India

Time after time N. China break S. China apart and rejoin over hundreds of millions of years, S. China resculpting ’s Australia ever-changing face. N. China

Siberia Congo/São Francisco Kalahari Laurentia

Baltica Congo/São W. Francisco Amazonia Amazonia W. Africa

Nuna 1.4 billion years ago 750 million years ago

New studies of magnetic minerals in rock from Brazil, for instance, are helping pin the ancient Amazon to a spot it once occupied in Nuna’s heart. Other recent research reveals the geo­ SUPERPUZZLE logic stresses that finally pulled Rodinia apart, some 750 million years ago. Scientists have even Geologists trace the comings and goings predicted when and where the next superconti­ nent may form — an amalgam of of Earth’s landmasses By Alexandra Witze and , evocatively named Amasia — some 250 million years from now. Look at any map of the , and you Reconstructing is like trying might feel the urge to slide and to assemble a 1,000-piece jigsaw puzzle after you’ve Africa together. The two continents just beg to lost a bunch of the pieces and your dog has chewed nestle next to each other, with Brazil’s bulge up others. Still, by figuring out which puzzle pieces locking into West Africa’s dimple. That visible went where, geologists have been able to illumi­ clue, along with several others, prompted Alfred nate some of Earth science’s most fundamental Wegener to propose over a century ago that questions. the continents had once been joined in a single For one thing, continental drift, that grad­ enormous landmass. He called it , or ual movement of landmasses across Earth’s “all lands.” surface, profoundly affected life by allowing spe­ Today, geologists know that Pangaea was just cies to move into different parts of the the most recent in a series of mighty super­ depending on what particular landmasses hap­ continents. Over hundreds of millions of years, pened to be joined. (The global distribution of enormous plates of Earth’s crust have drifted dinosaur fossils is dictated by how continents together and then apart. Pangaea ruled from were assembled when those great animals about 400 million to 200 million years ago. But roamed.) R. MITCHELL EVANS; 2016; D.A.D. wind the clock further back, and other superconti­ Supercontinents can also help geologists hunt­ nents emerge. Between 1.3 billion and 750 million ing for mineral deposits — imagine discovering years ago, all the continents amassed in a great gold ore of a certain age in the Amazon and using it land known as Rodinia. Go back even further, to find another gold deposit in a distant landmass about 1.4 billion years or more, and the crustal that was once joined to the Amazon. More broadly, shards had arranged themselves into a supercon­ shifting landmasses have reshaped the face of the tinent called Nuna. planet — as they form, supercontinents push up Using powerful computer programs and geologic mountains like the Appalachians, and as they clues from rocks around the world, researchers break apart, they create oceans like the Atlantic.

are painting a picture of these long-lost . “The assembly and breakup of these continents MURPHY (EDS.)/ LI AND J.B. Z.-X. EVANS, D.A.D. FULLER; SOURCES: NICOLLE RAGER ART: HISTORY EARTH THROUGH CYCLES SUPERCONTINENT

18 SCIENCE NEWS | January 21, 2017 Siberia

Baltica N. America

N. China China 2.1 billion years ago: Laurentia Landmasses begin Siberia assembling toward S. America the first true known supercontinent, Australia Nuna (also known as Columbia). W. Africa S. China

Antarctica 1.5 billion years ago: Nuna reaches its Amazonia maximum landmass.

