GLG101: Physical Geology Lecture Outlines for Exam#3:

G. Calderone- GLG101: Physical Geology Lecture Reviews, Exam#3, August 23, 2011 Page 18 of 20•Seismic velocity versus depth curves- Upper EarthSeismic refraction and reflection of the various waves allow us to construct a picture ofthe velocities versus depth within the Earth Basically, velocities increase with depthwith a few exceptions and jumps called discontinuities-Moho Discontinuity- crust-mantle boundary. Jump is consistent with change of rocktype from felsic/intermediate/mafic in crusts to ultramafic in mantle. confirmationfrom mantle xenoliths of peridotite-LVZ (Low Velocity Zone) = Asthenosphere- both S and P waves transmitted so materialmust behave like solid in short time frames. Large drop in wave velocities, however,suggests that material is not all that rigid.-400km Discontinuity- sharp velocity increase. Possibly due to mineral phase change orchemical change at this level. Not well known- either would work.-670km Discontinuity-sharp velocity increase. Possibly due to mineral phase change orchemical change at this level. Not well known- either would work.•Seismic velocity versus depth curves- Whole EarthShadow zones and velocities of the various waves allow us to construct a picture of thevelocities versus depth within the whole Earth•S-Wave shadow zone- Since S-Waves cannot transmit through liquids and no S-Wavesare observed to arrive beyond 103° from epicenter, then there must be a liquid layerof the Earth. Depth can be calculated by geometry. Evidence for existence of liquidcore.•P-Wave Shadow zone- No P-wave arrivals are know between 103 and 143° fromepicenter. P-Waves can transmit through liquids although their velocities will beslowed down. Therefore, refraction will make them enter core more steeply. This willcreate a shadow zone in the way portrayed by the illustration in book. Depth can becalculated by geometry. Consistent with S-Wave Shadow zone for existence of liquidcore.-Gutenberg discontinuity - major drop in P-wave velocity. S waves not transmitted.Therefore, material is most likely fluid. Core/mantle boundary-Slight discontinuity in p-wave arrivals traveling through inner most part of core -Chemical considerations dictate that extra pressure may allow liquid core to go to asolid at depth. Appears that there may be a solid inner core. Other evidence fromfree oscillations suggests solid inner core.