Scientists at the University of California at Berkeley have detected previously unknown channels of slow seismic waves in the Earth’s upper mantle, a discovery that helps explain the “hotspot volcanoes” that give rise to chains of islands like than Hawaii and Tahiti.
Unlike volcanoes that emerge from collision zones between tectonic plates, hot spot volcanoes form in the middle of the plates. The popular theory of the formation of a mid-plate volcano is that a single upwelling of hot, floating rock rises vertically in the form of a plume from the depths of the Earth’s mantle – the layer between the crust and the core of the planet – and provides heat to fuel volcanic eruptions.
However, some volcanic hotspot chains are not easily explained by this simple model, suggesting that a more complex interaction between the plumes and the upper mantle is at play, the study authors said.
The new slow seismic wave channels, described in an article published today (Thursday, September 5), in Science Express, provide an important piece of the puzzle in the formation of these hotspot volcanoes and other heat flux observations. unusually high. from the bottom of the ocean.
The formation of volcanoes on the edges of the plates is closely related to the movement of tectonic plates, which are created when hot magma rises through cracks in mid-ocean ridges and solidifies. As the plates move away from the ridges, they cool, harden and become heavier, eventually falling back into the mantle at the subduction zones.
But scientists have noticed large swathes of the seabed that are significantly warmer than expected from this tectonic plate cooling model. It had been suggested that the plumes responsible for the hotspot volcanism might also play a role in explaining these sightings, but it was not entirely clear how.
“We needed a clearer picture of where the extra heat comes from and how it behaves in the upper mantle,” said lead author of the study, Barbara Romanowicz, professor of Earth and Earth Sciences. planets at UC Berkeley and researcher at the Berkeley Seismological Laboratory. âOur new discovery helps bridge the gap between deep mantle processes and phenomena seen on the Earth’s surface, such as hot spots. “
The researchers used a new technique that takes waveform data from earthquakes around the world, then analyzed individual “ripples” in the seismograms to create a computer model of the Earth’s interior. The technology is like a scanner.
The model revealed channels – dubbed “low-speed fingers” by researchers – where the seismic waves moved unusually slowly. The fingers extended in bands approximately 600 miles wide and 1,200 miles apart, and moved to depths of 120 to 220 miles below the seabed.
Seismic waves typically travel at speeds of 2.5 to 3 miles per second at these depths, but the channels exhibited a 4 percent slowdown in average seismic speed.
“We know that the seismic speed is influenced by temperature, and we estimate that the slowdown we are seeing could represent an increase in temperature of up to 200 degrees Celsius,” said lead author of the study, Scott French, UC Berkeley graduate student in Earth and Planetary Sciences. .
Channel formation, similar to those revealed in the computer model, has been theoretically suggested to affect plumes in the Earth’s mantle, but has never been imaged before on a global scale. The fingers are also observed to align with the movement of the overlying tectonic plate, further evidence of the “channeling” of the plume, the researchers said.
“We believe that the plumes contribute to the generation of hot spots and high heat fluxes, accompanied by complex interactions with the shallow upper mantle,” French said. “The exact nature of these interactions will require further study, but we now have a clearer picture that can help us understand the” plumbing “of the earth’s mantle responsible for the hotspot’s volcanic islands like Tahiti, Reunion, and Samoa.”
Vedran Lekic, a graduate student at the Romanowicz lab at the time of this research and now an assistant professor of geology at the University of Maryland, co-authored this study.
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