Congo/São Francisco Australia 1.3 billion years ago: India Nuna begins to fragment and the parts reassemble as Rodinia. Pangaea Amasia 200 million years ago 250 million years from now 1 billion years ago: Rodinia reaches have profoundly influenced the evolution of the the Pacific Ocean is shrinking, its seafloor sucked maximum landmass. whole Earth,” says Johanna Salminen, a geophysi­ down by along the Ring of Fire — loop­ cist at the University of Helsinki in Finland. ing from New Zealand to Japan, Alaska and Chile. 750 million years ago: By running the process backward in time, geol­ Rodinia begins to Push or pull ogists can begin to see how oceans and continents break apart, creating the Pacific Ocean. For centuries, geologists, biogeographers and have jockeyed for position over millions of years. explorers have tried to explain various features Computers calculate how plate positions shifted of the natural world by invoking lost continents. over time, based on the movements of today’s 400 million years ago: Subducting ocean Some of the wilder concepts included , a plates as well as geologic data that hint at their crust once again sunken realm between Madagascar and India that past locations. shifts landmasses offered an out-there rationale for the presence of Those geologic clues — such as magnetic min­ toward one another, in early steps toward lemurs and lemurlike fossils in both places, and erals in ancient rocks — are few and far between. Pangaea. Mu, an underwater land supposedly described in But enough remain for researchers to start to ancient Mayan manuscripts. While those fantas­ cobble together the story of which crustal piece 300 million years ago: tic notions have fallen out of favor, scientists are went where. Pangaea reaches exploring the equally mind-bending story of the “To solve a jigsaw puzzle, you don’t necessar­ maximum landmass. supercontinents that actually existed. ily need 100 percent of the pieces before you Earth’s constantly shifting jigsaw puzzle of con­ can look at it and say it’s the Mona Lisa,” says 200 million years ago: tinents and oceans traces back to the fundamental Brendan Murphy, a geophysicist at St. Francis Pangaea begins to forces of plate tectonics. The story begins in the Xavier University in Antigonish, Nova Scotia. break apart, creating the Atlantic Ocean. centers of oceans, where hot molten rock wells “But you need some key pieces.” He adds: “With up from deep inside the Earth along underwater the eyes and nose, you have a chance.” mountain chains. The lava cools and solidifies into Next big event: About 250 million newborn ocean crust, which moves continually No place like Nuna years from now: away from either side of the mountain ridge as if For ancient Nuna, scientists are starting to find North America and carried outward on a conveyor belt. Eventually, the first of those key pieces. They may not reveal Asia will smash together with other the moving ocean crust bumps into a , the Mona Lisa’s enigmatic smile, but they are at land fragments to where it either stalls or begins diving beneath that least starting to fill in a portrait of a long-vanished create the supercon- continental crust in a process called subduction. supercontinent. tinent Amasia.

Those competing forces — pushing newborn Nuna came together starting around 2 billion SOURCES: D. EVANS; J. MEERT; Z.-X. LI, D.A.D. EVANS, J.B. MURPHY/ crust away from the mid-ocean mountains years ago, with its heart a mash-up of Baltica SUPERCONTINENT CYCLES THROUGH and pulling older crust down through subduc­ (the landmass that today contains Scandinavia), EARTH HISTORY 2016; M. YOSHIDA/ GEOLOGY 2016 tion — are constantly rearranging Earth’s crustal Laurentia (which is now much of North America) plates. That’s why North America and are and Siberia. Geologists argue over many things getting farther away from each other by a few cen­ involving this first supercontinent, starting with timeters each year as the Atlantic widens, and why its name. “Nuna” is from the Inuktitut language of

