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Chapter 2: Global Tectonics Our Dynamic Planet Introduction Each rocky body, whether planet or moon, started with a hot interior. Each has been kept warm over time by energy released by the decay of radioactive isotopes. Despite radioactive heating, rocky bodies have cooled considerably since their formation, so that their outer layers have stiffened into lithospheres (岩石圈). Introduction (2) The interior of Earth remains hot and geologically active. The mantles of Earth loses internal heat by convection (對流), the slow flow of solid rock. Hot rock rises upward to near the surface. Earth’s stiff lithosphere is broken into a collection of near-rigid plates. Introduction (3) Most large-scale geologic events, like earthquakes or volcanic eruptions, originate within Earth’s interior. Many other processes in the Earth system, such as the hydrologic and biogeochemical cycles, are profoundly affected by plate tectonics (板塊運動). Plate Tectonics (板塊學說): From Hypothesis to Theory Plate tectonics is a scientific theory that explains two centuries of often puzzling observations and hypotheses about our planet Earth. The continents are drifting very slowly across the face of our planet. Continental drift (大陸漂移) is a concept with a long history. Plate Tectonics: From Hypothesis to Theory (2) A century ago geologists puzzled over the fit of the shorelines of Africa and South America. They noted that fossils of extinct land-bound plants and animals, glacial deposits (冰河沉積), and ancient lava flows (熔岩流) could be matched together along coastlines that today are thousands of kilometers apart. Coal was found in Antarctica. Coal forms in tropical climates, implying that Antarctica has moved in the past. Plate Tectonics: From Hypothesis to Theory (3) Alfred Wegener proposed the most comprehensive early hypothesis for “Continental Drift (大陸漂移假 說) ” in 1912. His theory was widely rejected because: Ocean floor was too strong to be plowed aside. Wegener had not proposed a plausible force that could induce the continents to drift. Plate Tectonics: From Hypothesis to Theory (4) After many years of scientific observations, the theory of Plate Tectonics was born in 1960. Plate tectonics is the process by which Earth’s hot interior loses heat. Nowadays, we can measure the slow drift of plates worldwide using satellite navigation systems. The basic premises of plate theory are secure because they can be tested against a wide variety of observations. Continental Drift versus Plate Tectonics What Earth’s Surface Features Tell Us The rocks beneath our feet are solid, but they are not rigid. Topography: the relief and form of the land above sea level. Bathymetry: topography on the ocean floor. Earth bulges around its equator and is slightly flattened at the poles. Isostasy: Why Some Rocks Float Higher Than Others The continents average about 4.5 km elevation above the ocean floor. They stand notably higher than the ocean basins because the thick continental crust (大陸 地殼) is relatively light (average density 2.7 g/cm3). The thin oceanic crust (海洋地殼) is relatively heavy (average density 3.0g/cm3). The lithosphere (岩石圈) floats on the asthenosphere (軟流圈) . Isostasy (地殼均衡說) -similar to Principle of Archimedes' applied to the earth -first noted when French Bouguer in the 18th century surveyed the shape of the earth Isostasy (2) The principle of isostasy governs the rise or subsidence. All parts of the lithosphere are in a floating equilibrium. Low-density wood blocks float high and have deep “roots,” whereas highdensity blocks float low and have shallow “roots.” Fig. 2.2 Earth’s Surface: Land Versus Water The ocean covers 71 percent of Earth’s surface. Sea level fluctuates over time. When climate is colder and water is stored as ice: Sea level falls. The shoreline moves seaward. When climate gets warmer: The ice melts. Sea level rises. The shoreline advances inland. Fig. 2.1 Fig. 2.3 Earth’s Surface: Land Versus Water (2) Undersea mid-ocean ridges form a continuous feature more than 60,000 km long. Mid-ocean ridges mark where two oceanic plates spread apart. New lithosphere forms in the gap. Fig. 2.4 Continental shelves and slopes (light blue) take ~25% of the mass of the continental crust. Fig. 2.5 Topography across the northern Atlantic Ocean. The Atlantic coastline is a typical example for passive continental margin. Earth’s Surface: Land Versus Water (3) The continental shelf (大陸棚) steepens slightly at 100-200 meters below sea level. The continental slope (大陸斜坡) is the flooded continental margin. The continental rise (大陸隆起) descends more gently from the base of the continental slope Earth’s Surface: Land Versus Water (4) Ocean trenches (海溝) occurs where oceanic lithosphere and continental (or oceanic) lithosphere converge at the boundary