Rather, velocity will either…. 1. Decrease with increasing depth 2. Show sudden (step) increase with increasing depth
Shadow zones Caused by decrease of seismic wave velocity with increasing depth- occurs at mantle to (outer) core boundary. Fig 14.2 Note: 1. S-wave shadow zone is larger almost half the Earth 2. P-wave shadow zone is daoughnut shaped and smaller 3. Shadow zones are areas where no direct (P & S) waves can arrive BUT reflected waves do arrive in these areas. Fig 14.3 Internal layering and composition- distances from Earth's surface. Fig 14.5 1. At ~2900 km - P wave shows step decrease and S wave disappears Lower portion (outer core) must be liquid. Density of material increases suddenly- matches by (liquid) iron. Outer core consists of liquid iron (+ impurities) 2. At ~5100 km, P wave shows (small) step increase, S wave reappears. Lower portion must be solid. Inner core made of solid iron (Hotter here than outer core, but great pressure causes iron to solidify) Chapter 14 Wednesday, March 21, 2007 12:39 PM 3. At ~30 km, P and S waves show step increase. Solid rock on both sides, marks the crust to mantle boundary- the Mohorovicis Discontinuity Moho is ~10 km below the oceans and up to ~100 km below the continents. Oceanic crust ~10 km thick is dense (mafic). Whereas, continental crust ~30-100 km thick, is light (felsic) For the latter, the higher the elevation, the greater the Moho depth- "mountains have roots" Fig 14.6 Some form of flotation balance on "liquid" surface (asthenosphere) 4. At ~100 km, P and S waves show slight decrease with increasing depth. S- wave is more affected, suggests some liquid present. This marks the lithosphere to asthenosphere boundary 5. Generally, P and S wave velocity increases steadily with depth from ~200 to 2900 km. Mantle is made of solid rock.
Seismic tomography Fig 14.9 Like CAT scan, gives us glimpse of Earth's interior (temperature and composition) Shows that: A. At a given depth (say 2000 km) T varies with "latitude and longitude" (hotter areas show lower P/S velocities) B. In places, subducted plate can be detected down to ~2900 km (colder subducted plate has higher P/S velocities than surroundings Gravity measurements Varitions in (pull of) gravity from place to place gives us information about Earth's interior to ~100 km depth. Verifies variable depth to Moho noted above, and mountains have roots. Has also helped to understand "flotation" of difference segments of the Earth's "crust" Principle of Isostasy During glacial periods, crust is depressed by thick layer of ice. When this melts, load is removed, and crust "springs" back up (very slowly) Fig Box Called isostatic rebound and produces "raised beaches" in some coastal areas (Scandinavia) Temperature inside the Earth Near surface geothermal gradient is ~30 degrees C/ km, but falls with depth. At ~100 km (top of asthenosphere) rocks are close to melting point ~1200 degrees C Throughout the rest of the mantle, T must be lower than the melting point of (local) rocks- mantle is solid Fig 14.8 At ~2900 km, T must be high enough for iron to melt ~4000 degrees C. At 5100 km, T dictated by the fact that iron is solid under local pressure. T ~5000