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The Active Earth: Plate Tectonics Objectives of this Chapter: i. Describe plate tectonic theory ii. Discuss the development of plate tectonic theory iii. Draw the major types of plate boundaries and list their major features iv. Explain the forces that drive the plates. v. Describe how isostacy works The Earth evolved from a ball of dust and gas that formed our Solar System about 5 billion years ago to the relatively cool surface that we know today. However, the Earth’s interior remains hot, and drives volcanic eruptions, earthquakes, mountain building and other geologic processes that have created our physical environment. These processes can be explained by Plate Tectonics, a theory that revolutionized the Earth sciences. Unit 2-CO, p.121 Plate Tectonics View from Space Shuttle Columbia. Tectonic forces are widening the Red Sea and pushing the Sinai Peninsula further away from Africa at a rate of about 1 cm/year. Fig. 6-CO, p.122 Plate Tectonics Theory A simple, unifying theory that has revolutionized our understanding of the way the Earth works and how its systems interact to create our environment. This scientific revolution developed over many years, building on earlier observations, hypothesis and theories. The story shows how a scientific theory evolves, and how scientists rely on the work and discoveries of earlier scientists. Development of Plate Tectonics theory. Alfred Wegener (German Scientist; 1930) noticed, among other things, how the continents of South America and Africa, now separated by the Atlantic Ocean, fit together like a jigsaw puzzle. Fig. 6-1, p.123 Pangea: Laurasia and Gondwanaland In fact, all of the continents, when moved properly, fit together like a puzzle into a supercontinent he called Pangea. He then looked for other evidence, and could match fossils across continents. Fig. 6-2, p.124 Other evidence supporting Pangea (continued) Along with fossil evidence, he matched sedimentary rock deposits to climatic zones. By plotting 250 million year old glacial deposits on a map (as found today), they would have formed in tropical and subtropical areas, and crossed the oceans! What happens if you assemble the continents into Pangea (lower diagram)? Fig. 6-3, p.125 Glacial Evidence on modern distribution of the continents. Fig. 6-3a, p.125 Glacial deposits and other climate-sensitive sedimentary rocks plotted on Wegener’s map of Pangea. Now deposits match climate zones. Fig. 6-3b, p.125 Rock Correlation Wegener also matched certain sequences of rock across the continents. He plotted these rock types on his Pangea map and found those on the east side of the Atlantic Ocean were continuous with those on the west side. Continental Drift Wegener’s concept of a single supercontinent that broke apart to form the modern continents is called the theory of continental drift. However, he could not explain how the continents moved, and it would take 30 years after his death in 1930 before new evidence would be discovered that would rejuvenate his theory. The Earth’s Layers To understand Plate Tectonics, you must understand that the Earth is layered (how do we know this?). Fig. 6-4, p.127 Table 6-1, p.127 The seafloor and Mid-Oceanic Ridge After world war II, we began mapping the seafloor, and discovered the largest mountain chain on Earth (the Mid-Oceanic Ridge which Fig. 6-5, p.129 circles the globe like seams of a baseball). We discovered magnetic stripes: as lava cools and becomes basalt (which in rich in Fe) along the seafloor, the Ferich minerals become weak magnets and align parallel to the Earth’s magnetic field. With magnetometers, oceanographers can record magnetic patterns. Fig. 6-6, p.129 We found that rock records “normal” and “reversed” polarity, forming magnetic stripes symmetrical about the ridge axis. Vine, Matthews and Morley, three scientists, proposed the sea floor is spreading away from the Mid-Oceanic Ridge (continuously and like conveyor belts). New basalt lava rises at the ridge as the sea floor separates. When the basalt cools, it acquires the magnetic orientation of the Earth’s field. Fig. 6-7, p.130 Seafloor Spreading Reversals in the Earth’s magnetic field, which are known to occur about every 500,000 years, are shown by the magnetic stripes…each stripe represents new seafloor formed over ½ million years; where is the old seafloor? Discovered fossils in the seafloor sediments (overlying the basalt) were young at the ridges and older away from