Fig 14 6 Some form of flotation balance on liquid surface asthenosphere At 100

Fig 14 6 some form of flotation balance on liquid

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(Fig. 14-6) Some form of flotation balance on “liquid” surface (asthenosphere?) At ~100 km, P an S waves show light decrease with increasing depth. S-wave is more affected, suggests some liquid present. This marks the lithosphere to asthenosphere boundary. Generally P and S wave velocity increase steadily with depth from ~200 to 2900 km. Mantle is made of solid rock. (There are minor blips of velocity in the mantle, caused by atoms being packed more closely at certain depths. Fig. 14-5) Seismic tomography (Fig. 14-9) Like CAT scan, gives us glimpse of Earth’s interior (temperature and composition. Shows that: At a given depth (say 2000 km) T varies with “latitude and longitude” (hotter areas show lower P/S velocities) In places, subducted plate can be detected down to ~2900 km (colder subducted plate has higher P/S velocities than surrounding) Gravity measurement Variations in (pull of) gravity from place to place gives 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 “floatation” of different segments of the Earth’s “crust”. 49
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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 (v. slowly) (Fig. Box) Called isostatic rebound and produces “raised beaches” in some coastal areas (Scandinavia). Temperature inside 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 rest of 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 by the fact that iron is solid under local pressure. T ~5000 degrees C. T estimated at Earth’s center ~5500 degrees C- could be out by + or – 1000 degrees C. Heat loss from hot interior. Rocks are very poor conductors o hear- heat escapes slowly through mantle and crust. However, convection is an efficient mode of heat transfer (loss). “Liquid” portions of Earth allow faster hear loss, i.e. Outer core and asthenosphere. Near surface, a lot of heat is lost at divergent plate boundaries by upwelling magma that causes plates to be pushed apart (Fig. 1-11) 50
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Earth’s magnetic field. Use of compass, suggests Earth behaves as though giant (bar) magnet buried at depth (Fig. 1-12). When lava is erupted (containing magnetic mineral- magnetite ), at ~1200 C, it shows no magnetism. On cooling to 600 degrees C (Curie Point), it “takes” a snapshot of the Earth’s magnetic field. )This is permanently locked in unless rock is heated to >600 degrees C). Study of lava flows from many areas, shows that in the past, the magnetic field has reversed often.
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