In the canyon's sides, ancient metamorphic and igneous rocks that formed 1.7 billion years ago sit directly under younger sedimentary rocks laid down 500 million years ago. The 1.2 billion-year chunk of history in between wasn't preserved — leaving a geological gap known as the Great Unconformity.
The unconformity is sharply evident in the transition between Cambrian Tapeats Sandstone and Proterozoic Brahma Schist in the Grand Canyon. (Image credit: UCG via Getty Images)
Now, a new model of the movement of Earth's middle layer, the mantle, has revealed an explanation: The slow, heat-driven circulation of the mantle pushed these three regions upward during the time when the Great Unconformity was formed, according to new research published Nov. 4 in the journal Nature.
"We looked at, where are the best places that the mantle would help create a Great Unconformity?" study co-author Nicolas Flament, a geoscientist at the University of Wollongong in Australia, told Live Science. "And these three places turned out to be three places where it's known to be quite extensive."
Erosion during Neoproterozoic times, followed by sedimentation during early Phanerozoic times, formed the Great Unconformity. Bottom: Dynamic uplift was followed by subsidence for Laurentia ('LAU'), Baltica ('BAL') and North China ('NCH'). (Image credit: Author provided, CC BY-ND)Constant flux
Earth is in a constant state of recycling crust through plate tectonics, said Shanan Peters, a geologist at the University of Wisconsin-Madison who was not involved in the study. Oceanic crust sinks back into the mantle through the process of subduction — when one tectonic plate dives beneath another. The chemicals it carries down with it get released back into the atmosphere via volcanic eruptions, linking the mantle with the atmosphere.
It contributes to the environment in which complex life evolved.
Today, in contrast, most of Earth's subduction is around the Pacific Rim, and much of the Pacific seafloor is remote from continents, the seafloor is relatively starved of carbon. What's getting pushed into the mantle now is therefore a different chemical composition than what existed 200 million years ago.
The geological processes occurring during the time of the Great Unconformity may have set the stage for the Cambrian explosion, Peters said. For example, the crystalline rocks exposed on the surface at that time may have released elements like potassium, iron, manganese and carbon into the seas at an increased rate, prompting marine life to invent biomineralization — shells — in response and possibly shifting the composition of Earth's atmosphere to be more oxygen-rich, Peters said.
What's particularly interesting about the Great Unconformity is how it reveals not just a period of erosion but also a switch back to the deposition of sediments. The new paper provides an explanation for both, Peters said, and changes in the mantle are just the planetary-scale process to explain this big-picture phenomenon.
Related stories"This seems to me to be the only reasonable way to go back and forth between this state over time," he said.
"So far in Africa and Russia, we don’t have as comprehensive a database," he said. "Once that is done, we can go and check whether the idea works."
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