Seis­mol­o­gists get to the bot­tom of how deep Earth’s con­ti­nents go

Weekend Mirror - - CHILDREN’S CORNER -

Earth­quake vi­bra­tions are re­veal­ing just how deep the con­ti­nents be­neath our feet go.

Re­searchers an­a­lyzed seis­mic waves from earth­quakes that have rocked var­i­ous re­gions through­out the world, in­clud­ing the Amer­i­cas, Antarc­tica and Africa. In al­most every place, pat­terns in these waves in­di­cated a layer of par­tially melted ma­te­rial be­tween 130 and 190 kilo­me­ters un­der­ground.

That bound­ary marks the bot­tom of con­ti­nen­tal plates, ar­gue Saiki­ran Thari­mena, a seis­mol­o­gist at the Univer­sity of Southamp­ton in Eng­land, and col­leagues. Their find­ing, re­ported in the Aug. 11 Sci­ence, may help re­solve a long time de­bate over the thick­ness of Earth’s land­masses.

Es­ti­mat­ing con­ti­nen­tal depth “has been an is­sue that’s plagued sci­en­tists for quite a while,” says Tim Stern, a geo­physi­cist at Vic­to­ria Univer­sity of Welling­ton in New Zealand, who wasn’t in­volved in the work. Rock frag­ments belched up by vol­canic erup­tions sug­gest that the rigid rock of the con­ti­nents ex­tends about 175 kilo­me­ters un­der­ground, where it sits atop slightly run­nier ma­te­rial in Earth’s man­tle. But analy­ses of earth­quake vi­bra­tions along Earth’s sur­face have sug­gested that con­ti­nents could run 200 or 300 kilo­me­ters deep, very grad­u­ally tran­si­tion­ing from cold, hard rock to hot­ter, gooier ma­te­rial.

That dis­agree­ment may ex­ist, Thari­mena says, be­cause to study con­ti­nen­tal thick­ness, seis­mol­o­gists had pre­vi­ously an­a­lyzed fairly shal­low earth­quake vi­bra­tions that couldn’t show Earth’s struc­ture in fine de­tail at depths greater than

Seis­mic waves take dif­fer­ent routes from an earth­quake to a seis­mome­ter. Some TRAVEL DEEP THROUGH EARTH’S MAN­TLE. OTHERS RE­flECT OFF A LAYER OF MELT IN THE UP­PER MAN­TLE OR BOUNCE OFF THE UN­DER­SIDE OF EARTH’S SUR­FACE. BY CLOCK­ING THE TIME IT TAKES THESE RE­flECTED WAVES TO AR­RIVE, SCI­EN­TISTS SAY THEY CAN DIS­CERN THE DEPTH AND CON­SIS­TENCY OF A LAYER OF SLIGHTLY RUN­NIER ROCK THAT MARKS THE BOT­TOM OF CON­TI­NEN­TAL PLATES

about 150 kilo­me­ters.

Thari­mena’s team looked at waves that bounced off bound­aries be­tween dif­fer­ent lay­ers in Earth’s up­per man­tle and other waves that ric­o­cheted off the un­der­side of the planet’s sur­face be­fore ul­ti­mately reach­ing the same seis­mome­ter. By mea­sur­ing how long it took for each kind of wave to reach the seis­mome­ter, the re­searchers could map the depths and con­sis­ten­cies of dif­fer­ent lay­ers of ma­te­ri­als in the con­ti­nen­tal plates.

The data re­vealed a sharp tran­si­tion from rigid rock to slightly mushier ma­te­rial at a depth that was fairly sim­i­lar for all the con­ti­nents. For in­stance, the melt starts about 182 kilo­me­ters un­der South Africa and about 163 kilo­me­ters un­der Antarc­tica. This is about as deep as di­a­monds — thought only to re­side within con­ti­nents — are known to ex­ist, lead­ing re­searchers to con­clude this par­tially melted layer marked the bot­tom of the con­ti­nents.

Get­ting t his global es­ti­mate for con­ti­nen­tal thick­ness is “a big deal,” says Brian Sav­age, a geo­physi­cist at the Univer­sity of Rhode Is­land in Kingston who wrote a com­men­tary on this study in the same is­sue of Sci­ence. The fiND­ING COuLD HELP sCI­EN­tIsts make better sim­u­la­tions of plate tec­ton­ics, which could pro­vide in­sights into what Earth looked like in the past and what it might look like in the fu­ture.

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