between two plates (e.g., Ryukyu trench, Mariana trench). Because oceanic lithosphere is the denser of the two, it descends under the active continental margin and sinks into the deeper mantle. The large, flat abyssal floors (深海海床) of the open ocean lie 3 to 6 km below sea level. What Earth’s Internal Phenomena Tell Us Heat conduction (熱傳導) and convection (對流). Conduction is dominant when the temperature gradient in rocks is large. (earth’s surface and core-mantle boundary) Rocks are poor conductors of heat, so the internal heat is transferred by moving the rock itself. The circulation of hot rock is maintained by mantle convection (地涵對流或地幔對流). oceanic trench mid-ocean ridge Fig. 2.6 mantle convection that shapes the earth’s surface. Heat source comes from cooling of the earth itself since 4.55 Byr and decay of radioactive elements. Heat Conduction (熱傳導) Conduction is the process by which heat moves through solid rock via molecular collisions. It’s a diffusive (擴散) process wherein molecules transmit their kinetic energy to other molecules by colliding with them. Heat is conducted through a medium in which there is a spatial variation in the temperature or a steep temperature gradient. The loss of the earth’s internal heat through oceanic crust and lithosphere is largely controlled by conduction. Mantle Convection (1) Earth’s heat can move in a second process called convection (對流). Convection can happen in gases, in liquids, or, given enough time, in ductile solids. A prerequisite condition for mantle convection is the thermal expansion (熱膨脹) of hot rock. Convective heat is transported with the motion of ductile rock. Mantle Convection (2) Rock expands as its temperature increase, and its density thereby decreases slightly. The hot rock is buoyant relative to cooler rock in its immediate neighborhood. A 1 percent volume expansion requires an increase of 300-400oC and leads to a 1 percent decrease in density. Viscosity (黏滯係數) represents the tendency of rock to ductile flow (延展性流動). Unit: Newton.second/meter2 Rock Deformation: Elastic versus Viscous For an elastic solid, stress is linearly proportional to strain, E (elastic constant) In general, 300-1000 atmosphere pressure (1 atm ~ 1 bar = 105 Pascal =N/m2) is required to compress a rock by 1/1000. Viscosity (m): measures the resistance of a solid or fluid to ductile flow. m , (strain rate) d / dt, where is stress and is strain. Ductile deformation becomes important at larger depth, where rocks are hot and less rigid. 100 atm is estimated to cause the mantle rocks beneath the plates to deform at a steady rate of 1/106 per year. Mantle Convection (3) Rock does not need to melt before it can flow. The presence of H2O encourages flow in solid rock. Convection currents bring hot rocks upward from Earth’s interior. Geothermal Gradient (地溫梯度) of the Lithosphere Heat moves through the lithosphere primarily by conduction. The lithosphere-asthenosphere boundary is 13001350oC, depending on depth. Oceanic lithosphere is about 100 km thick. The average geothermal gradient in oceanic lithosphere is about 13oC/km. Average continental lithosphere is 200 km. The average geothermal gradient in continental lithosphere is about 6.70C/km. Fig. 2.7 Adiabatic Expansion (絕熱膨脹) of Rock Adiabatic expansion means “expansion without loss or gain of energy.” Rock is compressed and reduced in volume by increasing pressure with depth; it is also heated by the work done by the pressure force during the compression. The associated temperature rise causes adiabatic expansion. Adiabatic Expansion (絕熱膨脹) of Rock (2) In convective mantle, the mean temperature increases with depth along an adiabat (絕熱線). The adiabatic thermal gradient (絕熱溫度梯度) in the mantle is the rate of increase of temperature with depth as a result of compression of the rock by the weight of the overlying rock; it is approximately 0.5oC/km. Earth’s Convection: Driven From the Top Below the lithosphere, rock masses in the deeper mantle rise and fall according to differences in temperature and buoyancy. Earth’s convection is driven mainly by colder material sinking from the top. Earth’s Convection: Driven From the Top (2) The densest lithosphere is most likely to sink back into the asthenosphere and the deeper mantle while lighter continental lithoshere drifts across the earth’s surface. Ocean floor and the continents are slowly moving (up to 12 cm/yr). Fig. 2.8 Plates and Mantle Convection When continents split apart, a new ocean basin forms. The Red Sea was formed this way 30 million years ago. Subduction: the old lithosphere sinks beneath the edge of an adjacent plate. Global Positioning System In the 1960s, the U.S. Department of Defense established a network of satellites with orbits that could be used for reference in precisely determining location. The Global Positioning