It’s the only known planet with plate tectonics. Life on earth de­pends on it. How did we get so lucky? RICHARD A. LOVETT ex­plains.

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NOT ONLY DOES EARTH LIE in the ‘Goldilocks zone’ that al­lows wa­ter to ex­ist in the liq­uid form that life re­quires. It is also the only rocky planet we know of that con­stantly ren­o­vates its sur­face as its tec­tonic plates dive into the man­tle in some places and re-emerge as molten lava in oth­ers. Many as­tro­bi­ol­o­gists now think this con­stant re­newal is just as im­por­tant as liq­uid wa­ter for the flour­ish­ing of life as we know it.

The the­ory, ex­plains plan­e­tary sci­en­tist Adrian Le­nardic of Rice Univer­sity in Hous­ton, Texas, is that the Earth’s cli­mate has been buffered by the re­cy­cling of car­bon diox­ide (CO ) from the at­mos­phere into the planet’s in­te­rior via min­eral se­ques­tra­tion and then out again via vol­ca­noes. This has kept the cli­mate tem­per­ate even as the Sun’s heat has in­creased in in­ten­sity by about a third since the planet’s birth. With­out this buffer­ing, Earth might have heated so much that all the wa­ter in its oceans boiled away and huge quan­ti­ties of CO ac­cu­mu­lated in the at­mos­phere, much like Venus which has an av­er­age tem­per­a­ture of 462 C. Or it might never have re­cov­ered from be­ing a snow­ball, re­main­ing per­ma­nently frozen.

Among the rocky worlds we know, Earth’s tectonics are unique. Venus and Mer­cury have no sim­i­lar ge­o­log­i­cal ac­tiv­ity. Mars might have once, but not for bil­lions of years. So why are we so lucky?

Ac­cord­ing to geo­physi­cist David Ber­covici, of Yale Univer­sity, mod­els show the Earth sits right on the cusp be­tween be­ing a world with plate tectonics and one with a ‘stag­nant lid’, like mod­ern-day Mars or Venus. Some­thing must have kicked it in the di­rec­tion that pro­duced a ge­o­log­i­cally ac­tive world that even­tu­ally gave birth to us. Bizarrely, even as astronomers probe plan­ets hun­dreds of light-years dis­tant, ge­ol­o­gists still can’t pre­cisely ex­plain what trig­gered the events tak­ing place be­neath our feet. TECTONICS DE­RIVES FROM the Greek word ‘ tek­tonikos’, mean­ing to build. It points to what we do un­der­stand about the way Earth’s sur­face is con­stantly re­mod­elled. Our planet has a rigid shell called the litho­sphere that com­prises the crust and a hard­ened up­per slice of an other­wise play­doh-like man­tle (see di­a­gram). That shell is cracked into seven large plates and a num­ber of smaller ones that float on the man­tle in slow, con­stant mo­tion.

The first inkling that con­ti­nents moved dates back to the 1500s, when Flem­ish map­maker Abra­ham Ortelius noted that the east­ern and western coast­lines of the At­lantic Ocean looked as if they might have once fit­ted to­gether like pieces from a jig­saw puzzle.

In 1912 Ger­man geo­physi­cist Al­fred We­gener coined the term ‘con­ti­nen­tal drift’ to de­scribe how the lands on each side of the At­lantic had be­come so strangely sun­dered, but it wasn’t un­til 1963 that Bri­tish ma­rine ge­ol­o­gists Fred Vine and Drum­mond Matthews pro­vided the ex­pla­na­tion (see Cos­mos 54, p48). They

re­alised the in­te­rior of the Earth is in mo­tion. The rock of the man­tle is slightly plas­tic – enough so that it can rise and fall in slow, roil­ing mo­tions called convection cur­rents: hot rock rises from the depths, cools, be­come denser and then de­scends. The best anal­ogy is a lava lamp, which uses heat from a light bulb to in­duce the cir­cu­la­tion of coloured wax in liq­uid. While the lava lamp’s convection cur­rents are fast enough to pro­duce mes­meris­ing changes of colour, the rock of the man­tle moves “about as fast as your fin­ger­nails grow”, says Ber­covici – at a speed of less than 10 cm a year.

