Notice that Mauna Loa, the largest active volcano in the world, was going to erupt — as it did this week for the first time in nearly four decades — came to people on the Big Island of Hawaii an hour before the lava began to flow. Public officials scrambled to alert nearby residents. Scientists rushed to predict which areas of the island might be in danger. The curious made plans to observe what could shape up to be an event of a lifetime: the exhalation of a massive mountain.
The eruption was years in the making, matched not quite in scale by the ongoing effort to monitor the volcano with seismometers, spectrometers, tiltmeters, GPS units and other state-of-the-art tools. “Mauna Loa is one of the most well-instrumented volcanoes in the United States,” said Wendy Stovall, a volcanologist with the U.S. Geological Survey. Even still, so much about the inner workings of the mountain is unknown, Dr. Stovall and other scientists said.
Weston Thelen, a volcanologist with the U.S.G.S. who monitored the mountain from 2011 to 2016, said that sheer size, mineral composition and heat all presented logistical difficulties for scientists and public officials hoping to predict its movements. “Mauna Loa is a beast,” he said.
With the eruption underway, researchers on the Big Island, including Jim Kauahikaua, a volcanologist with the U.S.G.S. Hawaiian Volcano Observatory, have had to strike a careful balance between concern for public safety, given the many unknowns, and the desire to collect data.
“Our main mission is to mitigate these hazards scientifically,” Dr. Kauahikaua said. “An eruption is always exciting, but we learn to temper out excitement and professionally work toward our main mission.”
So far the eruption has posed little danger to surrounding communities — and thus has lent a sense of urgency to scientists who are eager to unlock Mauna Loa’s many mysteries. For how many weeks, months or years will the opportunity remain available? “Nobody really knows how long this eruption’s going to last,” said Gabi Laske, a geophysicist at the University of California, San Diego.
Dr. Thelen said: “We get very rare looks at what’s happening in the volcano. If we just station people in lawn chairs at the end of the lava flow and say, ‘It’s moved one meter,’ we’re blowing it.”
An ancient 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 the upper mantle through which rock — made molten and less dense by heat from the planet’s core — can push through to the surface. But the Hawaiian Islands are 2,000 miles from the nearest tectonic boundary, and their existence puzzled geologists for centuries.
In 1963, a geophysicist named John Tuzo Wilson proposed that the islands, which are covered with layers of volcanic stone, sit above a magma plume, which forms when rock from the deep mantle bubbles up and pools below the crust. This “hot spot” continually pushes toward the surface, sometimes bursting through the tectonic plate, melting and deforming the surrounding rock as it goes. The plate shifts over millions of years while the magma plume stays relatively still, creating new volcanoes atop the plate and leaving inactive ones in their wake. The results are archipelagoes like the Hawaiian-Emperor seamount chain and parts of the Iceland Plateau.
The hot spot theory gained broad consensus in the subsequent decades. “There is no other theory that is able to reconcile so many observations,” said Helge Gonnermann, a volcanologist at Rice University.
Some confirming observations came relatively recently, in the 2000s, after scientists began placing seismometers, which measure terrestrial energy waves, on the ocean floor. John Orcutt, a geophysicist at the University of California, San Diego, who helped lead that research, said that the seismometers had 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’s flow; the results pointed resoundingly toward the presence of a hot spot.
This hot spot has probably been fomenting volcanic activity for tens of millions of years, although it arrived in its current position under Mauna Loa only about 600,000 years ago. And as long as it remains there, Dr. 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 more smooth the behavior gets,” Dr. Orcutt said. “By the time you get this 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 fueling Mauna Loa is made primarily of molten basalt, which is less viscous than the magma beneath steeper stratovolcanoes like Mount St. Helens and Mount Vesuvius. This makes the average Mauna Loa 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 more difficult for seismometers to detect, which makes it harder for scientists to map the system of magma melts, rock, crystal and gas that feed eruptions.
Satellites, while ever-improving, are not sensitive enough under normal conditions to see deeper into Mauna Loa than the shallow magma reservoir a couple of miles below the summit. “It is not clear whether there are additional storage reservoirs at greater depths,” Dr. Gonnermann said.
Things change, though, when the volcano starts breathing. Magma pushes upward more quickly, cracking rock below 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 the molten pressure. From this, together with data about 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 to what’s going on there,” Dr. Thelen said. “For the most part, when these things are not erupting, they’re quiet.”
Mauna Loa last erupted in 1984, and in the years afterward, it stayed mostly silent, even as the smaller neighboring volcano, Kilauea, which shares the same magma source, erupted continuously. Rumblings in the ground beneath the volcano started increasing in frequency and intensity around 2013, and seismometers detected clusters of low-magnitude earthquakes deep underground.
“But it waxes and wanes and stops inflating and hangs out,” Dr. Thelen said. “You get lulled into this: ‘Here we go, another swarm up there.’”
Sean Solomon, a geophysicist at Columbia University, said that some earthquakes were caused by the volcano’s weight pushing down on the seafloor, but most result from rising magma, which presses up incessantly, fracturing rocks, creating new melts and forming paths of less resistance.
“Rocks retain memories of every fracture that’s happened before,” Dr. Solomon said. “There’s some kind of plumbing system underneath the volcanoes on Hawaii that leads to these preferred paths to rise.”
The details of this plumbing system are still relatively unclear, Dr. Thelen said: “All we can do is pass waves through the earth and see how they’re impacted, and try to make a model that explains how that wave is impacted underneath the volcano.” He added, “The closer we look, the more questions that we have.”
‘You can’t hold back the magma forever’
Late at night on Sunday, the seismometers around the summit of the volcano started showing more activity. “When they tried to locate where inside the seismicity was originating, they saw that it was originating shallower and shallower and shallower, and that is a telltale sign that the magma is moving upward,” Dr. Laske said.
At the surface of Mauna Loa are two rift zones, one on the northeast side of the mountain and the other on the southeast. These are imprints of previous eruptions, where magma pooled for miles down the slope in veiny, glowing streams. The northeast rift zone leads to an uninhabited area of the island. The southwest rift zone leads straight to the city of Hilo, population 45,000.
The eruption began at the summit of the mountain, when magma spurted through fissures in the rock and filled the bowl-like caldera. Previous eruptions had started in the summit and moved to a rift zone, but scientists did not know which of the two it would choose this time. The northeast flank would mean safety; the southwest would be could put thousands of people in danger. Even after the eruption started, Dr. Stovall said, “we didn’t know the eruption had moved to the northeast zone until we had eyes in the air,” flying over the rift zone and watching the lava spill out.
Since then, the lava flow has slowed in its progression down the sides of the mountain, although it does threaten to cross Saddle Road, a major highway on the Big Island. Magma continues to erupt from the northeast rift zone, spurting upward in red fountains, and scientists are unsure what might come next.
In the meantime, volcanologists and seismologists are trying to decipher the incoming data by placing more monitoring instruments around active zones and collecting more satellite images of the mountain’s surface. “We’re really trying to understand physically what’s happening in the volcano,” Dr. Thelen said.
There’s no knowing when the next eruption will occur. For some volcanologists on the Big Island, this is the first Mauna Loa eruption of their lifetimes. But, as Dr. Solomon noted, “on geological time scales, 38 years is pretty short.”
Dr. Orcutt said: “It’s just something that’s happened for thousands to millions of years, and it’s not going to stop doing that. You can’t hold back the magma forever.”