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Transcript
9. Our Living Earth • Earth’s atmosphere, oceans & surface • Earth’s interior & earthquakes • Earth’s plate tectonics activity • Earth’s magnetic field & magnetosphere • Earth’s evolving atmosphere • Earth’s human population & biosphere The Earth: A Portrait From Space Earth Data (Table 9-1) Earth From An Apollo Spacecraft Earth’s Environmental Spheres • Earth’s spheres – Geosphere – Hydrosphere – Atmosphere – Biosphere Rock & metallic Earth materials Water as ice, liquid & humidity ~78% nitrogen & ~21% oxygen All living things (biomass) • Earth’s ecosystem – Matter flows • A closed system for most practical purposes – Meteoroids enter daily, spacecraft leave occasionally – Energy flows • An open system for most practical purposes – Sunlight brings extremely large amounts of energy on one side – Radiant heat in extremely large amounts leaves on all sides Rocks • Definition – Consolidated mixture of one or more minerals • Monomineralic rocks have many crystals of one mineral • Other rocks have many crystals of two or more minerals • Making rocks – Igneous processes Fiery origins – Sedimentary processes Cemented small particles – Metamorphic processes Changed by heat/pressure • Destroying rocks – Physical / mechanical weathering – Chemical weathering Rock Cycle: Materials & Processes • Materials – Magma solidifies & becomes… – Igneous rock weathers & becomes… – Sediment lithifies & becomes… – Sedimentary rock metamorphoses & becomes… – Metamorphic rock melts & becomes… • Processes – Solidification – Weathering – Lithification – Metamorphism – Melting produces igneous rock produces sediment produces sedimentary rock produces metamorphic rock produces magma Rock Cycle Magma: Source of Igneous Rocks • Earth’s interior is hot – Residual heat of formation ~ 4.6 billion years ago – Decay of radioactive isotopes • Earth’s interior is mostly solid or “plastic” – Solid: Rigid / brittle under intense pressure – Plastic: Flows slowly under intense pressure • Localized areas are hot enough to melt rocks – Magma temperatures vary ~ 600°C to ~ 1,400°C – Iron turns red at ~ 600°C & melts at ~ 1,500°C • Magma has ~ 10% greater volume than source – Same mass ⇒ Greater volume ⇒ Lower density Some Common Igneous Rocks Sedimentary Rock Categories • Organic – Coal • Clastic Remains of plants & animals Fossilized fern leaves Broken rock & mineral fragments – Sandstone, shale & limestone • Bioclastic – Coquina • Chemical – Gypsum Broken shell fragments Limestone “fossil hash” Crystallization from water solution A common “evaporite” mineral Some Clastic Sedimentary Rocks Three Metamorphic Processes • Heat Absolutely essential – Hot enough for atoms & molecules to slowly migrate – Cool enough so that nothing melts • Pressure Common but not essential – Subduction zones Pacific Northwest – Regional subsidence Mississippi Delta • Fluids Only near active volcanoes – Volcanically active areas Eastern Oregon Foliated Metamorphic Rocks: Gneiss Oregon’s Metamorphic Environment Portland Astoria Earth’s “Chemical” Differentiation Characterizing Earth’s Interior • Chemical composition – Low Mineral composition density minerals • Granite continents & basalt ocean basins – Intermediate density minerals • Peridotite – High • Iron & nickel density minerals • Physical condition Crust Mantle Core Solid / plastic / liquid – A function of temperature & pressure • Temperature increases slowly with depth • Pressure increases rapidly with depth – Solid – Plastic – Solid – Liquid – Solid Lithosphere Asthenosphere Mantle Outer core Inner core Old & cool enough Lubricating layer Very slightly plastic Temperature wins Pressure wins Earth’s Interior Facts & Evidence • Some basic facts – Overall average density ~ 5.5 g . cm–3 – Surface average density ~ 2.7 to 3.0 g . cm–3 – Interior must have higher density materials • Much higher atomic number ⇒ Metals • Greater compression due to greater pressure • Some suggestive evidence – Asteroids orbiting the Sun • Range of materials from rock to iron/nickel • Proportions would produce a