Simplest assumption: suppose the Wait! How do we know this?? Earth!s interior was uniform... We measure the travel •! P wave, S wave, and times of seismic waves surface waves would from earthquakes, and arrive at all stations compare them with what we would expect •! we would compute from different layered their arrival times at models until we get a different seismometers match assuming a gradual increase in velocity due to pressure “inversion” of the seismic travel time •! this would work data for velocity perfectly structure If the Earth!s interior were uniform... Here is what we actually see! we could make a plot of P, S, and surface wave arrival time with distance “ ∆ “ surface ∆ = 50° S time (minutes) time P ∆ ∆ P wave velocities drop suddenly at 2900 km depth, “Seismic Phases” and S waves cannot pass through this layer • P,S : P,S waves in the mantle, e.g. PPP, SS • K : P-wave in the outer core, e.g. PKP, SKP • I,J : P,S waves in the inner core • c : reflection from CMB, e.g. PcP, ScS • i : reflection from IOB, e.g. PKiKS • SUMMARY: many phases are created by Earth’s stratification (reflections, and sometimes conversion from one wave type to another) Fig. 4.7 P wave shadow zone S wave shadow zone Fig. 4.8 Fig. 4.9 We get this picture by calculating arrival times for all seismic Why are there seismic velocity jumps inside phases in a stratified model of the Earth, and making sure they the mantle? match the observed arrival times at all points on the Earth • mantle has fairly uniform composition • same chemical elements arrange into different minerals at different depths • minerals that are stable dots (phase travel times) at great depth are the densest match curves (model- • seismic wave speeds predicted change as minerals change travel times) Engdahl and Kennett 1991 very well. • Low Velocity Zone: close to melting temperature We can get seismic wavespeeds, but does this tell us the chemical composition of the layers? Meteorites: analogues to METEORITES composition of the Earth!s interior? Abundances of elements in the solar system are estimated from meteorites, solar corona, etc. Crust and mantle: too little iron (and nickel and lead etc.) relative to oxygen, silicon, etc. Missing: we need lots of What is the Earth!s Core iron and nickel inside the Earth a small amount of made of? • liquid outer core: must be a liquid at the P,T + + conditions deep in the Earth • magnetic field generation: it must be a metal “Primordial” (never melted or re-processed) • densities: the core must be dense 4.5 BY old meteorite – Earth’s average: 5.5 g/cm^3 ~ – crust, mantle: 2.7-3.3 g/cm^3 (85% by volume) = Likely close to average – therefore, core: 10+ g/cm^3 composition of the Earth • meteorites suggest Fe core with trace O, Si, Ni, S Core - mantle boundary CORE-MANTLE BOUNDARY • dramatic density and seismic velocity change • slab graveyard • mantle plume birthplace • site of “anti-crust’’ (ULVZ) and “anti- lithosphere’’ (D’’ layer) • is the core reacting with the mantle? or is it melting the mantle? D’ ’ layer and ultra low velocity zone (ULVZ): what’s going on? Rising plumes, sinking slabs... The Earth Seismic tomography: let!s find is not exactly radially symmetric Texas seismographs suppose that the Earth is flat and that seismic waves travel unusually slowly through earthquakes Texas... Finding Texas. Global Seismic Tomography on-time late late late seismic velocity anomalies usually on-time differ by less than 2% from surroundings Global Seismic Tomography upper mantle • similar to CAT scan in medical imaging • compares real travel times with travel times predicted by the radially symmetric Earth model • small differences in travel times are translated to seismic velocity variations • Blue (cold) is fast & Red (hot) is slow • snapshots of mantle convection: hot core - mantle material rises and cold material sinks boundary Van der Hilst et al., 1998 Subducting slabs: stronger and colder Giant superplume rising from the CMB than their surroundings under the Pacific Ocean Do they all sink to the core-mantle boundary or not? There!s another one of these under Africa too. Van der Hilst et al., 1998 Seismic Tomography of the Mantle Tomography at a finer scale - P wave a “snapshot” of current mantle velocity velocity anomalies at 100 km depth • primary influences: • composition Subducting • temperature slab Yellowstone hot spot Strong mantle Maximum 3-D image: under the Sierra perturbation • orange-red: hot Nevada batholith (upward convection) is 2% Tomography at a finer scale - the Yellowstone hot spot they* could not resolve the bottom of this low-velocity feature small plumes associated with hot spots are too small to show up on the global tomographic images *Derek Schutt, Eugene Humphreys, Rebecca Salzer (P and S wave studies).