When ris­ing cur­rents hit the un­der­side of the solid litho­sphere, they de­flect side­ways, ex­ert­ing drag. If that drag is strong enough, it can rip the litho­sphere apart, cre­at­ing new plates and mak­ing old ones move, up­welling magma fill­ing in the gaps. When this hap­pens at the bot­tom of the ocean, the re­sult is ‘sea floor spread­ing’ – which is what Vine and Matthews ob­served. This is oc­cur­ring to­day in places such as the Mid-at­lantic Ridge and the Red Sea Rift be­tween Africa and Ara­bia.

As the spread­ing crust cools, it grows denser. Even­tu­ally the lead­ing edge fur­thest from the magma flow starts sink­ing back into the man­tle, pulling the rest of the slab be­hind it – a process called sub­duc­tion – and so com­plet­ing the convection cy­cle. Like the wax in the lava lamp, the cy­cle of ris­ing, spread­ing, fall­ing and ris­ing again is the en­gine that moves the plates, and with them the con­ti­nents, which ride atop like rafts.

Though these mo­tions oc­cur at a rate of only a few cen­time­tres per year, that is rapid enough to make even the old­est seafloor in the world star­tlingly young – less than 200 mil­lion years old. Con­ti­nen­tal crust, the buoy­ant crud that froths to the sur­face as ocean crust subducts, is much older.

The plates do not move in the same di­rec­tion or at the same speed. This causes some plates to crash into each other, driv­ing up moun­tain ranges, such as the Hi­malayas at the col­li­sion of the In­dian and Eurasian plates. They can also grind past one an­other, as along Cal­i­for­nia’s famed San An­dreas Fault. Or one can dive be­neath an­other, as oc­curs at the Pa­cific ‘Ring of Fire’ that cir­cles the Pa­cific Ocean in a belt of earthquake-prone re­gions and vol­canic ac­tiv­ity.

In this process, con­ti­nents tend to re­main on the sur­face. They are too buoy­ant to be eas­ily sub­ducted into the depths. But they still play an im­por­tant role via a process known as ‘weath­er­ing’, which pro­vides a vi­tal

ther­mo­stat that has helped keep the Earth tem­per­ate for bil­lions of years.

It be­gins when CO from the at­mos­phere dis­solves in rain­wa­ter to form car­bonic acid. This breaks down min­er­als in con­ti­nen­tal rocks, pro­duc­ing cal­cium and bi­car­bon­ate ions that wash into the sea. Ma­rine or­gan­isms take them up to form cal­cium car­bon­ate, the build­ing block for their shells and skele­tons, which ul­ti­mately set­tle to the seafloor and be­come lime­stone.

Each year the process re­moves about 300 mil­lion tonnes of CO from the at­mos­phere. But the car­bon isn’t se­questered for­ever, be­cause some of that lime­stone is sub­ducted along with the se­abed. It heats, melts and is in­cor­po­rated into magma for car­bon diox­ide-spew­ing vol­ca­noes to re­lease. This also pro­duces fresh rock for the next weath­er­ing cy­cle.

What makes this process func­tion like a ther­mo­stat is that the more CO there is in the at­mos­phere, the more car­bonic acid there is in rain (and the more rapidly weath­er­ing oc­curs). This re­moves CO from the at­mos­phere more swiftly, keep­ing the Earth from trans­form­ing into a Venu­sian run­away green­house. Con­versely, if at­mo­spheric CO lev­els fall,weath­er­ing slows, al­low­ing vol­canic CO to slowly build back up. It’s a slow, self-cor­rect­ing process that for bil­lions of years has kept the Earth’s tem­per­a­ture within a zone that is hos­pitable to life.

So what got Earth’s plate tectonics go­ing, rather than the planet end­ing up with a largely in­ert ‘stag­nant lid’ like Mars and Venus?

The ear­li­est Earth was all magma ocean with no solid sur­face to form plates, let alone plates that drift around and col­lide with one an­other. At a min­i­mum, plate tectonics couldn’t have be­gun un­til af­ter the Earth’s sur­face so­lid­i­fied, some­where about 4.5 to 4 bil­lion years ago. Just when the plate tectonics kicked in, though, still has ge­ol­o­gists squab­bling.

IF YOU’RE SEEK­ING the ear­li­est traces of plate tectonics, a good place to look is the Jack Hills in Western Australia. To the ca­sual trav­eller this range of low moun­tains about 800 km north of Perth is not hugely im­pres­sive. But to ge­ol­o­gists the hills are of tow­er­ing sig­nif­i­cance, con­tain­ing time cap­sules of the world’s old­est rocks in tiny crys­tals of zir­co­nium sil­i­cate (ZRSIO ).

