Around the world enormous chunks of the past have vanished, a phenomenon known as the “Great Unconformity”. At geologic sites where this has occurred, rocks formed relatively recently (by planetary standards) sit immediately above much, much older deposits with nothing in between. Last year scientists explained this, pinning the blame on “Snowball Earth”, but new evidence contradicts this claim.
The Grand Canyon is a geologist's paradise, with layer after layer of rocks forming a sort of time machine making the descent a passage into the past. Certain points are marked by unconformities, where there is a sudden jump, either because no new material was laid down or because some process stripped away the intervening rocks before the younger ones were deposited on top.
More than a century ago geologists noticed that across North America rocks around 550 million years old sit upon much older material – in some cases up to 3 billion years old. The phenomenon was dubbed The Great Unconformity, and appears to be global. The period before 550 million years ago is not missing everywhere, but is much rarer than would be expected.
Last year geologists attributed this absence to the period when the planet almost entirely froze over, known as Snowball Earth. The growth and then decline of glaciers scoured the rocks beneath, eventually causing most to be washed to the sea, they claimed, taking with them around 80 percent of the Earth's record of the era, a sort of geologic amnesia.
Dr Rebecca Flowers of the University of Colorado has challenged this idea in Proceedings of the National Academy of Sciences. Instead of a single Great Unconformity, Flowers claims, there were many localized unconformities, close enough in time to confuse geologists, but sufficiently separate they could not have all been caused by a single global event.
Flowers bases her claim on measurements at the base of Pikes Peak, Colorado, where older rocks were brought to the surface and experienced intense erosion. Flowers dates this as occurring more than 717 million years ago.
This makes the erosion that produced the unconformity too old to have been the product of Snowball Earth. Instead, Flowers thinks the cause may be a collision between the supercontinent Rodinia and neighboring tectonic plates, or the subsequent break up of Rodinia. Continental collisions cause mountain belts, just as India plowing into Asia has raised the Himalayas, followed by rapid erosion. Similar events probably occurred elsewhere, just close enough in time to create the illusion of being simultaneous to our eyes.
"Researchers have long seen this as a fundamental boundary in geologic history," Flowers said in a statement. "There is a lot of the geological record that is missing, but just because it's missing doesn't mean that this history is simple."