Wednesday, February 8, 2023
HomeScienceScientists just mapped Hawaii’s volcanic underbelly in stunning detail

Scientists just mapped Hawaii’s volcanic underbelly in stunning detail

“We were freaking out,” says John Wilding, a graduate researcher at the California Institute of Technology and lead author of a new study describing the geologic features in Science. “No one had ever directly observed magmatic activity at this scale before.”

The researchers used machine learning algorithms to search for earthquakes in seismic data from the Hawaii Volcano Observatory’s network of sensors, picking out trembles so small that previous methods missed them. The result is a stunningly detailed portrait of Hawaii’s fiery underworld, which promises to help scientists sort through the geological processes that drive the island’s volcanoes.

“This is probably going to be the future for volcano science,” says Matt Burgess, a former seismic analyst in Hawaii who has studied the deep earthquakes below Pahala.

Mysterious rumbles from the deep

The Pahala earthquake swarm has been rumbling since at least 1970. The quakes are located in the mantle, the layer of our planet between the crust and core, and most of them are too small and deep to joggle the surface with much force. Instead, the trembles feel more like a rolling or swaying of the ground. Sometimes Ka’u Coffee’s Daniele only realises that something’s amiss because ripples appear on the surface of his coffee. But in recent years the rattles beneath Pahala have become relentless.

“The seismicity was just continuing to go up and up,” says Ninfa Bennington, a volcano seismologist with the USGS Hawaiian Volcano Observatory who has been tracking the recent surge in activity.

Pahala is thought to sit above the head of a searing column of rock, called a hotspot, that built the Hawaiian Islands. As the Pacific Plate shifts across the stationary hotspot, new volcanoes—and eventually new islands—are born. The 15 volcanoes of the Hawaiian islands are the youngest in a chain of more than 129 that the hotspot has created, most of which have fallen silent and are now hidden beneath the waves.

Past studies have identified likely sources of molten rock below the earthquake swarm and suggested that an upward pulse of magma could be driving the deep rattles. Other studies have detailed the shallow volcanic plumbing. But exactly how molten rock flows up from the depths of the mantle isn’t known.

“We’re essentially missing this big piece,” Bennington says.

The swarm of earthquakes was a chance to get a closer look at the island’s fiery underbelly. While earthquakes can come from many sources, magma or fluid moving through cracks generates telltale seismic rumbles. And as the molten rock shifts, it can stress the ground nearby, causing it to crack and shift, which scientists can also spot in the earthquake data.

By plotting all these quakes in three dimensions—a bit like geologic pointillism—scientists have now sketched out a web of subterranean structures where magma may flow toward the surface, charging volcanic eruptions.

A seismic treasure trove

Amid the surge of earthquake activity in Hawaii, Wilding joined geophysicist Zachary Ross’s research group at Caltech. Ross had been developing methods that detect earthquakes using machine learning algorithms, which can pick out surprisingly small quakes and give stunning views into the spidery web of underground fault zones.

The team applied these methods to 3.5 years’ worth of Hawaiian seismic data, recorded between 2019 and 2022. The system identified nearly 200,000 earthquakes of the swarm, illuminating the stacked sill structures in the upper mantle. The extreme detail even allowed the scientists to track magma as it trickled into a sill, kicking off a cascade of quakes.

When Ross first saw the detail of the geological structures on his computer screen, he was dumbfounded. “It was kind of just like, oh my God, what are we looking at here?” he says. “It’s just shocking.”

He describes the complex of sills as the “gateway into the system,” providing a means to transport magma horizontally away from the area beneath Pahala. These underground features don’t contain empty space, instead representing a weak zone in the rock where magma has intruded and spread as a molten sheet. The complex links up with a zone of fractures that leads to Kilauea as well as an area that the team believes is connected to Mauna Loa.

There may be more than one route that molten rock follows to the surface, Ross says. He speculates that the sills might even be part of a broader layer of structures under the island that shuttles magma to the different volcanic peaks.

Abdullah Anaman
Abdullah Anaman
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