Zir­cons formed in cool­ing magma. Three things make them ge­o­log­i­cal gems. First, they carry a date stamp of for­ma­tion, based on the de­cay of traces of ura­nium trapped within them. Sec­ond, they are ex­tremely durable; the an­cient vol­canic rocks that gave birth to them eroded long ago and were re­con­sti­tuted

into sed­i­men­tary rocks in the Jack Hills’ out­crops. Third, they bear trace el­e­ments like ti­ta­nium and alu­minium, which re­veal the con­di­tions of their birth.

So far these zir­con time cap­sules have tele­graphed an ex­tra­or­di­nary mes­sage: 4.2 bil­lion years ago they were born kilo­me­tres be­low, crys­tallis­ing as they rose to Earth’s sur­face. This tells us the man­tle was start­ing to churn at that time.

But were these up­wellings the same as those that drive mod­ern plate tectonics? Craig O’neill thinks not. He’s a cheery geo­dy­nam­i­cist at Syd­ney’s Mac­quarie Univer­sity who has been study­ing Jack Hills zir­cons for many years. In his view, the zir­cons could have been formed by lo­calised up­wellings sim­i­lar to those oc­cur­ring to­day in places like Hawaii and Yel­low­stone. In other words, not an Earth-wide tec­tonic churn­ing but a lo­cal per­co­la­tion.

Vicki Hansen, a plan­e­tary ge­ol­o­gist at the Univer­sity of Min­nesota, Du­luth, has come to the same con­clu­sion based on “green­stone ter­ranes” found in Green­land, South Africa, Canada and Scan­di­navia.

These rock as­sem­blages, which mea­sure a few hun­dred kilo­me­tres across, date back to the Ar­chaean Eon, 4 to 2.5 bil­lion years ago. They are in­ter­est­ing be­cause the green­ish gran­ites that give them their name are mixed up hig­gledy-pig­gledy with se­abed sed­i­ments in ways we never see in more re­cent vol­canic prov­inces. If mod­ern-day rocks are like the veg­eta­bles dis­played at the su­per­mar­ket, the green­stone rocks are like stir-fry. This, Hansen says, in­di­cates that what­ever was go­ing on in the Achaean in­volved pro­cesses “fun­da­men­tally dif­fer­ent” to those to­day.

More ev­i­dence that mod­ern plate tectonics had not geared into ac­tion un­til rel­a­tively re­cently comes from the study of the his­tory of con­ti­nen­tal drift.

Su­per­con­ti­nents are formed when the plate­tec­tonic en­gine drives the Earth’s land masses to merge into one gi­gan­tic block. The clos­est we now have to a su­per­con­ti­nent is Eura­sia. But some re­mark­able de­tec­tive work – us­ing the age of rocks and mag­netic sig­na­tures that mark the lat­i­tudes where they first formed – re­veals at least four grand­daddy su­per­con­ti­nents that make Eura­sia look tiny.

The most re­cent is Pan­gaea, which formed about 335 mil­lion years ago and lasted through much of the age of the di­nosaurs. It was pre­ceded by Ro­dinia (1 bil­lion to 750 mil­lion years ago), then by Nuna (2 to 1.8 bil­lion years ago). The ear­li­est de­tectable su­per­con­ti­nent is Kenor­land (2.7 to 2.4 bil­lion years ago), relics of which are scat­tered across Western Australia, North Amer­ica, Green­land, Scan­di­navia and the Kala­hari Desert.

The fact we can’t find a su­per­con­ti­nent older than Kenor­land may sim­ply mean the sur­viv­ing bits are too scat­tered for ge­ol­o­gists to piece back to­gether. It’s like try­ing to fig­ure out the his­tory of a vase that has been bro­ken and re­assem­bled sev­eral times.

But with su­per­con­ti­nent for­ma­tion and break-up re­quir­ing mod­ern-style plate tectonics, the fact we haven’t found one be­fore Kenor­land might in­stead be telling us that for the Earth’s first 1.8 bil­lion years the lava lamp was not strong enough to pro­duce any­thing other than lo­calised per­co­la­tions, not the con­ti­nent­driv­ing process we have to­day.

IF THERE’S ANY CON­SEN­SUS amongst ge­ol­o­gists, it is that some­thing changed about 2.7 bil­lion years ago to kick tec­tonic plates into ac­tion. “There ap­pears to have been a ma­jor event,” says Kent Condie, a geochro­nol­o­gist at the New Mex­ico In­sti­tute of Min­ing and Tech­nol­ogy in So­corro.

