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This article explores the early evidence of continental drift, the rejection of Alfred Wegener's hypothesis, the discovery of seafloor spreading, the theory of plate tectonics, and the causes of plate motion.
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Plate Tectonics & the Changing Earth http://maps.unomaha.edu/maher/GEOL1010/lecture15/map_plate_tectonics_world.gif (aka Putting the Pieces Together!)
Drifting Continents • Describe one piece of early evidence to suggest that Earth’s continents may have once been joined. • Discuss the evidence of continental drift. • Explain why continental drift was not accepted when it was first proposed.
Early Mapmakers Many early mapmakers thought Earth’s continents had moved based on matching coastlines. Some early mapmakers thought that the coastline of South America matched the coastline of Africa.
Alfred Wegener A meteorologist, by trade.
Wegener’s Evidence The existence of coal beds in Antarctica indicates that the continent once had a temperate, rainy climate.
Wegener’s Evidence Glossopteris is a fossil fern that helped support Wegener’s hypothesis of continental drift.
Alfred Wegener "Doesn't the east coast of South America fit exactly against the west coast of Africa, as if they had once been joined?“ - "This is an idea I'll have to pursue." • Earth’s continents were once joined as a single landmass called Pangaea.
Pangaea The Argyle Sweater 3/11/11
Science’s Reaction "Utter, damned rot!" said the president of the prestigious American Philosophical Society. "If we are to believe [this] hypothesis, we must forget everything we have learned in the last 70 years and start all over again," said another American scientist. Anyone who "valued his reputation for scientific sanity" would never dare support such a theory, said a British geologist. The Scientific Community • Scientists at the time rejected Wegener’s hypothesis of continental drift because he could not explain how or why Earth’s continents move.
Seafloor Spreading • Explain the significance of magnetic patterns on the seafloor. • Explain the process of seafloor spreading. • Summarize the evidence that led to the discovery of seafloor spreading.
Mapping the Seafloor Mid-Atlantic Ridge
Paleomagnetism The study of Earth’s magnetic record is known as paleomagnetism.
Paleomagnetism(continued) The magnetic pattern of ocean-floor rocks on one side of an ocean ridge is a mirror image of that of the other side .
Isochrons A map line connecting points that have the same age is an isochron.
Isochrons(continued) Compared to ocean crust near deep-sea trenches, crusts near ocean ridges are younger .
Seafloor Spreading As new seafloor moves away from an ocean ridge, the seafloor cools and becomes more dense than the material beneath it.
Seafloor Spreading 1) The seafloor contracts and sinks. 2) Hot magma which is less dense than surrounding material, is forced toward the crust. 3) Crust spreads along an ocean ridge and magma fills the gap that is created. 5) New seafloor moves away from the ridge, cools, and becomes more dense than the material beneath it. 4) New ocean floor forms as the magma hardens.
Theory of Plate Tectonics • Explain the theory of plate tectonics. • Compare and contrast the three types of plate boundaries and the features associated with each.
The Theory The theory of plate tectonics states that Earth’s crust and rigid upper mantle are broken into enormous slabs called plates that move slowly over Earth’s surface.
The Theory (Continued) According to the theory of plate tectonics, plates interact at plate boundaries by coming toward each other, moving away from each other, or moving horizontally past each other.
Plate Boundaries DIVERGENT TRANSFORM CONVERGENT
Illustrations (continued) Interactive Map
Label the Plates - Quiz World Tectonic Map Quiz F I M C D N L G J K H E A O B
Causes of Plate Motion • Explain the process of convection. • Summarize how convection in the mantle is related to the movements of tectonic plates. • Compare and contrast the processes of ridge push and slab pull.
Convection The transfer of thermal energy by the movement of heated matter is convection. Convection currents transfer thermal energy from warmer regions to cooler regions.
Convection Currents The driving forces of tectonic plates are related to convection currents in Earth’s mantle.
Review Oceanic crust is composed mainly of basalt, and continental crust is composed mainly of granite. http://www.earth.northwestern.edu/people/seth/107/Gravity/contoceaniso.htm
The downward part of a convection current causes a sinking force that pulls tectonic plates toward one another . Slab Pull The weight of a subducting plate helps to pull the lithosphere into a subduction zone in a process called slab pull.
Ridge Push The rising part of a convection current causes both upward and lateral forces that lift and split the lithosphere at a divergent boundary.
Mountain Building Before I studied mountains, a mountain was just a mountain. While I studied mountains, a mountain was so much more than a mountain. When I understood mountains, a mountain was just a mountain. -Zen Saying-
OBJECTIVES • Describe the elevation distribution of Earth’s surface. • Explain isostasy and how it pertains to Earth’s mountains. • Describe how Earth’s crust responds to the addition and removal of mass. Crust-MantleRelationships
Thoughts?What is Earth’s highest elevation? • What is Earth’s lowest elevation? Earth’s Topography
About 70 % of Earth’s surface is below sea level. Majority of Topography
Isostasy between Earth’s mantle and crust exists when the mass of crust is balanced as a result of buoyancy and gravity. Isostasy
Displacement • The seafloor displaces more of the mantle than the same thickness of the continental crust. http://www.earth.northwestern.edu/people/seth/107/Gravity/contoceaniso.htm
