With the eruption of Mauna Loa, a rare look at the Earth

Note that Mauna Loa, the world’s largest active volcano, was about to erupt — as it did last week for the first time in nearly four decades — it came to people on the Big Island of Hawaii an hour before the lava started scrolling. Public officials were quick to alert nearby residents. Scientists were quick to predict which areas of the island could be in danger. The curious have made plans to watch what could amount to an event of a lifetime: the exhalation of a massive mountain.

The eruption took years to realize, matched not quite to scale by the ongoing effort to monitor the volcano with seismometers, spectrometers, inclinometers, GPS units and other state-of-the-art instruments. “Mauna Loa is one of the best-instrumentated volcanoes in the United States,” said Wendy Stovall, a volcanologist with the US Geological Survey. Even still, much about the mountain’s inner workings is unknown, Stovall and other scientists said.

Weston Thelen, a USGS volcanologist who monitored the mountain from 2011 to 2016, said the size, mineral composition and heat have presented logistical challenges for scientists and public officials hoping to predict its movements. “Mauna Loa is a beast,” he said.

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With the eruption underway, Big Island researchers, including Jim Kauahikaua, a volcanologist at the USGS Hawaiian Volcano Observatory, had to strike a careful balance between concern for public safety given the many unknowns and a desire to collect data.

“Our primary mission is to scientifically mitigate these risks,” Kauahikaua said. “An eruption is always exciting, but let’s learn to temper our excitement and work professionally towards our core mission.”

So far, the eruption has posed little danger to surrounding communities, and thus has given a sense of urgency to scientists eager to unravel Mauna Loa’s many mysteries. How many weeks, months or years will the opportunity be available? “Nobody really knows how long this eruption will last,” said Gabi Laske, a geophysicist at the University of California, San Diego.

Thelen said: “We get very rare glimpses of what is happening in the volcano. If we simply place people in lawn chairs at the end of the lava flow and say, “It moved 1 meter,” we’re blowing it up.

An ancient hot spot

Most volcanoes form above Earth’s tectonic plate boundaries, where collisions and separations can create anomalous areas in the crust and upper mantle through which rock, fused and less dense by heat from the core of the planet, can penetrate the surface. But the Hawaiian Islands are 2,000 miles from the nearest tectonic boundary, and their existence has baffled geologists for centuries.

In 1963, a geophysicist named John Tuzo Wilson proposed that the islands, which are covered in layers of volcanic rock, rest atop a plume of magma, which forms when deep-mantle rock boils and builds up under the crust. This “hot spot” pushes continuously towards the surface, sometimes breaking through the tectonic plate, melting and warping the surrounding rock as it goes. The plate shifts over millions of years while the magma plume remains relatively stationary, creating new volcanoes atop the plate and leaving dormant ones behind. The results are archipelagos such as the Hawaiian-Emperor seamount chain and parts of the Icelandic plateau.

The hot spot theory gained widespread acceptance in the following decades. “There is no other theory that can reconcile so many observations,” said Helge Gonnermann, a volcanologist at Rice University.

Some confirmatory observations came relatively recently, in the 2000s, after scientists began placing seismometers, which measure Earth’s energy waves, on the ocean floor. John Orcutt, a geophysicist at the University of California, San Diego who helped lead that research, said the seismometers provided an X-ray of the plume of magma rising beneath Hawaii. The instruments were able to accurately read the direction and speed of the magma flow; the results strongly indicated the presence of a hot spot.

This hotspot has likely been fomenting volcanic activity for tens of millions of years, although it only arrived at its present location below Mauna Loa about 600,000 years ago. And as long as it stays there, Orcutt said, it will reliably produce volcanic activity. “Few things on Earth are so predictable,” he added.

Closer to the surface, predicting when, where and how intense these eruptions will be becomes more difficult, despite the profusion of seismometers and satellite sensors. “The deeper you go, the smoother the behavior becomes,” Orcutt said. “When you get this interface between rock and molten rock and the ocean, the magma tends to sporadically leak out.”