System (GPS) detects small movements of the Earth’s surface. Fig. 2.9 C. surface motion from GPS measurement Global Positioning System (2) It is accurate within a few millimeters. Two measurement methods: A GPS campaign: researchers establish a network of fixed reference points on Earth’s surface, often attached to bedrock. The position is re-measured every few months or years. Continuous GPS measurement: the receivers are attached permanently to monuments, and position is estimated at fixed intervals of a few seconds or minutes. Four Types of Plate Margins and How They Move The lithosphere currently consists of 12 large plates. The seven largest plates are: North American Plate. South American Plate. African Plate. Pacific Plate. Eurasian Plate. Australian-Indian Plate. Antarctic Plate. Fig. 2.10 Fig. 2.9 A. Present-day plat motion based on many geological data, including lineation of magnetic anomaly on seafloor, relative motion along the strike of transform faults, earthquake slip direction and displacement, etc.. Red dots mark the location of significant earthquakes since 1965. Fig. 2.9 B. Surface motions from continuous GPS measurements. Fig. 2.9 Plates have four kinds of boundaries or margins (板塊邊界) Divergent margin/spreading center (分離板塊邊界/擴張中 心) (e.g. East Pacific Rise, Mid-Atlantic Ridge). Convergent margin/subduction zone (聚合板塊邊界/隱沒 帶) (e.g. Japan Trench, Aleutian Trench). Convergent margin/collision zone (聚合板塊邊界/碰撞帶) (e.g. Indo-Himalaya collision zone). Transform fault margin (轉型斷層邊界) Seismology and Plate Margin Earthquakes occur in portions of the lithosphere that are stiff and brittle. Earthquakes usually occur on pre-existing fracture surfaces, or faults. There are distinctive types of earthquakes that correlate nicely with motion at plate boundaries. Fig. 2.11 Four types of plate margines Fig. 2.12 three types of faults Type I: Divergent Margin Where two plates spread apart at a divergent boundary, hot asthenosphere rises to fill the gap. As it ascends, the rock experiences a decrease in pressure and partially melts. The molten rock from such pressure-release partial melting is called magma (岩漿). Type I: Divergent Margin (2) Oceanic crust is formed at mid-ocean ridges within 1-2 km of the ridge axes. Found in every ocean. Form a continuous chain that circles the globe. Oceanic crust is about 6-8 km thick worldwide. Animation of Seafloor Spreading Source: CD of the textbook Pressure-release partial melting Seafloor spreading and magnetic chron Birth of the Atlantic Ocean (1) When a spreading center splits continental crust: A great rift (裂谷) forms, such as the African Rift Valley (東非裂谷). As the two pieces of continental crust spread apart: The lithosphere thins. The underlying asthenosphere rises. Volcanism commences. The rift widens and deepens, eventually dropping below sea level. Then the sea enters to form a long, narrow water body (like the Red Sea). Birth of the Atlantic Ocean (2) Fig. 2.13 The continents that now border it were joined into a single vast continent that Wegener named Pangaea which means all lands. About 200 million years ago, new spreading centers split the huge continent. The Atlantic continues to widen today at 2-4 cm/yr. Characteristics of Spreading Centers (1) Earthquakes at midocean ridges occur only in the first 10 km beneath the seafloor and tend to be small. Volcanic activity occurs at midocean ridges and continental rift. The midocean ridges rise 2 km or more above surrounding seafloor.The principle of isostasy applies: lower-density rock rises to form a higher elevation at ridges and the cooling results the subsidence of seafloor Characteristics of Spreading Centers (2) If the spreading rate is fast: A larger amount of young warm oceanic lithosphere is produced, and the ridge will be wider. A slow-spreading ridge will be narrower. The Atlantic Ocean spreads slowly (2-4 cm/yr). The Pacific spreading center (East Pacific Rise) is fast by comparison: 6-20 cm/yr. The Pacific Ocean basin is shrinking because … Seafloor Spreading and Age Map http://www.windows.ucar.edu/tour/link=/earth/interior/seafloor_spreading.html Animation of Topography and Subduction Angles at Fast and Slow Moving Plates (from CD of textbook) Role of Seawater at Spreading Centers Seawater circulates through cracks beneath the ocean floor. Cold water percolates (滲透) through these cracks, warms in contact with subsurface rock, and rises convectively to form undersea hot springs. A small fraction of the seawater remains in the rock, chemically bound within hydrous (water-bearing) minerals like serpentine and clays. The CO2 Connection As oceanic lithosphere ages, it accumulates a thick layer of sediments such as clay and calcium carbonate (CaCO3) from the shells and internal skeletons of marine organisms. The formation of calcium carbonate