When the famed explorer John Wesley Powell bumped, splashed and thrashed his way down the Colorado River in 1869, he discovered one of the most striking geologic features on Earth. Not the Grand Canyon—although that too is a marvel—but a conspicuous boundary between the sunset-colored sediments of the upper walls and the dark, jagged rocks below them.
Powell had learned to read the layers of desert rocks like pages in a book, and he recognized that the boundary represented a missing chapter in Earth's geological history. Later, researchers realized it was more like an entire lost volume, spanning roughly one-fifth of Earth's existence, and that a similar gap existed in many places around the world.
"There must have been some sort of special event in Earth's history that led to widespread erosion," said Steve Marshak, a geologist at the University of Illinois at Urbana-Champaign who studies what has come to be known as the Great Unconformity.
New research suggests it was something special indeed. Scientists propose that several freak episodes of global glaciation scoured away miles of continental crust, obliterating a billion years of geologic history in the process.
The idea was first proposed back in 1973 by a geologist named William White, but no one took him seriously, said C. Brenhin Keller of the Berkeley Geochronology Center, who led the study published Monday in the Proceedings of the National Academy of Sciences.
"It's an extraordinary claim that requires extraordinary evidence," Keller said.
Today, however, researchers have come to accept the outlandish notion that, a few times in its 4.6 billion-year history, the planet froze over and became a "Snowball Earth." Now Keller and his colleagues hope to convince their peers that the glaciers that crawled across the continents between 720 million and 580 million years ago were responsible for the Great Unconformity.
Since there are so few rocks from that period, the researchers had to look for other kinds of clues to figure out what happened. They reasoned that the missing layers probably went through the full geologic spin cycle: They would have been broken down into sediment and washed out to sea, then deposited on the ocean floor and recycled into the mantle during subduction before finally melting into the magma that feeds volcanoes.
If so, a record of this activity should hide in tiny time capsules called zircons. These indestructible crystals grow in magma, and they contain the elements oxygen and hafnium. Oceanic and continental crust have distinct signatures of these elements. Therefore, a huge spike in the amount of recycled continental material should have left a clear chemical signal in zircons that formed at that time—and it did.
Keller's team found stark variations in the oxygen and hafnium in zircons, consistent with the continents losing an average of 2 to 3 vertical miles of rock.
"We are talking about an absolutely a huge amount of crust being eroded," he said. "In which case, we should have noticed it missing—and we have."
The zircons also show that the amount of rock getting recycled ramped up just as Snowball Earth set in, suggesting the two events were connected. If so, this explanation solves the long-standing riddle of why erosion increased so dramatically in so many places at the same time.
Usually, rocks start to break down in a particular region when a mountain range gets muscled into existence by plate tectonics. But it's hard to imagine that happening simultaneously on all the continents, Keller said. "Glaciation"—during Snowball Earth, at least—"would apply everywhere."
The researchers offer other data to support their hypothesis. They point to the dearth of meteor craters dating back before 700 million years, suggesting these dents got Brillo-ed out in the making of the Great Unconformity. The researchers also highlight the close agreement between their results and records of ancient sea levels, which would have changed as the continents lost rock and bobbed higher in the mantle, the way an unloaded ship rides higher in the water.
"It's a really cool and provocative idea," said Galen Halverson, a geologist at McGill University in Montreal who was not involved in the work.
Halverson said the researchers have clearly uncovered evidence that something wacky happened at that time in Earth's history. However, he said other scientists may quibble over whether it really corresponds to the Great Unconformity and whether global glaciation was to blame.
For instance, it's not clear that the ice sheets of Snowball Earth could have eaten away so much crust. "If we look at the actual geological record that we have of these glaciations, the sedimentation rates appear to be very, very slow," he said.
Marshak agrees that there could be a connection between Snowball Earth and the Great Unconformity but said questions remain about the order of events. He and several colleagues published a study last year suggesting that widespread erosion—caused by the breakup of the supercontinent Rodinia—triggered chemical reactions that drew down atmospheric carbon dioxide, cooling the planet and plunging it into a Snowball Earth state.
Researchers will have to nail down the timing of erosion and glaciation to see which came first, Marshak said. And, he added, it's possible that both ideas are correct and multiple forces conspired to create this strange moment in Earth history.
"It doesn't mean that it has to be one or the other," he said.
What's clear is that two unusual events occurred around the same time, and a third soon followed: the evolution of complex life, which exploded roughly 540 million years ago.
Around the world, scientists have noticed that the rocks above the Great Unconformity contain obvious fossils, like trilobites and brachiopods, while those below do not. Many early researchers figured those older fossils had just disappeared with the lost rocks, but now scientists know that no such creatures existed back then. That may be more than a coincidence.
The Great Unconformity may have set the stage for this transition by providing vast shallow seas where marine life could flourish. And Snowball Earth appears to have bulldozed loads of critical nutrients into the ocean that nourished the diversification of life.
"It's just really mixing up the pot in terms of what is possible and what is available," Keller said.
It also goes to show that the planet doesn't always change slowly and gradually, Marshak said. "There are punctuation points in Earth's history where things change in new directions," he said.
And from that capriciousness came the world—and perhaps life—as we know it.