Under the hood of the volcano

The magma plume that feeds Mauna Loa consists primarily of molten basalt, which is less viscous than the magma beneath steeper stratovolcanoes like Mount St. Helena and Vesuvius. This makes Mauna Loa’s average eruption less explosive and contributes to the mountain’s long profile: about 10 miles from base to summit and covering 2,000 square miles.

The movement of thinner magma is also harder for seismometers to detect, making it harder for scientists to map the melting system of the magma, rock, crystal and gas that fuel eruptions.

The satellites, while still improving, aren’t sensitive enough under normal conditions to see deeper into Mauna Loa than the shallow magma reservoir a couple of miles below the summit. “It’s not clear whether there are additional storage tanks at greater depths,” Gonnermann said.

Things change, however, when the volcano begins to breathe. The magma pushes up more rapidly, cracking rock beneath the ground and causing the volcano’s surface to bulge. Such deformations can be detected by seismometers, which detect the depth and intensity of minerals as they vibrate and split under the molten pressure. From this, together with data on the gases and crystals emitted during the eruption and the tiny inflections of gravitational force, a picture from the chaos begins to emerge.

“We’re lucky if the pressure is high enough or the system is moving fast enough that we can get clues about what’s going on there,” Thelen said. “For the most part, when these things aren’t erupting, they’re silent.”

Mauna Loa last erupted in 1984 and has remained mostly silent for the next several years, although the smaller neighboring volcano, Kilauea, which shares the same magma source, has erupted continuously. Noises in the ground beneath the volcano began to increase in frequency and intensity around 2013, and seismometers detected clusters of low-magnitude earthquakes deep underground.

“But it waxes and wanes and stops swelling and freezes,” Thelen said. “You get lulled into this ‘Here we go, another swarm up there.'”

Sean Solomon, a geophysicist at Columbia University, said some earthquakes were caused by the volcano’s weight pushing down on the seabed, but most stem from rising magma, which pushes upward relentlessly, fracturing rocks. creating new melts and forming paths of least resistance.

“Rocks hold memories of every fracture that happened before,” Solomon said. “There’s kind of a hydraulic system underneath the volcanoes in Hawaii that leads to these favorite routes to climb.”

The details of this hydraulic system are still relatively unclear, Thelen said: ‘All we can do is run waves through the land and see how they get hit, and try to create a model that explains how that wave gets hit under the volcano.” He added, “The closer we look, the more questions we have.”

“You can’t hold magma forever”

On Sunday, the seismometers around the volcano’s summit started showing more activity. “When they tried to pinpoint where within the seismicity was originating, they saw that it was originating deeper and deeper, and that’s a telltale sign that the magma is moving upward,” Laske said.

There are two rift zones on Mauna Loa’s surface, one on the northeast side of the mountain and the other on the southeast side. These are footprints from previous eruptions, where magma has accumulated miles down the slope in bright, veiny flows. The northeast rift zone leads to an uninhabited area of ​​the island. The southwest rift zone leads to several communities along the Kona coast.

The eruption began at the mountain’s summit, when magma shot through fissures in the rock and filled the bowl-like caldera. Previous eruptions had started at the top and moved into a rift zone, but scientists didn’t know which one it would choose this time. The northeast flank would mean security; the Southwest could endanger thousands of people. Even after the eruption began, Stovall said, “we didn’t know the eruption had moved to the northeast area until we got our eyes in the air,” flying over the rift area and watching for the eruption. lava.

Since then, the lava flow has slowed its progress down the mountainside, even as it threatens to cross Saddle Road, a major highway on the Big Island. Magma continues to erupt from the rift zone to the northeast, spurting upward in red fountains, and scientists aren’t sure what might happen next.

Meanwhile, volcanologists and seismologists are trying to decipher the incoming data by placing more monitoring tools around active patches and collecting more satellite images of the mountain’s surface. “We’re really trying to physically understand what’s going on in the volcano,” Thelen said.

It is not known when the next eruption will occur. For some Big Island volcanologists, this is the first Mauna Loa eruption of their lives. But, as Solomon noted, “on geological time scales, 38 years is pretty short.”

Orcutt said, “It’s just something that’s been going on for thousands to millions of years, and it’s not going to stop happening. You can’t hold magma forever.

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