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) 2
Types of Seismic Waves Two types of (body) Seismic Waves Primary Waves Slinky Secondary Waves Whip 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 3
Secondary Waves: (play w/rope) Shear waves Particles move perpendicular to propagation Slow: 3.5 km/s Only travel through solids! Called S-Waves 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 4
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: sinq 1 /sinq 2 = V 1 /V 2 Or sinq 1 / V 1 = sinq 2 /V 2 5
What we observe: S-Waves and the outer core In outer portion, P-waves refract as predicted by Snell s Law. (mantle) Rays that intersect central region stop propagating! S-wave shadow zone 105º to 105º Why? Because the Earth has a liquid core! 6
What we observe: P-waves & The Liquid Outer Core In outer portion, P- waves refract as predicted by Snell s Law. (mantle) Rays that intersect central region 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 Solid 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. Thus, evidence for solid inner core. 7
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 (~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 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 Crust vs. Lithosphere? 8
Draw composition vs. rheologic layers 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: ~3000 km thick Si - Mg - Fe (Dense) Core: ~3500 km thick Fe (Ni-S) (very dense) 9
Crust vs. Lithosphere or Composition vs. Rheologic layers Rheologic layers on left: Lithosphere: 100-150 km thick Includes crust and rigid upper mantle This is the rigid plate of plate tectonics Base is defined by 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 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 This is the concept of isostacy Continental lithosphere sinks deeper into the asthenosphere and rides higher than oceanic lithosphere due to it s thickness 10
concept of isostacy Continental lithosphere sinks deeper and rides higher than oceanic lithosphere due to 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 bobs up above the oceanic lithosphere. Isostacy: More Examples of Isostacy: Thick ice sinks deeper and rides higher than thin ice 11
More Isostacy: Demonstration 1 Demonstration 2 Demonstration 3 Now we understand this graph! 12