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Transcript
Structure and Composition of the Earth (Why?) Recall Hypsometric Curve? Continental crust is very different from oceanic crust. To understand this, we need to know more about the structure & composition of Earth. But how do you look into the Earth to see its structure? 1 Earthquakes provide key information on structure of Earth Consider how Earthquakes work: (break pencils) Types of Seismic Waves • Two types of (body) Seismic Waves • Primary Waves – Slinky • Secondary Waves – Whip 2 Primary Waves: (play w/slinky) • Compression waves – Particles move parallel to propagation • Fast: 6 km/s • Travel through all media (gas, liquid or solid) • Sometimes called P-waves Secondary Waves: (play w/rope) • Shear waves – Particles move perpendicular to propagation • Slow: 3.5 km/s • Only travel through solids! • Called S-Waves 3 Characteristics of seismic waves are key to understanding earth’s structure. • Difference in Velocity allows geologist to locate earthquakes (not for us). • Particle motion provides important information about structure If Earth was homogenous : • Waves would travel ‘straight’ straight’ through without ‘bending’ bending’ (refracting) • If Density increased with depth, waves would refract away from center (Snell’ (Snell’s Law). 4 Draw what happens according to Snell’s Law in Homogenous Earth as velocity increases with depth (density). Snell’s Law: sinq1/sinq2 = V1/V2 Or sinq1/ V1 = sinq2/V2 What we observe: S-Waves and the outer core • In outer portion, P-waves refract as predicted by Snell’ Snell’s Law. (mantle) • Rays that intersect ‘central region’ region’ stop propagating! – S-wave shadow zone – 105º 105º to 105º 105º • Why? Because the Earth has a liquid core! 5 What we observe: P-waves & The Liquid Outer Core • In outer portion, Pwaves refract as predicted by Snell’ Snell’s Law. (mantle) • Rays that intersect ‘central region’ region’ refract ‘strangely’ strangely’. – Result in P-shadow zone (no P-waves from 103º 103º to 143º 143º – Evidence for liquid core • More… More… What we observe: P-Waves & the Solid inner core • ‘Complex’ Complex’ refraction of Ray B (and those traveling closer to center of earth) indicates additional structure. • Direct wave (180º (180º) arrives anomalously fast compared to whole liquid core model. • Thus, evidence for solid inner core. 6 Summarizing structure and dimensions of the Core • S-wave shadow indicates the outer core is liquid (s-waves don’ don’t travel through liquids). – From geometry we can calculate depth to outer core (~3000 km). – This is also, therefore, an estimate of the thickness of the mantle. • P-wave shadow zone and early arrival of direct wave indicate the solid inner core – From geometry we can calculate the depth to the inner solid core (~5000 km). – Also support the s-wave evidence for liquid outer core. Seismic waves and the Mantle • Mantle is complex… complex… • Notice the upper 250 km – 10-35 km: Sharp increase in velocity marks the Crust-mantle boundary – Decrease in velocity at ~100-150 km marks base of lithosphere… lithosphere… • Crust vs. vs. Lithosphere? 7 Crust vs. vs. Lithosphere or Composition vs. . vs Rheologic layers • Composition layers on left: • Ocean Crust: 0-10 km thick – Si - Mg - Fe (Dense) • Continental Crust: 35-75 km thick – Si - Al - Ca - Mg - Fe (light) • Mantle: ~3000 km thick – Si - Mg - Fe (Dense) • Core: ~3500 km thick – Fe (Ni-S) (very dense) Crust vs. vs. Lithosphere or Composition vs. vs. Rheologic layers • Rheologic layers on left: • Lithosphere: 100-150 km thick – Includes crust and rigid upper mantle – This is the rigid plate in plate tectonics – Base is defined by 1200º 1200ºC boundary • Asthenosphere: – Top Marked by decrease in seismic velocity – no defined base (here it is 700 km or base of transition zone) – This is plastic region that lithosphere plates ride on • Mesosphere extends to ~3000 km depth - we will call this Lower Mantle • Outer core and inner core - discussed previously 8 Now we can understand hypsometric curve • Ocean floor is ‘deeper’ deeper’ than continents because – Ocean lithosphere is thinner than continental lithosphere – Oceanic lithosphere is denser than continental lithosphere • This is the concept of isostacy… isostacy… – Continental lithosphere sinks deeper into the asthenosphere and rides higher than oceanic lithosphere due to it’ it’s thickness Isostacy: • concept of isostacy… isostacy… – Continental lithosphere sinks deeper and rides higher than oceanic lithosphere due to it’ it’s thickness. – In this picture the thick board represents the continental lithosphere. – The thinner boards represent the oceanic lithosphere. – Note, if you remove the top of a continent, it ‘bubs’ bubs’ up above the oceanic lithosphere. 9 More Examples of Isostacy: Thick ice sinks deeper and rides higher than thin ice More Isostacy: 10 Now we understand this graph! 11