by Adityarup "Rup" Chakravorty Tuesday, August 16, 2016
Coral reefs at Cap Miné near the northern end of Madagascar attest to uplift in the area caused by underlying mantle upwelling: While a living reef forms underwater today, the remnants of two former reefs — one upon which the lighthouse sits and one capping the hill at left — were raised out of the water. Credit: Mark Hoggard.
From the earthquake-prone Himalayas and Andes to the volcanically active “Ring of Fire” around the Pacific, the boundaries of Earth’s tectonic plates are often sites of considerable geologic activity. In contrast, the interiors of tectonic plates have been thought to be relatively rigid and quiet.
But these “quieter interiors of tectonic plates are way more active than we previously realized,” says Mark Hoggard, a geologist at the University of Cambridge in England and lead author of a new study in Nature Geoscience investigating the rise and fall of plate interiors induced by Earth’s mantle.
Convection of the mantle — powered by the escape of heat from the planet’s interior — is known to drive tectonic activity in parts of the surface far from plate boundaries. Above areas where relatively warm mantle upwells toward Earth’s surface, “the surface can get pushed upward,” Hoggard says, “and it can get pulled downward over sinking mantle currents.” What’s surprising from the findings of the new study is how often the surface appears to be pushed up or pulled down.
Hoggard and his colleagues used a database of more than 1,100 seismic reflection profiles and other seismic data to determine the depth and structure of Earth’s crust at locations around the world. Then they used a computer model to look for potential correlations between these traits and mantle convection.
Analysis of this detailed dataset showed that Earth’s crust was often higher or lower than expected, even when the researchers accounted for varying sediment loads, glacial rebound and other factors that affect crustal elevations. For example, the crust on the west coast of India appeared to have been pushed up by about 500 meters, whereas off the country’s eastern coast it was almost a kilometer lower than expected.
Older models had predicted that mantle convection could cause the surface to rise or sink by a maximum of 2 kilometers over distances of roughly 10,000 kilometers, or about two and a half times the distance between Los Angeles and New York. Such bumps or troughs would barely be discernible because the relatively small elevation changes would be spread over massive distances.
In contrast, Hoggard and his colleagues found that the peaks and troughs in surface elevation caused by mantle convection were separated by only about 1,000 kilometers. “If these spatial variations occur at distances of only 1,000 kilometers, it means that … over millions of years, any point on [Earth’s] surface is likely to be bobbing up and down much quicker than the predictive models would suggest,” Hoggard says.
But the cause of the unexpected mismatch between the new observations and previous models is difficult to unravel. Hoggard says he and his colleagues suspect it is due to “a lot more convection in the shallow parts of the mantle than people had previously predicted.”
Up-and-down movement of the surface can have massive global impacts. “It could influence where sediments are being eroded and deposited, which is important for the hydrocarbon industry; the depth of the oceans and therefore where oceanic currents flow or get cut off; and the stability of ice sheets, which has big effects on the past and present climate of Earth,” Hoggard says.
Hans-Peter Bunge, a geophysicist at Ludwig Maximilian University of Munich in Germany who was not involved in the new study, agrees that mantle convection is likely the driving force behind the frequent ups and downs observed by Hoggard and his colleagues, but, he says, “the conclusions of the new study will have to be tested with computer models of global mantle flow” to further assess their validity.
In the future, Hoggard says he plans to expand this study of present conditions back into the past. “We need to add more geological observations to quantify exactly how fast the surface is bobbing up and down through time at any given location,” he says.
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