But what could that have been? The­o­ries range from the mun­dane to the dra­matic, but all re­quire the Earth to have over­come the same ba­sic hur­dles. Ei­ther the power of the lava lamp that makes man­tle cur­rents rise and swirl must have in­creased or the Earth’s crust must have weak­ened, al­low­ing it to break into plates; or per­haps both oc­curred si­mul­ta­ne­ously.

One view, favoured by Matt Wel­ller of Rice Univer­sity, is that feed­back loops in magma cur­rents

Time cap­sule: Jack Hills zir­con. Born 4 bil­lion years ago, it shows earth was al­ready churn­ing. If there’s any con­sen­sus among ge­ol­o­gists, it is that some­thing changed about 2.7 bil­lion years ago to kick tec­tonic plates in ac­tion.

grad­u­ally built up to a level strong enough to pro­duce self-sus­tain­ing plate tectonics via what en­gi­neers call a ‘hys­tere­sis loop’. A hys­tere­sis loop oc­curs when there is a lag be­tween cause and ef­fect. It is anal­o­gous to an out-of-tune au­to­mo­bile en­gine. When you press down on the ac­cel­er­a­tor, at first the en­gine barely re­acts, then it lurches for­ward.

Sup­pose the deep convection cur­rents driv­ing the Earth’s plate tectonics were to sud­denly shut down. That would re­duce the amount of heat that can es­cape, caus­ing man­tle rocks to heat up and be­come more plas­tic. Softer rocks can sup­port more vig­or­ous convection, so the lava-lamp ef­fect in­ten­si­fies, car­ry­ing heat more rapidly from the in­te­rior –un­til enough has es­caped, the man­tle cools and its cur­rents slow again.

“You can shift back and forth as you heat up and cool down, heat up and cool down,” says Ju­lian Low­man, a geo­dy­nam­i­cist from the Univer­sity of Toronto. Ac­cord­ing to this view, the ju­ve­nile Earth ex­pe­ri­enced these on-and-off episodes on a small scale, pro­duc­ing the lo­calised tectonics sug­gested by the Jack Hills zir­cons and the green­stone ter­rains. Then, about 2.7 bil­lion years ago, these shifts be­came locked into a self-sus­tain­ing Earth-wide convection cy­cle.

Hansen, on the other hand, opts for a more dra­matic sce­nario. The event that kicked off the tec­tonic plates might lit­er­ally have been a kick – in the form of an as­ter­oid or comet strike. Not as big as the one that formed the Moon, but far larger than the one that killed the di­nosaurs.

She first de­scribed her the­ory in 2007 in the jour­nal Ge­ol­ogy, ar­gu­ing such an ob­ject would have punched right through the crust, heat­ing the man­tle and set­ting cur­rents in mo­tion, drag­ging the plates along with them and start­ing tec­tonic move­ments. Once plates be­gan col­lid­ing and sink­ing, the process ex­panded un­til

it spread across the planet. “Sub­duc­tion is like a virus,” the pa­per states. “Once be­gun it can eas­ily spread.”

Al­ter­na­tively, the dra­matic event might have come from be­low. In a 2015 pa­per in Na­ture, a team led by Teras Gerya, of the Swiss Fed­eral In­sti­tute of Tech­nol­ogy in Zurich, ar­gued that hot spots on the Earth’s core could have caused plumes of hot man­tle to rise be­neath a con­ti­nent. Un­der the right cir­cum­stances, they cal­cu­lated, this could break up the con­ti­nent and cause pieces to sink, cre­at­ing a self­sus­tain­ing cy­cle that be­came plate tectonics.

Yet an­other pos­si­bil­ity is that some­thing changed the dis­tri­bu­tion of heat deep within the Earth. That heat comes from two sources: the long-lived ra­dioac­tive de­cay of atoms such as ura­nium and tho­rium trapped in the man­tle; and from the core, which con­tains a vast reser­voir of heat re­main­ing from the for­ma­tion of the Earth. Both are slowly de­clin­ing as the Earth ages.

One might think a cool­ing Earth would have weaker tectonics. But it’s not that sim­ple. “There are lines of re­search,” Low­man says, “sug­gest­ing that plate tectonics has a bet­ter chance of man­i­fest­ing it­self as a planet cools.” That’s be­cause the lava-lamp en­gine that drives plate tectonics de­pends less on how hot the Earth’s in­te­rior is as on how rapidly it can trans­fer heat to the sur­face. The faster heat is trans­ferred, the stronger the en­gine, and the stronger the man­tle cur­rents that drive tectonics.

