With the eruption of Mauna Loa, a rare glimpse of Earth

Note that Mauna Loa, the world’s largest active volcano, about to erupt – as it did last week for the first time in nearly four decades – has come to residents of the Big Island of Hawaii a hour before the lava begins to flow. . The public authorities hastened to alert the residents. Scientists raced to predict which areas of the island could be at risk. The curious have planned to observe what could become the event of a lifetime: the exhalation of a massive mountain.

The eruption lasted for years, but not quite on the scale of ongoing efforts to monitor the volcano with seismometers, spectrometers, tiltmeters, GPS units and other advanced tools. “Mauna Loa is one of the best-instrumented volcanoes in the United States,” said Wendy Stovall, volcanologist at the US Geological Survey. Even still, so much about the inner workings of the mountain 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 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, researchers on the Big Island, including Jim Kauahikaua, a volcanologist at the USGS Hawaiian Volcano Observatory, had to strike a delicate balance between concern for public safety, given the many unknowns, and the desire to collect data.

“Our primary mission is to scientifically mitigate these dangers,” Kauahikaua said. “An eruption is always exciting, but we are learning to temper our enthusiasm and work professionally for our primary mission.”

So far, the eruption has posed little danger to surrounding communities – and thus provided a sense of urgency for scientists keen to unravel Mauna Loa’s many mysteries. For how many weeks, months or years will the opportunity remain 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 have very rare looks at what is happening in the volcano. If we just put people on lawn chairs at the end of the lava flow and say, “It moved a meter,” we detonate it. »

An old hot spot

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

In 1963, a geophysicist named John Tuzo Wilson proposed that the islands, which are covered in layers of volcanic stone, sit above a magma plume, which forms when deep mantle rock bubbles up and accumulates under the crust. This “hot spot” continually pushes towards the surface, sometimes puncturing the tectonic plate, melting and deforming the surrounding rock as it goes. The plate moves over millions of years while the magma plume remains relatively still, creating new volcanoes at the top of the plate and leaving dormant volcanoes in their wake. The results are archipelagos such as the Hawaiian-Emperor Seamount Chain and parts of the Icelandic Shelf.

The hotspot theory gained broad consensus over the following decades. “No other theory is able to reconcile so many observations,” said Helge Gonnermann, a volcanologist at Rice University.

Some confirming sightings 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 conduct the research, said seismometers provided an X-ray of the magma plume rising beneath Hawaii. The instruments were able to accurately read the direction and speed of the magma flow; the results clearly pointed to the presence of a hot spot.

This hotspot likely fomented volcanic activity for tens of millions of years, although it came to its present position beneath Mauna Loa only 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. “By the time you get that interface between rock and molten rock and the ocean, the magma tends to come out sporadically.”

Under the Hood of the Volcano

The magma plume that feeds Mauna Loa is composed primarily of molten basalt, which is less viscous than the magma beneath steeper stratovolcanoes such as Mount St. Helens and Mount 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 system of melting magma, rock, crystal and gas that fuel eruptions.

The satellites, while constantly improving, are not sensitive enough under normal conditions to see deeper into Mauna Loa than the shallow magma reservoir a few miles below the summit. “It’s not clear if there are additional storage reservoirs at greater depths,” Gonnermann said.

Things change, however, when the volcano begins to breathe. The magma pushes upward faster, cracking rock beneath the ground and causing the surface of the volcano to swell. Such deformations can be picked up by seismometers, which detect the depth and intensity of minerals vibrating and splitting under melting pressure. From this, along with data on the gases and crystals emitted during the eruption and tiny inflections in gravitational force, a picture begins to emerge from the chaos.

“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 popping, they’re quiet.”

Mauna Loa last erupted in 1984, and in the years since has remained virtually silent, even as the nearby smaller volcano, Kilauea, which shares the same source of magma, has erupted continuously. Rumblings 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 drags,” 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 weight of the volcano pushing on the seafloor, but most result from rising magma, which presses down constantly, fracturing rocks , creating new casts and forming paths of less resistance.

“Rocks hold memories of every fracture that has happened before,” Solomon said. “There’s some sort of plumbing system under the volcanoes in Hawaii that leads to these favorite paths to climb.”

The details of this plumbing system are still relatively hazy, Thelen said: “All we can do is run waves through the earth and see how they’re impacted, and try to make a model that explains how this wave is impacted under the volcano.” He added, “The closer we look, the more questions we have.”

“You can’t hold back magma forever”

Late last Sunday, seismometers around the volcano’s summit began to show more activity. “When they tried to locate the origin of the seismicity inside, they saw that it was coming from shallower and shallower, and that’s a telltale sign that the magma moves up,” Laske said.

On the surface of Mauna Loa are two fault zones, one on the northeast side of the mountain and the other on the southeast. These are imprints of previous eruptions, where magma has accumulated for miles down the slope in veiny, glistening streams. 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 top of the mountain, when magma gushed through cracks in the rock and filled the bowl-shaped caldera. Previous eruptions had started at the summit and moved towards a fault zone, but scientists were unsure which of the two it would choose this time. The northeast flank would mean security; the southwest could put thousands at risk. Even after the eruption started, Stovall said, “we didn’t know the eruption had moved to the northeast area until we got our eyes up,” flying over the area rift and watching the lava spread.

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

In the meantime, volcanologists and seismologists are trying to decipher the incoming data by placing more monitoring instruments around active areas 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 volcanologists on the Big Island, this is the first eruption of Mauna Loa in their lives. But, as Solomon noted, “on the scale of geologic time, 38 years is pretty short.”

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

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