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1 Chapters 5, 6, 13, 14, 21, 22 Section 4 for Hominid evolution 2 Earth’s Interior Layers • Crust: • 5-90 km thick • Continental and oceanic • Mantle • composed largely of peridotite • dark, dense igneous rock • rich in iron and magnesium • Core • iron and a small amount of nickel 3 Earth’s Interior Layers • Lithosphere • solid upper mantle and crust • broken into plates that move over the asthenosphere • Asthenosphere • part of upper mantle • behaves plastically and slowly flows 4 Earth’s Crust • Continental • (20-90 km thick) • density 2.7 g/cm3 • contains Si, Al • Oceanic • (5-10 km thick) • density 3.0 g/cm3 • composed of basalt 5 Plate Tectonic Theory • Lithosphere is broken into individual pieces called plates • Plates move over the asthenosphere – as a result of underlying convection cells 6 Geology and the formulation of theories • What is a theory? • It is arrived at through the scientific method, which involves: • • • • gathering and analyzing facts formulating hypotheses to explain the phenomenon testing the hypotheses and finally proposing a theory. • The hypotheses is a tentative explanation. • A scientific theory is a testable explanation for some natural phenomenon, that is supported by a large body of evidence. 7 Modern Plate Map 8 Plate Tectonic Theory • At plate boundaries • Volcanic activity occurs • Earthquakes occur • Movement at plate boundaries • Plates diverge • Plates converge • Plates slide sideways past each other 9 Plate Tectonic Theory • Types of plate boundaries Ridge Transform Divergent Cont.-Cont. Convergent Cont.-Ocean Convergent Ocean-ocean Convergent 10 The upper part of the mantle and the crust • There are convection cells (currents) in the mantle 11 • There are currentsZone in the mantle A Subduction • When the currents in the mantle carry one plate down • It melts and volcanoes are produced 12 A Subduction Zone • Sometimes the molten rock cools down below the surface 13 Continental Collision • When two plates carrying continents collide mountain chains are built 14 An Oceanic Ridge • If plates are being destroyed, new plate material must be being made somewhere else • At new plate margins 15 What Wegener knew: an example of ‘how science works’ Click on the image to launch ‘What Wegener Knew’ PowerPoint 16 17 18 19 Evidence for the structure of the Earth 20 Launch of a depth charge 21 Earthquake damage 22 23 24 The lithosphere (!) 25 Evidence for plate tectonics 26 Convection in the lab 27 28 Modelling the mantle 29 Mid-Atlantic ridge http://maps.grida.no/go/graphic/world-ocean-bathymetric-map (Hugo Ahlenius, UNEP/GRID-Arendal) 30 Icelandic-type eruption Reproduced with kind permission of U.S. Department of Interior, U.S. Geological Survey 31 Black Smokers 32 Pillow lavas 33 34 Research ship used to tow magnetometer 35 The equipment used 36 37 Age of the sea floor © Dale S. Sawyer http://zephyr.rice.edu/plateboundary/home.html 38 39 40 41 42 Island arc volcanism: Zavadovski Island 43 44 Ocean-continent convergence: Mount St Helens Courtesy of USGS/Cascades Volcano Observatory 45 46 Folds at Lhotse (Himalayas) 47 48 Plate Tectonic Theory influence on geological sciences: • Revolutionary concept • comparable to evolution • Provides a framework for • interpreting many aspects of Earth on a global scale • relating many seemingly unrelated phenomena • interpreting Earth history 49 Atmosphere Solid Earth Plate Tectonics and Earth Systems • plate tectonics is driven by convection in the mantle and in turn drives mountain building • and associated igneous and metamorphic activity • arrangement of continents affects: • solar heating and cooling, • and thus winds and weather systems • rapid plate spreading and hot-spot activity may release volcanic carbon dioxide and affect global climate 50 Biosphere Hydrosphere Plate Tectonics and Earth Systems • continental arrangement affects ocean currents • rate of spreading affects volume of mid-oceanic ridges and hence sea level • placement of continents contributes to the onset of ice ages • movement of continents creates corridors or barriers to migration, the creation of ecological niches, and transport of habitats into more or less favorable climates