIGURE 7-27 Midoceanic ridges (red) and trenches (blue). 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 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 61 WILSON CYCLES 2.Expansion of ocean basin as seafloor spreading continues and subduction begins. Sedimentary deposits include: a.Graywacke b.Turbidites c.Volcanic rocks Also mélange, thrust faults, and ophiolite sequences near the subduction zone. 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. 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 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. 65 RIDGE-PUSH MODEL 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.. 66 FIGURE 7-40 The ridge-push/slab-pull mechanism for plate movement. SLAB-PULL MODEL 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. 67 FIGURE 7-40 The ridge-push/slab-pull mechanism for plate movement. 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. 68 THERMAL PLUMES A triple junction over a thermal plume. Afar Triangle. 69 FIGURE 7-39 Rising plumes of hot mantle may severely rift the crust, often at 120 angles. 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. 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). 71 PALEOMAGNETIC EVIDENCE Normal (+) and reversed (-) magnetization of the seafloor about the mid-ocean ridge. Note the symmetry on either side of the ridge. Magnetic stripes on the sea floor are symmetrical about the mid-ocean ridges. 72 FIGURE 7-42 Normal (+) and reversed (-) magnetizations of the seafloor. FIGURE 7-41 Magnetic field of seafloor near Iceland. 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. 73 FIGURE 7-43 Reversals of Earth’s magnetic field during the past 70 million years. 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). 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. 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. 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. 77 GRAVITY EVIDENCE 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. 78 FIGURE 7-50 Gravity variation over a deep-sea trench. 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. 79 THERMAL PLUMES, HOT SPOTS, AND HAWAII A new volcano, Lo'ihi, is forming above the hot spot, SE of the island of Hawaii. 80 FIGURE 7-51 The Hawaiian Island chain. 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. 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. 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. 83 FIGURE 7-55 The western margin of North America is a jumble of exotic terranes. • FIGURE 7-2 P- and S-type seismic waves. Source: . • FIGURE 7-1 Typical seismograph record. Source: . • FIGURE 7-6 Seismic waves refract (bend) as they travel through Earth. Source: . • FIGURE 7-5 What’s inside Earth. Source: . • FIGURE 7-10 Generalized cross-section showing Mohorovi?i? discontinuity. Source:. • FIGURE 7-57 Types of faults. Source: . • FIGURE 7-63 Types of folds. Source: . • 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: . • FIGURE 7-23 Highly mobile locations of Earth’s north magnetic pole during the past half-billion years. Source: . • FIGURE 7-25 Earth’s major tectonic plates. Source: . • FIGURE 7-26 Earth’s lithosphere. Source: . • FIGURE 7-32 Convergence: two types of convergent plate boundaries. Source: . • FIGURE 7-31 The juncture of North American and Pacific plates. Source: . • FIGURE 7-30 Three types of transform faults. Source: . •FIGURE 7-27 Midoceanic ridges (red) and trenches (blue). Source: . • FIGURE 7-40 The ridge-push/slab-pull mechanism for plate movement. Source: . • FIGURE 7-39 Rising plumes of hot mantle may severely rift the crust, often at 120 angles. Source: . • FIGURE 7-52 Major worldwide hotspots. Source: . •FIGURE 7-41 Magnetic field of seafloor near Iceland. Source: . • FIGURE 7-42 Normal (+) and reversed ( -- ) magnetizations of the seafloor. Source: . • 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: . • FIGURE 7-51 The Hawaiian Island chain. Source: . • 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.