planet like Earth – Meteorites found on Earth • Range of materials from rock to iron/nickel • Proportions would produce a planet like Earth Earth’s Layers: The Lithosphere Earth’s Layers: Crust/Mantle/Core Earthquake Focus & Epicenter The focus is also called the hypocenter Seismic (Earthquake) Waves • Body waves – Source location: Focus • Place of maximum underground shaking • Place where the earthquake begins Usually ! ! ! – Varieties • Compressional waves • Transverse waves P-waves S-waves Primary waves Secondary waves • Surface waves – Source location: Epicenter • Place of maximum surface shaking • Place directly above the focus Usually ! ! ! – Varieties • Compressional waves • Transverse waves Sideways jolting Up & down jiggling Compressional & Transverse Waves Compressional Seismic Waves Transverse Seismic Waves Body Seismic Waves Surface Seismic Waves Seismicity & Earth’s Internal Structure Plate Tectonics • Tectonic plates = Lithospheric plates – Rigid & brittle • “Glide” over the asthenosphere – Sizes vary greatly • Micro plates Juan de Fuca plate • Macro plates Pacific plate • Three kinds of tectonic plates – Oceanic plates Basaltic composition – Continental plates Granitic composition – Composite Both basalt & granite plates Mantle Convection & Plate Motion • Thermal gradient: Hotter at core than at crust – Results in a density gradient • Heat sources – Planetesimal impact Dominant as a protoplanet – Radioactive decay Ongoing exponential decay – Gravitational collapse Minimal as a protoplanet • Point of origin – Thought to be the core-mantle boundary • Shape – Elongated “curtains” of rising material A Model of Mantle Convection A Global View of Mantle Convection Tectonic Plates Tectonic Plate Boundary Processes Divergent Plate Boundaries Convergent Plate Boundaries Transform Plate Boundaries Mid-Atlantic Ridge Spreading Zone Ridge offset by transform faults Effects of Plate Motion: Volcanoes • Divergent tectonic plate boundaries – Most rising magma spreads out under lithosphere • Lithosphere warms ⇒ Lowers density ⇒ Floats higher • Penetrates the lithosphere, causing eruptions • Convergent tectonic plate boundaries – Highest density plate subducts • Ocean ⇒⇐ ocean collision – Oldest (i.e., coldest & densest) basaltic plate subducts – Basaltic to andesitic lavas build gently curving line of volcanoes • Ocean ⇒⇐ continent collision – Basaltic (therefore most dense) oceanic plate subducts – Andesitic to rhyolitic lavas build gently curving line of volcanoes Plate Motion Effects: Earthquakes • Divergent tectonic plate boundaries – All activity is near the Earth’s surface • Virtually all earthquakes are shallow – Most rock is relatively warm & soft • Absence of brittle rock reduces earthquake strength • Convergent tectonic plate boundaries – Ocean – ocean boundaries • Deep & strong earthquakes are very common – Ocean – continent boundaries • All depths & strong earthquakes are very common • Transform tectonic plate boundaries – Ocean – ocean boundaries • Absence of brittle rock reduces earthquake strength – Ocean – ocean boundaries • Presence of brittle rock increases earthquake strength Plate Motion Effects: Mountains • Volcanoes – Usually occur at convergent & divergent boundaries • At least one plate must have basaltic oceanic crust – Factors contributing to solid rock melting • Thermal gradient • Friction • Addition of water ⇒ Deeper is hotter ⇒ Subducting slab ⇔ country rock ⇒ Under-sea subduction trenches • Folded mountains – Occur primarily at convergent boundaries • Both plates must have granitic continental crust – Thrust faulting is also very common • Significant crustal shortening Plate Motion Effects: Geography • Continent ⇔ ocean configuration very dynamic – Three probable Pangaea episodes • All major landmasses gather into one supercontinent • Remaining 70% of Earth’s surface is one super-ocean – The present situation • Major continental landmasses are relatively stable • Major ocean basins are – Atlantic Ocean is increasing in size – Pacific Ocean is decreasing in size very dynamic Earth’s Magnetic Field • Basic physical