PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Polar Glaciers Melting AnimationSlide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20thick flood basalt sequence in BrazilSlide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.isostasy, gravity, magnetism, and internal heatEarth’s gravity fieldisostasyequilibrium of adjacent blocks of brittle crust“floating” on underlying upper mantleouter layers of Earth divided into 2 based on their strengthlithosphere: rigid, solid outer layer (brittle) --strong crust and uppermost mantleDO NOT CONFUSE WITHCRUST AND MANTLEWHICH ARE BASED ON COMPOSITIONasthenosphere: underlying denser, heat-softened, partially melted (plastic) -- weakupper mantletransition from lithosphere to asthenosphere reflectstemperature and rocks response to increased temperatureisostasyequilibrium of adjacent blocks of brittle crust“floating” on underlying upper mantlei.e. mass above a certain depth must be the samethink of wood blocks in waterblock that sticks up higheralso extends farther in waterdensity of wood < density of waterfor masses to be the same above the isostatic compensation depth: compensation depthmass in column 1 = mass in column 2masses in both columns in 2 dimensions equal (density wood x thickness wood) + (density water x thickness water)density water > density woodwood that replaces water in the column must be thicker than water it replacesisostasycontinental crust isless dense thanoceanic crustmass in column 1 = mass in column 2 = mass in column 3density mantle > density oceanic crust > density continental crustcrust isless dense thanmantlesame concept as wood blocks applies to lithospheric blocks(crust and uppermost mantle)floating on asthenosphere above the compensation depthcompensation depthif more mantle in column -- column will be thinnerif more continental crust in column -- column will be thickerimplication is that mountains have “roots” -- crust is thicker below themisostasya more detailed view of density differencesinclude sea water&sedimentsisostasyleads to “isostatic adjustment” if mass is redistributederosion redistributes rockfrom mountain (high)to sediment depositedin basin (low)less mass on mountaincauses uplift of crust below mountain(thins and rises)andsubsidence of basinas mass ofsediment is addednote mountain andcrustal root below iterosion of mountainas mountain erodes,column becomes shorter thus, mantle mass in column increases over time(mass A = mass B = mass C)A B CABCXmantlecrustAXeffect on mass columnsisostasy“see” isostatic adjustment today from load of glaciers oncrust during last glaciation and unloading from melting(possible because response of asthenosphere is slow)process is called post-glacial reboundisostasypost-glacial rebound still occurs in Canada & northern Europei.e. crust is rising -- (not isostatically balanced)(can measure uplift rates with highly precise GPS receivers--mm’s/yr)amount of uplift since glaciationPolar Glaciers Melting AnimationFrom: http://www.uni-geophys.gwdg.de/~gkaufman/work/onset/onset_ice3g.htmlgravitygravitational force between two objects determinedby their masses and distance between themgravitydifferences in density of materials (rocks) in Earth’s interior produces small differences in local gravity field (anomalies)can be measured with a gravimeter (attraction of spring to mass)dense materialattractsand extends springvoid (cave) has nomass to attractspringmass uniformand springis neutralcan find buried, dense things (abandoned gas station tanks)and empty spaces (caves -- don’t build)gravitydensity differences also occur over larger areas: mountainsmass above compensation depth is uniform (isostatically balanced)--no excess or deficiency in mass; no gravity anomalies--compensation depthgravitymass above compensation depth is not uniform -- excess mass of dense mantle below mountain (no crustal root)compensation depthgenerates increased gravity and, thus, a positive gravity anomalygravitymass above compensation depth is not uniform -- deficiency of mass below low area (too much crust)compensation depthgenerates decreased gravity and, thus, a negative gravity anomalyEarth’s gravity field measured from spacemass in Earth “pulls” on satellites as they orbit, causing “wobbles” in orbit paths, which are measured--amount of wobble related to amount of mass--GRACE--NASA--mission toexamineEarth’sgravityfieldEarth’s magnetic fieldsurrounds the Earth• has north and south magnetic poles• is detected by compasses• is recorded in rocks and minerals as they cool• is generated in the Earth’s liquid outer core asit spins and produces electrical currentsEarth’s field similar to that forbar magnet (left)magnetic N and Sis not the sameas geographicN and S poles(bar magnet “tilted”)Earth’s magnetic fieldchanges through timechange in magnetic north relative to true north1580-19701831-2001migration of magnetic northconsequence of rotation of outer coreNSEarth’s magnetic fieldreverses over time (north and south poles flip) --magnetic field lines reverse--“normal” polarity: north is north and south is south“reversed” polarity: north is south and south is northafter next reversal, compass needle will point southEarth’s magnetic fieldhow do rocks and minerals acquire magnetism?rocks and minerals at high temperatures (e.g. molten) must cool through their Curie temperatures • above Curie temperature, atoms are random• below Curie temperature, atoms align in domainsthat are independent of each other• below Curie temperature, atoms align withmagnetic field if one is present (e.g. Earth)Earth’s magnetic fieldhow do rocks and minerals acquire magnetism?rocks and minerals that cool through Curie temperature and stay below that temperature through timerecord magnetic field AT THE TIME OF THEIR COOLINGmagnetite common mineral in basaltpaleomagnetism: study of ancientmagnetic fields in rocks--reconstruction of past fields--thick flood basalt sequence in BrazilEarth’s magnetic fieldexamine thick sequences of basalts to identify reversalsthrough time (paleomagnetism)Earth’s magnetic fieldre-construct “normal” and “reversed” for lava sequenceEarth’s magnetic fieldcreate time-scale for magnetism from many observationsblack = normal polarityblue =