Structure 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? Earthquakes provide key information on structure of Earth Consider how Earthquakes work: (elastic rebound) 1
Types of Seismic Waves Two types of (body) Seismic Waves Primary Waves Slinky Secondary Waves Whip Primary Waves: (slinky) Compression waves Particles move parallel to propagation Fast: 6 km/s Travel through all media (gas, liquid or solid) Sometimes called P- waves 2
Secondary Waves: (whip) Shear waves Particles move perpendicular to propagation Slow: 3.5 km/s Only travel through solids! Called S-Waves animation Characteristics of seismic waves are key to understanding earth s structure. Difference in Velocity allows geologist to locate earthquakes (not for us). Wave paths provides important information about structure Wave velocity Sound waves (compression) travel fastest through A. Gas B. Liquid C. Solid 3
If Earth was homogenous : Waves would travel straight through without bending (refracting) If Density increased with depth, waves would refract away from center (Snell s Law). Draw what happens according to Snell s Law in Homogenous Earth as velocity increases with depth (density). Snell s Law: sinθ 1 /sinθ 2 = V 1 /V 2 Or sinθ 1 / V 1 = sinθ 2 /V 2 Velocity increases with depth, so V 1 <V 2 4
What we observe: S-Waves and the outer core In the mantle, S-waves refract as predicted by Snell s Law. Rays that intersect central region stop propagating! S-wave shadow zone 105º to 105º Why? What we observe: P-waves & The Liquid Outer Core In the mantle, P-waves refract as predicted by Snell s Law. Rays that intersect the core refract strangely. Result in P-shadow zone (no P-waves from 103º to 143º Evidence for liquid core More What we observe: P-Waves & the inner core Complex refraction of Ray B (and those traveling closer to center of earth) indicates additional structure. Direct wave (180º) arrives anomalously fast compared to whole liquid core model. Why? 5
Summarizing structure and dimensions of the Core S-wave shadow indicates the outer core is liquid (s-waves don t travel through liquids). From geometry we can calculate depth to outer core (~2900 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 Notice the upper 250 km 7-35 km: Sharp increase in velocity marks the Crust-mantle boundary Decrease in velocity at ~100-150 km coincides with base of lithosphere Crust vs. Lithosphere? Crust vs. Lithosphere or Composition 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: ~2900 km thick Si - Mg - Fe (very dense) Core: ~3500 km thick Fe (Ni-S) (supper dense) 6
Crust vs. Lithosphere or Composition vs. Rheologic layers Rheologic layers on right: Lithosphere: 100-150 km thick Includes crust and rigid upper mantle This is the rigid plate of plate tectonics Base is defined by 1280ºC boundary Asthenosphere: Top Marked by decrease in seismic velocity Base is set at 700 km This is plastic region that lithosphere plates ride on Mesosphere extends to ~2900 km depth - we will also call this Lower Mantle Outer core and inner core - discussed previously The lithosphere includes crust and part of the mantle. A. True B. False The Asthenosphere is composed entirely of mantle. A. True B. False 7
Now we can understand hypsometric curve Ocean floor is deeper than continents because Ocean lithosphere is thinner than continental lithosphere Oceanic lithosphere is denser than continental lithosphere Thus, due to relative buoyancy the oceanic lithosphere does not ride as high on the asthenosphere compared to the continental lithosphere. This is the concept of isostacy Continental lithosphere sinks deeper into the asthenosphere and rides higher than oceanic lithosphere due to it s thickness and lower density Buoyancy Model What will happen if the thickness of the block is increased? A. The base of the block will sink deeper. B. The top of the block will rise higher C. Both A and B D. Nothing will happen, the position of the block will not change. Z U Z A Z B concept of isostacy Continental lithosphere sinks deeper and rides higher than oceanic lithosphere due to it s thickness and lower density. 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 bobs up above the oceanic lithosphere. Isostacy: 8
More Examples of Isostacy: Thick ice sinks deeper and rides higher than thin ice More Isostacy: animation Now we understand this graph and image! 9
Review Questions What does the hypsometric curve show you? How is breaking a pencil analogous to an earthquake? How are seismic waves generated according to the elastic rebound theory? Seismic waves: why don t S-waves travel through liquids? How are P-waves different from S-waves Why do waves refract when they pass from one media to another? Review Questions How do we know Earth has a liquid outer core? How do we know Earth s inner core is solid? How is the mesosphere different from (a) the asthenosphere and (b) the lithosphere? Why are oceans deep compared to continents? 10