processes – Slow circulation of the liquid metallic outer core – Rapid axial rotation of once per day • Basic properties – Combined magnetic field of many smaller “cells” – Reverses on average ~ 0.5 million years • May be in the initial stages of a reversal now – Not perfectly aligned with Earth’s rotational axis • True of almost every planet in the Solar System • Magnetic declination – Deviation of magnetic North [compass] away from true North • Magnetic inclination – Angle between Earth’s surface & Earth’s magnetic field lines Visualizing Earth’s Magnetic Field Earth’s Magnetosphere • Basic physical processes – Earth’s relatively strong magnetic field – The ever-changing solar wind • Ionized hydrogen atoms • This is an electric current Free protons & electrons Generates a magnetic field – Strong interaction between two magnetic fields • Basic properties – Earth’s magnetosphere shaped like a teardrop • Blunt side faces Sun, pointed side faces opposite Sun – Solar wind gusts produce striking effects • Geomagnetic storms Disrupt radio signals – Occasionally strong enough to disrupt electric power distribution • Aurorae Ionize atmospheric atoms – Occasionally strong enough to be seen in Florida & Texas Visualizing Earth’s Magnetosphere Aurora Australis From Space Aurora borelis from Fairbanks, Alaska Aurora Boralis: Bear Lake, Alaska Aurora Borealis: Scotland 2014/02/28 Aurora Borealis: Finland So-Called “Greenhouse” Effect Earth’s 3-D Atmospheric Circulation Earth’s Vertical Atmospheric Structure Terrestrial Planetary Atmospheres • Venus – ~100 times more atmosphere than Earth – ~ 96.5% CO2 & ~3.5% N2 • Runaway global warming • Earth – Very large amount of CO2 & relatively close to the Sun – ~ 78% N2 + ~21% O2 + ~1% Ar • Moderate global warming • Mars – Very small amount of CO2 & moderately close to the Sun – ~100 times less atmosphere than Earth – ~ 95.3% CO2 & ~2.7% N2 • Minimal global warming – Very small amount of CO2 & relatively far from the Sun Source of Planetary Atmospheres • Volcanic outgassing – Venus • Abundant with no oceans to assimilate gases – Earth • Abundant with oceans to assimilate gases – Mars • Absent with no oceans to assimilate gases • Comet impacts – Very common in the young Solar System – Very rare in today’s Solar System Growth of Earth’s Atmospheric O2 Human Population & the Biosphere • Earth’s rapidly increasing human population – Burning fossil fuels returns CO2 to the atmosphere • General upward trend Increasing use of fossil fuels • Seasonal fluctuations Summer CO2 uptake by plants – Removing forest cover (deforestation) • Reduces CO2 uptake – Partially offset by ocean absorption • Radically changes local climate – Much hotter & much drier – Body heat contributes to “urban heat island” effect • Only recognized very recently Earth’s Growing Human Population Northern Hemisphere CO2 Increase Earth’s Changing Temperatures Earth’s Antarctic Ozone Hole Important Concepts • – Geosp., hydrosp., atmosp. & biosp. • • Location of continents & ocean basins • Volcanic & earthquake activity Earth’s environmental spheres The rock cycle • – Basic causes – Five materials & five processes • Rotation & outer core convection • Geomagnetic field reversals • Magma as Earth’s initial condition – Three basic rock types – Generation of the magnetosphere • Igneous, sedimentary & metamorphic • • Interactions with the solar wind • Aurora Borealis & Aurora Australis Earth’s internal structure – Chemical & physical classifications – Interactions between temp. & pressure – Information from seismic waves • Plate tectonics – Driven by mantle convection – Plate boundary types & properties • Convergent, divergent & transform – Effects of plate tectonic activity Terrestrial planetary atmospheres – Venus, Earth & Mars compared • Atmospheric gases & amounts • Closeness to the Sun • Compressional & transverse waves • Surface & body waves • Earth’s magnetic field – Atmospheric structure & circulation • Earth’s human population – Rapid growth in numbers – Use of fossil fuels • CO2 returned to Earth’s atmosphere • Global warming & ozone depletion