More than a billion years of geological history is missing from the walls of the Grand Canyon, and a new model of Earth's mantle may explain why, Live Science reported.
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 stretch in between was not preserved, a gap known as the Great Unconformity. That missing interval came just before and during the Cambrian explosion, when life on Earth diversified rapidly. Similar gaps from that time appear across North America, the Baltic region of Europe and northern China. Researchers have long debated the cause.
A new model of mantle circulation, published Nov. 4 in the journal Nature by S. Dhungana and Nicolas Flament, points to slow, heat-driven flow that pushed those three regions upward while the unconformity was forming. The uplift would have encouraged erosion, so sediments from that period sloughed off the continents and into the ocean. The same model finds that in the past 540 million years those areas started sinking instead of rising, creating space for sediments to pile up in neat layers — the point where the unconformity ends, matching the rock record. "We looked at, where are the best places that the mantle would help create a Great Unconformity?" 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."
Snowball Earth, and a stage for complex life
Some researchers have argued that a global deep freeze known as Snowball Earth could have shoveled sediment off the continents. Glaciation probably played a major role in erosion, Flament said, but the new study suggests the key driver was deep in the planet: without the mantle lifting the continents, glaciers and other forces would have had no slope to push sediments down.
Shanan Peters, a geologist at the University of Wisconsin-Madison who was not involved in the study, told Live Science that calling the gap "missing information" is not quite right. The absence itself reveals what Earth was doing, and may even have helped set the stage for the Cambrian explosion. Crystalline rocks exposed at the surface may have released potassium, iron, manganese and carbon into the seas at a higher rate, he said, prompting marine life to invent biomineralization — shells — and possibly helping make the atmosphere more oxygen-rich. "It contributes to the environment in which complex life evolved," Flament said. Peters added that the paper is valuable because it explains both the era of erosion and the later return to deposition. Flament said he hopes a global rock-layer database led by Peters, called Macrostrat, will help test the model in places such as Africa and Russia, where the record is less complete.