www.sciencenews.org | January 21, 2017 19 FEATURE | SUPERCONTINENT SUPERPUZZLE

the Arctic. It means lands bordering the northern we can say Congo could have been there.” Like oceans, so dubbed for the supercontinent’s Arctic- building out a jigsaw puzzle from its center, the fringing components. But some researchers prefer work essentially expands Nuna’s core. to call it Columbia after the Columbia of North America’s Pacific Northwest. Rodinia’s radioactive decay Whatever its moniker, Nuna/Columbia is an By around 1.3 billion years ago, Nuna was breaking exercise in trying to get all the puzzle pieces to apart, the pieces of the Mona Lisa face shattering fit. Because Nuna existed so long ago, subduction and drifting away from each other. It took another has recycled many rocks of that age back into the 200 million years before they rejoined in the con­ deep Earth, erasing any record of what they were figuration known as Rodinia. doing at the time. Geologists travel to rocks that Recent research suggests that Rodinia may not remain in places like India, South America and have looked much different than Nuna, though. “As soon as you North China, analyzing them for clues to where The Mona Lisa in its second incarnation may still they were at the time of Nuna. have looked like the portrait of a woman — just start asking One of the most promising techniques targets maybe with a set of earrings dangling from her why [Pangaea] magnetic minerals. Paleomagnetic studies use the lobes. formed, how minerals as tiny, time capsule compasses, which A team led by geologist Richard Ernst of Carleton it formed and recorded the direction of the magnetic field at the University in Ottawa, Canada, recently explored time the rocks formed. The minerals can reveal the relative positions of Laurentia and Siberia what processes information about where those rocks used to between 1.9 billion and 720 million years ago, a are involved ... be, including their latitude relative to where the period that spans both Nuna and Rodinia. Ernst’s you run into Earth’s north magnetic pole was at the time. group specializes in studying “large igneous prov­ problems.” Salminen has been gathering paleomagnetic inces” — the huge outpourings of lava that build up data from Nuna-age rocks in Brazil and western over millions of years. Often the molten rock flows BRENDAN MURPHY Africa. Not surprisingly, given their current lock- along sheetlike structures known as dikes, which and-key configuration, these two chunks were funnel magma from deep in the Earth upward. once united as a single ancient continental block, By using the radioactive decay of elements in known as the Congo/São Francisco craton. For the dike rock, such as uranium decaying to lead, millions of years, it shuffled around as a single scientists can precisely date when a dike formed. geologic unit, occasionally merging with other With enough dates on a particular dike, research­ blocks and then later splitting away. ers can produce a sort of bar code that is unique to Salminen has now figured out where the Congo/ each dike. Later, when the dikes are broken apart São Francisco puzzle piece fit in the jigsaw that and shifted over time, geologists can pinpoint the made up Nuna. In 1.5-billion-year-old rocks in bar codes that match and thus line up parts of the Brazil, she unearthed magnetic clues that placed crust that used to be together. the Congo/São Francisco craton at the southeast­ Ernst’s team found that dikes from Laurentia ern tip of Baltica all those years ago. She and her and Siberia matched during four periods between colleagues reported the findings in November in 1.87 billion and 720 million years ago — suggest­ Precambrian Research. ing they were connected for that entire span, the It is the first time scientists have gotten paleo­ team reported in June in Nature Geoscience. Such magnetic information about where the craton a long-term relationship suggests that Siberia and may have been as far back as Nuna. “This is quite Laurentia may have stuck together through the remarkable — it was really needed,” she says. “Now Nuna-Rodinia transition, Ernst says. Other parts of the puzzle tend to end up in the Subduction’s drag Supercontinents as- same relative locations as well, says Joseph Meert, semble (top) as pieces of continental crust a paleomagnetist at the University of Florida in continental crust amass. Gainesville. In each supercontinent, Laurentia, Later they may break oceanic oceanic apart (bottom) when crust mantle crust Siberia and Baltica knit themselves together slabs of plunging oceanic in roughly the same arrangement: Siberia and crust (light blue) begin rift zones Baltica nestle like two opposing knobs on one end SCIENCE LETTERS 2016 AND PLANETARY EARTH pulling the continent from either side, creating continental crust under tension of Laurentia’s elongated blob. Meert calls these tensional forces that rift three continental fragments “strange attractors,” it apart. oceanic oceanic since they appear conjoined time after time. crust mantle crust

It’s the outer edges of the jigsaw puzzle that ET AL / CAWOOD P.A.

