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Transcript
RESTLESS EARTH – PART I GLG-190 – The Planets Chapter 4 LECTURE OUTLINE Introduction Heat transfer Seismic waves Earth’s interior Crust, mantle, core Continental drift and plate tectonics HEAT TRANSFER Heat is transferred by three main processes Radiation Conduction Convection Radiation involves transfer of heat by radiant energy Energy transferred from hot body by photons Photons hit cooler body and impart energy, heating it Example: Sunlight heating Earth Conduction involves transfer by collisions of atoms and molecules with immediate neighbors Heat present in the form of vibrations Vibrational energy transferred by movement of electrons inside material Example: heat movement through metal handle of a pot Convection is movement of heat through by movement of a “fluid” Due to density differences caused by temperature differences Any material can move by convection as long as there is a heat source The less fluid the material, the slower the convection Rocks in mantle (heat from core) Liquid metal in outer core (heat from formation of inner core) Air in atmosphere (heat from radiation hitting the surface) Main way terrestrial planets lose heat Single cell atmospheric convection in a nonrotating planet SEISMIC WAVES Produced by earthquakes and explosions Energy in form of sound waves expands from point of origin (focus) Used to map interior of Earth Wave velocities depend on type of rock, pressure, and temperature Waves move more slowly through hot, low density rock Waves change speed and direction (are refracted) when moving from one layer to another Waves also are reflected when they encounter new layers Reveal internally layered Earth EARTH’S LAYERED STRUCTURE Thin rigid rocky lithosphere Crust + non-convecting upper mantle Hot convecting rocky mantle Extra weak layer just below lithosphere asthenosphere Lower mantle Dense metallic core Liquid outer part (convecting) Solid inner part LITHOSPHERE Term “lithosphere” refers to non-convecting part of a body’s rocky layer (heat moves by conduction) Also used to describe rigid ice layer on floating on liquid internal ocean (Europa, Enceladus) May constitute entire rocky layer if body has lost too much heat to allow convection (Moon, Mercury) EARTH’S CORE Core is 54.7% Earth’s radius 1/6 Earth’s total volume Liquid outer part Solid inner part suspended in middle of liquid part Dense material 32.5% Earth’s mass Cosmochemical and experimental data indicate iron-nickel (FeNi) alloy Less dense than pure NiFe alloy light elements (Si, O, S) also in core decrease density Variations in seismic wave velocities (anisotropy) inner core is crystalline Temperature of inner core 6900 K (hotter than surface of Sun) ORIGIN OF MAGNETIC FIELD Magnetic field originates in liquid outer core Convection of liquid metal driven by heat from inner core helical flow due to Earth’s rotation Self-sustaining: magnetic field generates electrical currents magnetic field electric currents magnetic field, etc. Magnetic field changes orientation at random intervals magnetic reversals MAGNETIC REVERSALS Reversals used for… Age dating Establishing directions and rates of plate motions Paleolatitude determinations Geomagnetic Polarity Time Scale Ages based upon isotopic and paleontological dating EARTH’S LITHOSPHERE & ASTHENOSPHERE Lithosphere consists of crust + uppermost part of mantle (100-200 km thick) Oceanic lithosphere Continental lithosphere Thin with young (<200 Ma) basaltic crust Thick with old (up to 4 Ga) granitic crust Weak asthenosphere lies below lithosphere Rigid lithosphere slides over asthenosphere Asthenosphere is unique to Earth MOVEMENT OF EARTH’S SURFACE Earth’s surface is in constant motion Movement rates are very slow not easily observable Lithospheric “plates” shift and interact Continental “drift” proposed to account for marching coastlines of Africa and South America Abraham Ortelius (1596), Francis Bacon (1625), Benjamin Franklin (1780s) Evidence of large scale surface movements rare in solar system Early Mars? Few larger icy moons CONTINENTAL DRIFT Wegener (1915) proposed all continents were once connected (Pangaea) Fit of edges of continents Fossils match across oceans Mountain chains match across oceans Ancient climates (glaciers, coal) Pangaea broke up about 200 my ago and continents slowly moved to present positions Matching fossils Glacial evidence SEAFLOOR TOPOGRAPHY Recent discovery (post 1940s) Ships equipped with sonar discovered oceanic ridge system (yellows and greens above) Mid-ocean ridge is longest mountain range on Earth (75,000 km) SIGNIFICANCE OF MID-OCEAN RIDGES Hess (1960s) proposed ridges represent zones of hot mantle upwelling Rising mantle produces magma Decompression melting of mantle rock rising under ridges Lithosphere splits apart earthquakes Magma fills cracks in lithosphere forms new (young) oceanic lithosphere If lithosphere is forming at ridges, it must be lost elsewhere Lithosphere sinks into mantle at deep sea trenches earthquakes Reversals of Earth’s magnetic field are recorded in newly solidified magma at mid-ocean ridges Magnetic stripes formed (symmetrical across ridge) Width depends on how fast new lithosphere is formed AGE OF OCEAN FLOOR DEFINING PLATE BOUNDARIES Earthquake zones, volcanism, and mountain building (tectonic activity) plate boundaries DRIVING PLATE TECTONICS Lithosphere moves over asthenosphere Subduction recycles lithosphere into Earth’s interior Seafloor spreading adds new material to plate margins (makes new lithosphere) Plate tectonics is Earth’s way of losing heat from interior Convection driven by density (temperature) differences WHAT MOVES THE PLATES? Ridge push – lithosphere slides off high thermal budge Slab pull – dense lithosphere sinks and pulls plate after it Other forces, such as convection (movement of material) in mantle Types of plate boundaries Divergent (plates pull apart) seafloor spreading (mid-ocean ridges); continental rifting Convergent (plates collide) subduction (volcanic arc) and mountain building Transform (plates slide past one another) San Andreas Fault DIVERGENCE: CONTINENTAL RIFTING Tensional forces stretch and thin continental crust rift valley Upwelling of asthenosphere (decompression melting) volcanism Rift may develop into ocean basin PLATE CONVERGENCE Ocean-continent Ocean-ocean Trench along continent margin Volcanism and deformation of continental margin (Andes) Trench in ocean Volcanism creates island arc (Aleutians) Continent-continent Mountain building (Himalayas) Preceded by ocean-continent collision MOUNTAIN BUILDING Continents do not subduct Mountain building produces Densities are too low for them to sink into mantle Collide forming mountains Bending (folding) Breaking (faulting) Change of shape (deformation) Mountain heights limited by gravity, strength of rock, erosion rates Appalachian Mountains folds from space MANTLE PLUMES & HOT SPOTS Mantle plumes are columns or rising hot material that seem to originate near CMB Long-lived structures Decompression melting forms magma Plume can come up under any part of a plate (except subduction zones where they are blocked by the descending slab) Continental hot spot: Yellowstone Oceanic hot spot: Hawaii Spreading center hotspot: Iceland Important heat loss mechanism on other bodies (e.g., Venus, Mars, Io) EARTH’S CRATERS Only 180 confirmed impact craters Earth experienced same rates of cratering as Moon most craters removed by surface processes SURFACE PROCESSES Very important on Earth (and other bodies with atmosphere) Weathering Physical breakdown and chemical alteration of rock at or near the surface Liquid water plays dominant role Erosion Transport removal of weathered material by water, wind, and ice Movement of material (sediment) by water (streams), wind, and ice (glaciers) Sedimentation Deposition of sediment in low areas