consumes carbon dioxide (CO2) that is dissolved in seawater. Seafloor sediments remove CO2 from the atmosphere, and thus have a long-term influence on the greenhouse effect and Earth’s climate. Type II: Convergent Margin/Subduction Zone As the plate cools, it grows denser and the principle of isostasy demands that the plate subsides. The process by which lithosphere sinks into the asthenosphere is called subduction. The margins along which plates are subducted are called subduction zones. The sinking slab warms, softens, and exchanges material with the surrounding mantle. Type II: Convergent Margin/Subduction Zone (2) Under elevated temperature and pressure, the crust expels a number of chemical compounds. Water (H2O), Carbon dioxide (CO2), and Sulfur compounds (S). A small addition of these volatile substances can lower the melting point of rock by several hundred degrees Celsius The hot mantle rock immediately above the sinking slab starts to melt. Magma rises to the surface to form volcanoes. Subduction zones are marked by an arc of volcanoes parallel to the edge of the plate. The CO2 Connection, Again Water, CO2, and sulfuric gases like SO2 and H2S return to the atmosphere. Subduction zone volcanic activity raises the carbon dioxide level in the atmosphere, exerting a strong influence on the greenhouse effect and Earth’s climate. Volcanism tends to replace the CO2 that is lost from the atmosphere into the ocean and stored in the seafloor.. Volcanoes At Subduction Zones At a plate boundary, the plunging plate draws the seafloor down into an ocean trench. When the slab gets down to about 100 km, water squeezed out of the subducted materials begins to react with the ambient mantle rock and causes some of the mantle to melt. Molten rock that makes it all the way to the surface erupts to form a line of volcanoes spaced about 70 km apart from the trench. Volcanoes At Subduction Zones (2) If the overriding plate is oceanic lithosphere, volcanoes form a series of islands called a volcanic island arc (火 山島弧) , e.g., Mariana Islands, Aleutian Islands. If the overriding plate is continental lithosphere, a continental volcanic arc forms. Sediment washed from the continent tends to fill the offshore trench, e.g., Cascade Range of the Pacific Northwest, the Andes of South America. Earthquakes in Subduction Zones The largest and the deepest earthquakes occur in subduction zones. The location of most earthquakes define the top surface of a slab as it slides into the mantle (the surface to as deep as 670 km). Quakes deeper than 100 km are more likely associated with faults caused by stresses within the slab. Distribution of earthquake epicenters from 1975 to 1995. Depth of the earthquake focus is indicated by color. Animation of Subduction Process (from CD of textbook) Fig. 2.16 Wadati-Benioff Zone Type III: Convergent Margin/Collision Zone Continental crust is not recycled into the mantle. Continental crust is lighter (less dense) and thicker than oceanic crust. When two fragments of continental lithosphere converge, the surface rocks crumple (擠壓摺皺) together to form a collision zone. Type III: Convergent Margin/Collision Zone (2) Collision zones that mark the closure of a former ocean form spectacular mountain ranges. For example, the Alps, Himalayas, and Appalachians. Fig. 2.17 Type IV: Transform Fault Margin Along a transform fault margin, two plates grind(摩擦) past each other in horizontal motion. These margins involve strike-slip faults in the shallow lithosphere and often a broader shear zone deeper in the lithosphere. Most transform fault occur underwater between oceanic plates. Type IV: Transform Fault Margin (2) Two of Earth’s most notorious and dangerous transform faults are on land. The North Anatolian Fault in Turkey. The San Andreas Fault in California. Both transform faults are similar in slip rate, length and straightness. (Right-lateral strike-slip, fault lengths of ~1000 km.) The fault slip rate is about 24 mm/yr for NAF and 20-34 mm/yr for SAF. Animation of Movement Along Transform Fault (from CD of textbook) Fig. 2.10 Fig. 2.18 Topography of the Ocean Floor Two main features: Midocean ridges Some 64,000 km in length. The oceanic ridge with its central rift reaches sea level and forms volcanic islands, e.g., Iceland. Oceanic trenches the deepest parts of the ocean. The deepest spot on Earth is located in the western Pacific, near Guam in the Mariana Trench. (depth 11,033 m at the CHALLENGER DEEP) Iceland Comparing Venusian Topography Venus resembles Earth in size and chemical composition. The “Magellan” project mapped its surface over several years. Venusian tectonics is not plate tectonics. Venusian topography does not exhibit long midocean ridges and subduction zones. Venus has no water ocean or ocean floor because of the extreme temperature of