20 SCIENCE NEWS | January 21, 2017 change. Fragments like north China and south­ happened as Rodinia began to break apart. ern Africa end up in different locations around the They note that crustal plates began subduct­ supercontinent core. “I call those bits the lonely ing around the edges of Rodinia right around the wanderers,” Meert says. time of its breakup. That sucking down of crust caused the supercontinent to be pulled from all Getting to know Pangaea directions and eventually break apart, Cawood While some puzzle-makers try to sort out the and his colleagues wrote in Earth and Planetary reconstructions of past supercontinents, other Science Letters in September. “The timing of major geologists are exploring deeper questions about breakup corresponds with this timing of opposing why big landmasses come together in the first subduction zones,” he says. place. And one place to look is Pangaea. Volcanic features called “Most people would accept what Pangaea looks The future is Amasia dikes form when molten like,” Murphy says. “But as soon as you start ask­ That stressful situation is similar to what the rock squirts into a crack and cools to form a long ing why it formed, how it formed and what pro­ Pacific Ocean finds itself in today. Because it is linear feature. Matching cesses are involved — then all of a sudden you run flanked by subduction zones around the Ring of dikes of the same age and into problems.” Fire, the Pacific Plate is shrinking over time. Some chemical composition across continents can Around 550 million years ago, subduction zones geologists predict that it will vanish entirely in the help geologists recon- around the edges of an ancient ocean began drag­ future, leaving North America and Asia to merge struct maps of long-lost ging that oceanic crust under continental crust. into the next supercontinent, Amasia. Others have supercontinents. The dike above, at Singhbhum But around 400 million years ago, that subduction devised different possible paths to Amasia, such as craton near Keonjhar, suddenly stopped. In a major shift, a different, closing the Arctic Ocean rather than the Pacific. India, is about 1.7 billion much younger seafloor began to subduct instead “Speculation about the future supercontinent years old. beneath the continents. That young ocean crust Amasia is exactly that, speculation,” says geolo­ kept getting sucked up until it all disappeared, and gist Ross Mitchell of Curtin University in Perth, the continents were left merged together in the Australia, who in 2012 helped describe the giant mass of Pangaea. mechanics of how Amasia might arise. “But there’s Imagine in today’s world, if the Pacific stopped hard science behind the conjecture.” shrinking and all of a sudden the Atlantic started For instance, Masaki Yoshida of the Japan shrinking instead. “That’s quite a significant Agency for Marine-Earth Science and Technology problem,” Murphy says. In unpublished work, he in Yokosuka, recently used sophisticated com­ has been exploring the physics of how plates of puter models to analyze how today’s continents oceanic and continental crust — which have dif­ would continue to move atop the flowing heat of ferent densities, buoyancies and other physical the deep Earth. He combined modern-day plate characteristics — could have interacted with one motions with information on how that internal another in the run-up to Pangaea. planetary heat churns in three dimensions, then Supercontinent breakups are similarly com­ ran the whole scenario into the future. In a paper plicated. Once all the land amasses in a single in the September Geology, Yoshida describes how big chunk, it cannot stay together forever. In one North America, , Australia and Africa will scenario, its sheer bulk acts as an electric blan­ end up merged in the Northern Hemisphere. ket, allowing heat from the deep Earth to pond up No matter where the continents are headed, beneath it until things get too hot and the super­ they are destined to reassemble. Plate tectonics continent splinters (SN 1/21/17, p. 14). In another, says it will happen — and a new supercontinent physical stressors pull the supercontinent apart. will shape the face of the Earth. It might not look Peter Cawood, a geologist at the University of like the Mona Lisa, but it might just be another St. Andrews in Fife, Scotland, likes the second masterpiece. s option. He has been studying mountain ranges that arose when the crustal plates that made up Explore more Rodinia collided, pushing up soaring peaks where ss Z.-X. Li, D.A.D. Evans and J.B. Murphy (eds). they met. These include the Grenville mountain- Supercontinent Cycles through Earth History. building event of about 1 billion years ago, traces Geological Society of London Special of which linger today in the eroded peaks of the Publication, Vol. 424, 2016. Appalachians. Cawood and his colleagues ana­ ss UNESCO International Geoscience Programme. lyzed the times at which such mountains appeared Supercontinent Cycles & Global Geodynamics.

COURTESY OF J. MEERT OF J. COURTESY and put together a detailed timeline of what http://bit.ly/supercontinents

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