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Transcript
Our Blue-Green Earth! Topics • • • • • Core and Structure Plate Tectonics Atmosphere Magnetosphere Tides Basic Structure • Mass = ~ 6x1024kg • Average Density ~ 5500kg/m3 • Surface Water! • Surface rocks ~ 2000-4000kg/m3 Hence Dense Core Crust, Mantle, Outer Core and Inner Core Seismic Activity Earthquakes send shock waves through the Earth. Probe of Earth’s internal structure! Primary and Secondary Waves P-Wave and S-Wave Behaviour • P- Waves travel ~ 5-6km/s • Travel through both liquid and solid • Pressure wave like sound (Longitudinal) • S-waves travel slower ~ 3-4 km/s • Absorbed in Liquid • Shear waves (Transverse) Internal Structure • Note Jump in density at the Core/Mantle boundary. • Dense core rich in iron and nickel. • Metallic core, Rocky (silicon-based) mantle. • Crust is light – “floats”! Basalts (3000-3300kg/m3) Granites (2700-3000kg/m3) Cooling Planet • “Differentiation” – not uniform density • Slow cooling from outside in • Radioactive heating – Rock acts as insulator – hot core. Planet’s History • Debris, dust and rock coalesce. • Heat generated by external Bombardment melts surface, not the core. • Radioactive heating – trapped by insulating rocks - melts iron-rich rocks. • Gravity causes heavy iron to sink to centre. • Crust is cooling ‘skin’ on the surface. Seismic Studies Reveal… More Surprises! • Core is rotating faster than the planet! By ~0.67 sec/day = ¼ turn/century. • Evidence of solid iron ‘crystal’ for the inner core. – P wave speed depends on orientation. Surface Activity Earthquakes and Volcanoes ….Ancient and Modern Internal stresses caused this terrain to be folded upwards. Erosion by wind and rain have exposed the different constituents. Plate Tectonics Spots indicate sites of major volcanoes and earthquakes….taken together outline ‘plates. Plate Tectonics • Still drifting today. • Plates ≠ Continents • Move ~cm/year (like fingernail growth!) Seems small but moves ~4000km over ~200 million years (5% of Earth age) Continental Drift Estimates of previous positions of plates given current drift rates and directions. Some Plates collide! They don’t stop… but build major mountain ranges… or deep ocean trenches Some Plates Move Apart! Seafloor spreading in the Mid-Atlantic Ridge What Drives Plate Tectonics? • Probably gigantic thermal convection cells that drag the surface with its motion. • ‘Soft’ hot rock takes millions of years to cycle. Magnetic Field Direction Reversal • Age increases with distance from Ridge. • Magnetic field in rock at the time it cooled maps Earth’s field. • Field reversal every ~ 0.5 million years! Colour = Same Direction The Earth’s Atmosphere N2 ~78%, O2 ~21% Ar ~0.9%, CO2 ~0.03% Water (variable) – 0.1-3% • Layers defined by temperature inversions. • • • • • Atmospheric Layers • • • • Ionosphere Mesosphere Stratosphere Troposphere Complex Convection Cells Winds driven by Sun’s heat and Earth’s rotation. Origin and Evolution of the Atmosphere • Primary atmosphere: light gases H2, He, CH4 (methane), NH3 (ammonia) • Secondary Atmosphere: Outgassed via volcanoes: H2O, CH4 , SO2, NO, N2, NH3 • Hot light gases escaped + UV ionisation. • Temperature cooled - Water condensed. • CO2 and SO2 dissolved in rocks. • Life produces oxygen....and O3 (ozone) layer. The Greenhouse Effect • Clouds reflect and absorb sunlight. • Sunlight heats up surface. • Hot bodies radiate energy. • CO2 and H2O absorb heat efficiently… “Greenhouse Gases”. Global Warming • CO2 concentration increased by ~16% since 1958. • Temperatures predicted to rise by ~0.9-3.5o by 2100. • Long time variations in orbit parameters contribute…. …..So do humans. Saw-tooth is due to seasons Warning Signs… …for Climate Change • Deforestation • Ozone Depletion • El Niño Effect ~50% Reduction, ~10-20% in midlatitudes ~20,000km2 per year We need to take this seriously! Why is the Sky Blue? • Scattering of light by dust particles. • Scattering depends on particle size. • Blue ~10x more likely to scatter. • Red Sunsets. Earth’s Magnetic Field • Magnetic Poles aligned ~11o from rotation axis • Drift ~ 10km / year • Created by electric currents in liquid iron core: Dynamo = Convection + rapid rotation. • North is South! Magnetosphere • Discovered in 1950’s by satellites. • Protects our atmosphere from solar wind. Van Allen Belts Spiral motion traps ions in converging magnetic fields lines. • Two main doughnut-shaped regions - pronounced at equator: ~3000km protons, 20,000km electrons. • New belt (1993) = O, N and Ne ions. Aurora Borealis & Australis UV image - 1996 Views from Space Earth’s Tides Daily, monthly and yearly cycles. Consider Just Earth-Moon System • Gravity ∝ 1/r2 • Lunar gravity: Pull is greatest on side closest Moon. • Variations – compared to that at Earth centre. Differential or Tidal force ∝ 1/r3. Result: Tidal Bulge High and Low tides twice a day. Solar and Lunar Tides • New/Full Moon enhances Tides. = SPRING • 1 & 3 Quarter Moons reduced tides = NEAP Sun partially cancels Moon’s effect Earth’s Tidal Bulge • Earth rapid spin causes a tidal bulge – oceans depth offset from Earth-Moon line by ~10°. • WHY? Friction – Rotating Earth ‘drags’ tidal bugle around with it. • RESULT? – Slowing Earth down – longer days! Furthermore: Moon Drifting Away • Laser Pulses bounced of the Moon show it is receding at a rate of ~3.8cm/year. • Eventually… ……Earth ‘day’ will be 47x longer than now and Moon will stay fixed in sky at ~1.43x present distance… ….in billions of years time.