By most accounts, the birth of continents was a violent affair. Lighter minerals floated to the top of the hot, turbulent mantle, forming volcanic islands in the primeval ocean. But these bits of protocrust did not just sit there: they were slammed together again and again by the churning mantle and often plunged back into it. With so much remodeling and recycling going on, recognizable remains of early continental crust are scarce. That’s why geophysicist Roger Buick was so excited when he came upon a wedge of continental crust in northwestern Australia, hundreds or even thousands of square miles in extent, that had barely been disturbed-- and that turned out to be three and a half billion years old. It’s a very old bit of the Earth, and it just happens to have sat there for three and a half billion years while nothing happened to it, says Buick, who works at the University of Sydney. Here we have the very deepest time we can access through the geologic record--about three-quarters of the way back to the beginning.
While the arrangement of continents continues to shift, the actual volume of continental crust, many geologists believe, has been about the same for the past two and a half billion years. Before then there was a burst of continent formation: volcanic island arcs that had been plastered together received a rapid infusion of molten granite. As the granite rose up through fissures in the crust and solidified, it thickened and stabilized the crust and made it more buoyant. It’s the infusion of all that granite that turns island arcs into continental crust, explains Buick. Granite gives them the permanent buoyancy that keeps them above sea level.
What Buick’s find in the geologic province known as the Pilbara craton reveals, though, is that this whole process started at least a billion years earlier than had been thought. While doing some mapping work for a mining company, Buick and a colleague stumbled across a boundary between two rock layers that were not parallel. We went up and looked at the boundary, says Buick, and within five minutes we realized how significant it was. The researchers recognized the boundary as an unconformity: an ancient land surface that had been eroded, tilted, and then at some later time buried under a deposit of new sedimentary and volcanic rock. They also saw that the older rock below the unconformity was loaded with granite.
Buick knew the Pilbara craton was ancient terrain. But he didn’t know just how old his ancient land surface was until he extracted zircon crystals from the rock and precisely measured the amount of uranium and lead they contained. Using the known rate at which uranium decays radioactively into lead, he could calculate when the rock had crystallized from the molten state. The granitoid layer below the unconformity turned out to be 3.52 billion years old. The rocks above had been deposited some 50 or 60 million years later.
The rock layers had been tilted and turned on one side--which is what exposed the unconformity to Buick’s view--but otherwise they were mostly undisturbed. The younger rocks above the boundary were especially pristine: they were riddled with fragments of volcanic pumice whose pores hadn’t been squashed at all. You can look at the minerals in the pumice-- some of which are very sensitive to heat--and see that they have never been subjected to temperatures of more than 500 to 600 degrees Fahrenheit, says Buick. That’s a level of heating that would only occur very near the surface; they’ve never been deeply buried. Indeed, whereas the rocks well below the unconformity were deposited on the deep ocean floor, the rocks above it, judging from their mineral composition and from ripples in them that look like ripples on a beach, were deposited in shallow water or maybe no water at all--that is, on an emerging continent.
Buick believes he can even see a pale layer of fossilized soil, or paleosol, peeping out from between the layers of rock covering his unconformity. The soil, if that’s what it proves to be, would have accumulated on the 3.52-billion-year-old continental surface in the 60 million years before rock was deposited on top. It would be proof that the ancient surface had indeed been exposed to the air; soil, at least the inorganic kind, is made when atmospheric gases dissolved in rainwater attack exposed rock. And a 3.5-billion-year-old paleosol could help answer fundamental questions about the composition of the ancient atmosphere. For instance, it might reveal which greenhouse gas was most responsible for warming the planet at a time when, astronomers tell us, the sun was much fainter than it is today--and yet Earth was warm enough to have a liquid ocean.
There has been much debate on that question, but the debate has been largely unconstrained by direct evidence. Buick’s find could change that. Both carbon dioxide and methane in very high concentrations could have produced the early greenhouse effect, but according to work by Harvard geologist Heinrich Holland, CO2 would have reacted with iron-rich rocks to produce a mineral called siderite. If Buick does find fossilized soil, he and Holland will be looking for siderite in it. The thing that’s most interesting is, what if it’s not CO2? says Buick. The people who think about the origin of life would like a methane-rich atmosphere, since you can make the basic chemicals of life a lot more easily in a methane-rich environment.
As it happens, the oldest fossil cells--the oldest solid evidence of life on Earth--were found a decade ago in the very rock formation that lies above Buick’s unconformity, the Warrawoona Group, and just a dozen miles away from his main research site. The diversity of those 3.5-billion- year-old fossils suggests that life began even earlier. Along with geochemist David Des Marais of the NASA Ames Research Center, Buick plans to search for evidence of that earlier life in the rocks below the unconformity; they’ll be looking for the distinctive ratio of carbon isotopes that photosynthesizing organisms leave behind. Des Marais thinks there’s a good chance they’ll find something. Buick has found some of the best-preserved stuff anyone has come up with, he says. He’s done an amazing thing here that I don’t think will be paralleled for quite a while. It sets the stage for a number of important discoveries that will come.