It has been known since the 1930s that the Earth’s core has two lay­ers: an outer one com­posed of molten metal, and an in­ner one made of solid metal. As the Earth cools, the in­ner core grows. In the process it re­leases heat energy – equal to the amount it took to melt all that ma­te­rial in the first place. That energy rises through the core, in­creas­ing the rate at which it heats the man­tle and, ul­ti­mately, rises to the sur­face.

Sup­port­ing the the­ory that the cool­ing core may power the tec­tonic en­gine, a 2015 study by Condie and col­leagues in the jour­nal Pre­cam­brian Re­search traced the mo­tions of con­ti­nents over the past 2 bil­lion years. They con­cluded that plate tectonics have been slowly speed­ing up, with av­er­age plate speed nearly dou­bling over that time.

But even the strong­est man­tle cur­rents would not have trig­gered tec­tonic ac­tiv­ity if the Earth’s crust was too strong to break into plates. As it was, ap­par­ently, on Mars. For Berkovici, the key fac­tor for the emer­gence of plate tectonics was there­fore the weak­en­ing of Earth’s crust. It might have started grad­u­ally, be­gin­ning with the type of plume tectonics re­flected in an­cient green­stone ter­ranes. Each of these magma break­throughs would have cre­ated fault lines along which rocks slipped against each other, just as they do in to­day’s earth­quakes. These mo­tions would have pro­duced weak spots that might have be­come fo­cal points for later break­throughs. Ber­covici com­pares it to re­peat­edly bend­ing a pa­per clip. “It gets softer,” he says. “Even­tu­ally you can bend it eas­ily.”

Grad­u­ally these weak zones would have spread un­til they merged into plate bound­aries sim­i­lar to to­day’s, and the process went from lo­cal and in­ter­mit­tent to global and con­tin­u­ous. “A meteor might have got­ten it started,” Ber­covici says in a nod to Hansen, “but it needs these feed­backs to keep go­ing.” IT IS EASY TO try to fold all of this into a nice, co­her­ent story. It would be­gin with a magma ocean, fol­lowed by weak, in­ter­mit­tent plume-style tectonics. These would even­tu­ally reach some tip­ping point that shifted the process to its present state, ei­ther due to changes in the core, an as­ter­oid im­pact, the ac­cu­mu­la­tion of Ber­covici’s weak spots, or some com­bi­na­tion of all three. But the plethora of op­tions sug­gests cau­tion. We may not yet have all the pieces to the puzzle.

Lindy Elkins-tan­ton, di­rec­tor of the School of Earth and Space Ex­plo­ration at Ari­zona State Univer­sity in Tempe, re­mem­bers be­ing a grad­u­ate stu­dent at a con­fer­ence, won­der­ing what made sci­en­tists who dis­agreed with her own pre­sen­ta­tion so sure of them­selves.

“I sat there think­ing per­haps I just didn’t know enough yet,” she re­calls. “But now, 15 years later, I see that none of us know enough. We can only make small in­cre­men­tal progress in this very com­pli­cated prob­lem.” RICHARD A. LOVETT is a sci­ence writer and sci­ence fic­tion au­thor based in Port­land, Oregon.

IMAGES 01 Vi­talij Cere­pok / Getty Images 02 John Val­ley, Univer­sity Wisconsin 01 Pic­ture Press / Getty Images

Even the strong­est man­tle cur­rents wouldn’t have trig­gered tec­tonic ac­tiv­ity if the Earth’s crust was too strong to break into plates.

Ice­land’s lava fields: ev­i­dence of the rift be­tween the North Amer­i­can and Eurasian tec­tonic plates.

Tec­tonic ther­mo­stat: con­ti­nen­tal weath­er­ing re­moves 300 mil­lion tonnes from the at­mos­phere each year. It’s a vi­tal part of the car­bon cy­cle that has kept the Earth tem­per­ate.

The Earth’s rigid tec­tonic plates float on a play­doh-like man­tle in slow, con­stant mo­tion. With­out this move­ment, the planet might well have ended up with a ‘stag­nant lid’ no more con­ducive to sus­tain­ing life than Mars or Venus.

A slice through the earth. The crust and up­per man­tle form the brit­tle litho­sphere which cracks into tec­tonic plates.

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