its surface (around 450-500oC). Fig. 2.19 An Icy Analogue to Earth Tectonics (1) The closest approximation to Earth tectonic in our solar system is found on Europa (one of Jupiter’s (木星) four largest moons). Europa is 3138 km in diameter, large enough to be discovered in 1610 by Galileo with his early telescope. Europa’s interior has rocky composition with density similar to Earth. An Icy Analogue to Earth Tectonics (2) Its surface layer consists mainly of water ice, perhaps more than 100 km deep. Plates on Europa are much smaller than Earth’s plates. Topography at Europa’s plate margins suggests convergence, divergence, and transform-fault motion, just as with Earth’s plate margins. HOT SPOT Hot Spots And Absolute Motion (1) American geologist James Dwight Dana (1813-1895) observed that the age of extinct volcanoes in the Hawaiian Island chain increases as one gets farther away from the active volcanoes on the “big island.” The only active volcanoes are at the southeast end of the island chain, and the seamounts to the northwest are long extinct. Hot Spots And Absolute Motion (2) In the 1960’s, J. Tuzo Wilson proposed that a long-lived hot spot lies anchored deep in the mantle beneath Hawaii. A hot buoyant plume of mantle rock continually rises from the hot spot, partially melting to form magma at the bottom of the lithosphere—magma that feeds Hawaii’s active volcanoes. If the seafloor moves over the mantle plume, an active volcano could remain over the magma source only for about a million years. Hot Spots And Absolute Motion (3) As the plate moves, the old volcano would pass beyond the plume and become dormant, and a new volcano would sprout periodically through the plate above the hot spot, fed by plume magma. Animation of Formation of Hotspot Track (from CD of textbook) Fig. 2.22 Yellow dots mark hotspot volcanism associated with rising mantle plumes. Volcanism Associated with Plate Tectonics Hot Spots, mantle plumes Several dozen hot spots have been identified. Because hot spot volcanoes do not form tracks on the African Plate, geologists conclude that this plate must be very nearly stationary. Hot spots transport roughly 10 percent of the total heat that escapes Earth. Mantle plumes were probably more numerous 90-110 million years ago than today, because extinct seamount volcanoes of that age crowd together in the central Pacific. What Causes Plate Tectonics? (1) There is more agreement on how plate tectonics works than on why it works. Hot, buoyant, low-viscosity material rises in narrow columns that resemble hot spot plumes. Cooler, stiffer material from the surface sinks into the mantle. What Causes Plate Tectonics? (2) Three forces seem likely to have a part in moving the lithosphere: Ridge push: the young lithosphere sits atop a topographic high, where gravity causes it to slide down the gentle slopes of the ridge. Slab pull: at a subduction zone, as the cold, dense slab is free to sink into the mantle, it pulls the rest of the lithosphere into the oceanic trench behind it. Friction: Slab friction drags the top, the bottom, and the leading edge of descending lithosphere in the subduction zone. Plate friction drags elsewhere at the base of the plate. Why Does Plate Tectonics Work? The theory of plate tectonics does not explain why the plates exist. At the present time, a number of scientific clues point to water as the missing ingredient in the plate tectonics. Water molecules can diffuse slowly through solid rock. Water can weaken rock in several ways. Fig. 2.25 Computer simulation of mantle convection. Viscosity decreases as temperature increases by 800C from the coolest (blue) to warmest (red) regions. Hot buoyant rock (red) flows more readily and rises upward in narrow plumes. Cooler rock (blue) is stiffer and sinks in interconnected sheets. The arrows show the lateral velocity of ductile flow, diverging from the hot plumes and converging on the cooler sheets. Evolution of Mantle Plumes Fig. 2.26 Computer simulation of mantle convection with stiff plates. Rock viscosity is formulated to maintain narrow weak zones at the plate boundaries, so that plates remain distinct and can move relative to each other. The viscosity varies by a factor of 30 between red (stiff) and blue (weak) regions. Arrows indicate surface velocity. http://www.geos.ed.ac.uk/undergraduate/prospectus/earthsci/PlumesCartoon_Helge_Gonnermann.jpg Cartoon of Earth’s interior Seismic tomography 低速 (熱) 高速 (冷) Images of Subducting slabs(隱沒板塊) Grand S.P., van der Hilst R.D., and Widiyantoro S., 1997. Global seismic tomography a snapshot of convection in the Earth, GSA Today. Images of Subducting slabs(隱沒板塊) The thickness of continental lithosphere ? SAW16AN Seismically defined lithosphere is at most 200250 km thick under old continents. - consistent to other geophysical studies.