the ridge axis Discovered the layer of mud that overlies basalt at the seafloor becomes progressively thicker away from the ridge axis. The Seafloor Spreading hypothesis became the general model for the origin of all oceanic crust, and the basis for the Theory of Plate Tectonics (along with Continental Drift). Tectonic Plates and Movement The lithosphere is a hard shell of strong rock about 100 km thick that floats on the hot, plastic asthenosphere. The lithosphere is broken into several large plates (and several smaller ones) that glide slowly (1-16 cm/yr), and move continents and oceans with them. Fig. 6-8, p.131 Three types of plate boundaries are known (fractures that separate one plate from another). Plates move relative to one another in three ways: Fig. 6-9, p.132 Table 6-2, p.133 Divergent Plate Boundary (spreading center) Lithospheric plates move away from a spreading center by gliding over the weak, plastic asthenosphere. New lithosphere forms at the ridge, and is thin at this area, but thickens as it becomes cooler away from the ridge. Note subduction zones. Fig. 6-10, p.133 Divergent Plate Boundary: Rift Zone The continent of Africa is splitting apart along the East African Rift. Where else in the world is this occurring? Fig. 6-11, p.134 Convergent Plate Boundary One type is continentalcontinental convergence. Collision between India and Asia created the Himalayan mountain chain. Fig. 6-12, p.135 Transform Plate Boundary Continentalcontinental transform plate boundary; plates slide horizontally past one another. What is a plate? Segment of the lithosphere (uppermost rigid mantle and crust). Can carry both oceanic and continental crust. Avg. thickness of lithosphere covered by oceanic crust is 75 km; covered by continental crust is 125 km (how thick at spreading centers?). Composed of hard, strong rock Floats and glides on underlying hot, plastic asthenosphere (like a slab of ice on a pond). Margins are tectonically active (EQ, Volcanoes); interiors are generally stable. Move at rates from 1-16 cm/yr. Continents and oceans then move across the Earth at the rate plates move. Manhattan Island then is 9 meters farther from London than it was 225 years ago when the Declaration of Independence was written. Why do plates move? Recall Wegener couldn’t explain this. Today convection of the mantle is thought to drive plate motion. Heat is from the core and from radioactivity. Fig. 6-13, p.136 New lithosphere glides downslope away from a spreading center. The old, cool part of the plate sinks into the mantle at a subduction zone, pulling the rest of the plate along with it. Fig. 6-14, p.137 Mantle Plumes and Hot Spots Rising column of hot, plastic mantle rock. As pressure decreases in a rising plume, magma forms at a hot spot in the mantle just below the lithosphere, and rises to erupt from volcanoes on the Earths surface. Can you name a place where this occurs, in the middle of an Ocean? Isostasy: Vertical Movement of the lithosphere Add mass to the lithosphere, it settles and underlying asthenosphere flows laterally away to make room (like when you step onto a boat). Fig. 6-15, p.138 Isostasy (continued) How is mass added or subtracted from the lithosphere? The lithosphere in floating equilibrium on the asthenosphere is isostasy. Fig. 6-15a, p.138 Isostasy (continued) Vertical movement in response to changing burden is called isostatic adjustment. Fig. 6-15b, p.138 Just like a large iceberg, continental crust extends more deeply into the mantle beneath high mountain ranges than it does under the plains. Oceanic lithosphere is thinner and more dense, so it floats at a lower level. Fig. 6-16, p.138 How Plate Movements affect Earth’s Systems Generate volcanic eruptions, earthquakes, build mountains and change global distribution of continents and oceans. Where do volcanoes occur? Eruptions can alter the atmosphere to change global climate. How? What are consequences of ash, sulfur compounds and Co2 in the atmosphere? How did the eruption of Mount Pinatubo (1991) affect the Earth’s overall temperature? What about the Permian extinction 248 million years ago? What about 120 million years ago during the formation of a lava plateau near Peru? Mountains strongly affect global wind and precipitation patterns. Migration of continents and oceans affects wind and ocean currents. Probably not much of an affect during our lifetime. Earthquakes alter the human environment. Eruption of Mount St. Helens, 1980 Fig. 6-17, p.139 Fig. 6-18, p.142 p.144