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A Cool Early Earth? the Textbook View That the Earth Spent Its First Half a Billion Years Drenched in Magma Could Be Wrong

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A Cool Early Earth? the Textbook View That the Earth Spent Its First Half a Billion Years Drenched in Magma Could Be Wrong NEW VIEW of the young earth covered in oceans of liquid water as early as 4.4 billion years ago contrasts sharply with the hot, hostile world typically depicted in textbooks. 58 SCIENTIFIC AMERICAN COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. A Cool Early Earth? The textbook view that the earth spent its first half a billion years drenched in magma could be wrong. The surface may have cooled quickly—with oceans, nascent continents and the opportunity for life to form much earlier By John W. Valley SCIENTIFIC AMERICAN 59 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. In its infancy, beginning about 4.5 billion years ago, the earth glowed like a faint star. Incan- ed for as long as 500 million years, an hardened over, the thickening layer of descent yellow-orange oceans of magma era thus named the Hadean. Major sup- consolidated rock would have insulated roiled the surface following repeated col- port for this view comes from the appar- the exterior from the high temperatures lisions with immense boulders, some the ent absence of any intact rocks older deep within the interior. If there were size of small planets, orbiting the new- than four billion years—and from the suitably quiescent periods between ma- ly formed sun. Averaging 75 times the fi rst fossilized signs of life, which are jor meteorite impacts, if the crust was speed of sound, each impactor scorched much younger still. stable, and if the early hothouse atmo- the surface—shattering, melting and In the past fi ve years, however, ge- sphere did not trap too much heat, sur- even vaporizing on contact. ologists—including my group at the face temperatures could have quickly Early on, dense iron sank out of the University of Wisconsin–Madison— fallen below the boiling point of water. magma oceans to form the metallic have discovered dozens of ancient crys- Furthermore, the primitive sun was core, liberating enough tals of the mineral zircon with chemical fainter and contributed less energy. gravitational energy compositions that are changing our Still, for most geologists, an undis- to melt the entire pla n- thinking about the earth’s beginnings. puted fi ery birth and scant clues in the et. Massive meteorite The unusual properties of these durable geologic record seemed to point instead strikes continued for minerals—each the size of the period in to a prolonged ultrahot climate. The old- hundreds of millions of this sentence— enable the crystals to est known intact rock is the four-billion- years, some blasting preserve surprisingly robust clues about year-old Acasta gneiss in Canada’s craters more than 1,000 what the environment was like when Northwest Territories. This rock formed kilometers in diameter. they formed. These tiny time capsules deep underground and bears no infor- At the same time, deep bear evidence that oceans habitable to mation about conditions on the surface. OLD VIEW of underground, the de- primitive life and perhaps continents Most investigators assumed hellish con- a hot young cay of radioactive ele- could have appeared 400 million years ditions at the planet’s surface must have earth: Life ments produced heat at earlier than generally thought. obliterated any rocks that formed earli- magazine, rates more than six er. The oldest rocks known to have orig- December 8, 1952. times greater than they Cooling Down inated underwater (and thus in relatively do today. since the 19th century, scien- cool environs) did not form until 3.8 bil- These fi ery conditions had to subside tists have attempted to calculate how lion years ago. Those sediments, which before molten rock could harden into a quickly the earth cooled, but few expect- are exposed at Isua in southwestern crust, before continents could form, be- ed to fi nd solid evidence. Although mag- Greenland, also contain the earliest evi- fore the dense, steamy atmosphere could ma oceans initially glowed at tempera- dence of life [see “Questioning the Old- ) pool as liquid water, and before the tures exceeding 1,000 degrees Celsius, a est Signs of Life,” by Sarah Simpson; earth’s fi rst primitive life could evolve tantalizing suggestion of a more temper- Scientifi c American, April 2003]. this page and survive. But just how quickly did ate early earth came from thermody- Single crystals of zircon began to add the surface of the earth cool after its lu- namic calculations showing that crust new information about the early earth in minous birth? Most scientists have as- could have solidified on the surface the 1980s, when a few rare grains from sumed that the hellish environment last- within 10 million years. As the planet the Jack Hills and Mount Narryer re- gions of Western Australia became the Zircon Time Capsules most ancient terrestrial material known Overview/ at that time—the oldest dating back al- ■ Geologists have long thought that the fi ery conditions of our planet’s most 4.3 billion years. But the informa- birth 4.5 billion years ago gave way to a more hospitable climate by about tion these zircons carried seemed am- ( PICTURES/GETTY IMAGES LIFE TIME ); 3.8 billion years ago. biguous, in part because geologists were ■ Now tiny crystals of the mineral zircon, which retain clear evidence of when unsure of the identity of their parent and how they formed, suggest that the earth cooled far sooner—perhaps as rock. Once formed, zircon crystals are early as 4.4 billion years ago. so durable that they can persist even if pages preceding ■ Some ancient zircons even bear chemical compositions inherited from the their parent rock is exposed at the sur- cooler, wet surroundings necessary for life to evolve. face and destroyed by weathering and erosion. Wind or water can then trans- ( DIXON DON 60 SCIENTIFIC AMERICAN OCTOBER 2005 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. OLDEST PIECES OF THE PLANET Isua sediments (oldest evidence for life, 3.8 billion years old) Acasta Gneiss Fossilized gravel bed in the Jack Hills (oldest intact rock, (above) contained the world’s oldest zir- 4 billion years old) cons yet discovered. Geologists crushed and sorted hundreds of kilograms of this below Rocks older than rock ( ) to fi nd the 20 crystals that 2.5 billion years bear signs of cool conditions more than four billion years ago. Inferred Jack Hills zircons Exposed (oldest earth material, 4.4 billion years old) Ancient rocks older than 2.5 billion years crop out or lie just underneath the soil in many spots around the globe (red) and are probably hidden below younger rocks across even broader regions (pink). Zircon crystals as old as those discovered in the Jack Hills of Western Australia may eventually be discovered at another of these locations. port the surviving grains great distances The Jack Hills conglomerate was depos- to lead. When a zircon forms from a so- ) before they become incorporated into ited three billion years ago and marks lidifying magma, atoms of zirconium, deposits of sand and gravel that may the northwestern edge of a widespread silicon and oxygen combine in exact later solidify into sedimentary rock. In- assembly of rock formations that are all proportions (ZrSiO4) to create a crystal deed, the Jack Hills zircons—separated older than 2.6 billion years. To recover structure unique to zircon; uranium oc- photographs by perhaps thousands of kilometers from less than a thimbleful of zircons, my col- casionally substitutes as a trace impu- their source—were found embedded in a leagues and I collected hundreds of kilo- rity. Atoms of lead, on the other hand, fossilized gravel bar called the Jack Hills grams of rock from these remote out- are too large to comfortably replace any conglomerate. crops and hauled them back to the labo- of the elements in the lattice, so zircons ); JOHN W. VALLEY ( So, despite the excitement of fi nding ratory for crushing and sorting, similar start out virtually lead-free. The urani- map ( such primeval pieces of the earth, most to searching for a few special grains of um-lead clock starts ticking as soon as scientists, including me, continued to sand on a beach. the zircon crystallizes. Thus, the ratio of accept the view that the climate of our Once extracted from their source lead to uranium increases with the age young planet was Hadean. It was not rock, individual crystals could be dated of the crystal. Scientists can reliably de- until 1999 that technological advances because zircons make ideal timekeep- termine the age of an undamaged zircon Colgate University University Colgate allowed further study of the ancient zir- ers. In addition to their longevity, they within 1 percent accuracy, which for the con crystals from Western Australia— contain trace amounts of radioactive early earth is about plus or minus 40 and challenged conventional wisdom uranium, which decays at a known rate million years. about the earth’s earliest history. JOHN W. VALLEY received his Ph.D. in 1980 from the University of Michigan at Ann Arbor, Digging Deep where he fi rst became interested in the early earth. He and his students have since ex- the australian zircons did not plored the ancient rock record throughout North America and in Western Australia, give up their secrets easily. For one thing, Greenland and Scotland. Currently Valley is president of the Mineralogical Society of the Jack Hills and their surroundings are America and Charles R. Van Hise Professor of Geology at the University of Wisconsin– dusty barrens at the edge of vast sheep THE AUTHOR Madison, where he founded a multimillion-dollar laboratory called WiscSIMS. The cut- stations, called Berringarra and Mileu- ting-edge capabilities of the lab’s new CAMECA IMS 1280 ion microprobe will enable a ra, situated some 800 kilometers north diverse range of research; besides zircons, Valley and his colleagues will probe many LUCY READING-IKKANDA; SOURCE: WILLIAM PECK H.
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