Therearesomeearthphenomenayoucancounton, butthemagneticfield, somedayisnotofthem. Itfluctuatesinstrength, driftsfromitsaxis, andeveryfew 100,000 yearsundergo, dramaticpolarityreversal—aperiodwhennorthpolebecomessouthpoleandsouthpolebecomesnorthpole. Buthowisthefieldgenerated , andwhyisitsounstable?
GroundbreakingresearchbytwoFrenchgeophysicistspromisestoshedsomelightonthemystery. Using 80 metersofdeepseasedimentcore, theyhaveobtainedmeasurementsofmagnetic?fieldintensitythatspan 11 polarityreversalsandfourmillionyears. Theanalysisrevealsthatintensityappearstofluctuatewithaclear, well?definedrhythm. Althoughthestrengthofthemagneticfieldvariesirregularlyduringtheshortterm, thereseemstobeaninevitablelongtermdeclineprecedingeachpolarityreversal. Whenthepolesflip—aprocessthattakesseveralhundredthousandyears—themagneticfieldrapidlyregainsitsstrengthandthecycleisrepeated.
Theresultshavecausedastiramonggeophysicists. Themagneticfieldisthoughttooriginatefrommoltenironintheoutercore,3,000 kilometersbeneaththeearth'ssurface. Bystudyingmineralgrainsfoundinmaterialrangingfromrockstoclayarticles, previousresearchershavealreadybeenabletoidentifyreversalsdatingback 170 millionyears, includingthemostrecentswitch 730,000 yearsago. Howandwhytheyoccur, however, hasbeenwidelydebated. Severaltheorieslinkpolarityflipstoexternaldisasterssuchasmeteorimpacts. ButPeterOlson, ageophysicistattheJohnsHopkinsUniversityinBaltimore, saysthisisunlikelyiftheFrenchresearchersareright. Infact, Olsonsaysintensitythatpredictablydeclinesfromonereversaltothenextcontradicts 90 percentofthemodelscurrentlyunderstudy. Iftheresultsprovetobevalidgeophysicistswillhaveanewtheorytoguidethemintheirquesttounderstandtheearth'sinnerphysics. Itcertainlypointsthedirectionforfutureresearch.?
1. Whichofthefollowingtitlesismostappropriatetothepassage??
A. PolarityReversal: AFantasticPhenomenonofNature.?
B. MeasurementoftheEarth'sMagnetic?FieldIntensity.?
C. FormationoftheTwoPolesoftheEarth.?
D. ANewApproachtothestudyofGeophysics. ?
2. WhathavethetwoFrenchgeophysicistsdiscoveredintheirresearch?
A. Someregularityinthechangesoftheearth'smagneticfield.?
B. Somecausesofthefluctuationoftheearth'smagneticfield.?
C. Theoriginoftheearth'smagneticfield.?
D. Thefrequencyofpolarityreversals. ?
3. TheFrenchgeophysicists'studyisdifferentfromcurrentlyprevailingtheoriesin ?______?.?
A. itsidentificationoftheoriginoftheearth'smagneticfield?
B. thewaytheearth'smagneticintensityismeasured?
C. itsexplanationoftheshiftintheearth'spolarity?
D. thewaytheearth'sfluctuationrhythmisdefined ?
4. InPeterOlson'sopiniontheFrenchexperiment ?______.?
A. islikelytodirectfurtherresearchintheinnerphysicsoftheearth
B. hassuccessfullysolvedthemysteryofpolarityreversals?
C. iscertaintohelppredictexternaldisasters?
D. hascausedgreatconfusionamongtheworld'sgeophysicists