Some volcanic hot spots may have a surprisingly shallow heat source

By | January 6, 2022

Some of the world’s volcanic hot spots may be fueled by molten material that originates surprisingly close to Earth’s surface.

While some of the hottest spots are fueled by plumes of buoyant material welling up from deep within Earth, as expected, molten flows driving activity at the coolest hot spots may result from relatively shallow geophysical processes, a new study suggests.

A lot of our planet’s volcanic activity occurs at or near the edges of the tectonic plates that make up Earth’s crust (SN: 1/13/21). At mid-ocean ridges, which often form the boundaries between some tectonic plates, hot material wells up from the mantle — the hot, thick layer that lies between the Earth’s core and its crust — to create fresh crust.

But more mysterious volcanic activity also occurs in many locales in the middle of a tectonic plate, far from mid-ocean ridges, says Xiyuan Bao, a geophysicist at UCLA. The islands of Hawaii, Ascension Island in the South Atlantic and the Pitcairn Islands in the South Pacific are just a few examples of volcanoes created by such activity (SN: 1/29/19).

Scientists suspect that many of these sites of isolated volcanism are fed by plumes of hot material rising from deep within the mantle, somewhat akin to small packets of water rising to the surface in a pot of near-boiling water (SN: 9/16/13). But a new analysis by Bao and colleagues, described in the Jan. 7 Science, suggests that some of these isolated hot spots are fueled by material that isn’t as hot as expected, casting doubt that volcanic activity there is driven by deep-mantle plumes. The results could help scientists figure out the mysterious processes unfolding at various sites of volcanism in the interior of plates.

“This study helps sort out which volcanic plumes are deep-seated and which are not,” says Keith Putirka, an igneous petrologist at California State University, Fresno who wasn’t involved in the work.

The team focused on 26 volcanic hot spots in oceanic areas that previous studies had suggested were fed by deep-mantle plumes. The researchers used seismic data to estimate the temperature of mantle material at various depths from 260 to 600 kilometers. In general, the hotter the material is, the slower that seismic waves travel through it.

The team then compared the temperature estimate for each hot spot with the average temperature of mantle material welling up at mid-ocean ridges. Because tectonic plates are pulling apart there, there’s no resistance to upwelling of hot rock from deep in the mantle. That, in turn, provides a baseline against which scientists can compare temperatures of rocks deep beneath isolated hot spots.

Temperatures at mid-ocean ridges average about 1388° Celsius (2530° Fahrenheit). For a dozen of the hot spots the team studied, deep-mantle material was more than 155° C warmer than mid-ocean ridge material, Bao and his team report. Material that hot is more than warm enough to rise to Earth’s surface, chew through overlying crust and create prodigious volcanic activity.

But for 10 hot spots, deep-mantle material ranged between only 50° C and 135° C warmer than mid-ocean ridge material, just warm enough to rise to the surface and through crust. And four of the hot spots were less than 36° C warmer than mid-ocean ridge material, which suggests the hot spot material wouldn’t be able to rise fast enough to sustain buoyancy and break through the crust. Other sorts of geophysical processes occurring closer to Earth’s surface are fueling volcanic activity at these 14 cool-to-middling hot spots, the researchers propose.

“The evidence for mantle plumes under most volcanic islands is lacking,” says Godfrey Fitton, a geochemist at the University of Edinburgh who wasn’t involved in the work. An alternate source of molten material, he suggests, could be areas where tectonic plates collided to help create past supercontinents (SN: 1/11/17).

In those crumpled zones, Fitton explains, Earth’s crust would be thicker and thus help insulate the flow of heat from the mantle to the surface. The buildup of heat in the crust, in turn, could lead to local melting of carbonate-rich rocks that could fuel volcanism. In 2020, he and his colleagues suggested that such processes have fueled volcanism at hot spots off the western coast of Africa and off the northeastern coast of Brazil for the last 50 million years or more.