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, but a conspicuous boundary between the sunset-colored sediments of the upper walls and the dark, jagged rocks below them.
Powell 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.
“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.
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.
The idea was proposed in 1973 by William White, but no one took him seriously, said C. Brenhin Keller of the Berkeley Geochronology Center, who led the study published in the Proceedings of the National Academy of Sciences. “It’s an extraordinary claim that requires extraordinary evidence.”
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 what is now known as the Great Unconformity.
Since there are so few rocks from that period, the researchers reasoned that the missing layers probably went through the full geologic spin cycle. They would have been broken down into sediment and washed to sea, deposited on the ocean floor and recycled into the mantle before melting into the magma that feeds volcanoes.
If so, a record of this activity should hide in zircons, indestructible crystals that grow in magma. Oceanic and continental crust have distinct signatures of oxygen and hafnium. Therefore, a huge spike in the amount of recycled continental material should have left a clear chemical signal in zircons formed at that time — and it did.
They found variations consistent with the continents losing an average of 2 to 3 vertical miles of rock. “We are talking about … a huge amount of crust being eroded,” Keller said.
The zircons also show that the amount of rock getting recycled ramped up just as Snowball Earth set in, suggesting the events were connected. If so, this would solve the long-standing riddle of why erosion increased so dramatically in so many places at the same time.
Usually, rocks start to break down 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.”