By Oliver Whang
Notice that Mauna Loa, the largest active volcano in the world, was going to erupt — as it did this past 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, Stovall and other scientists said.
Weston Thelen, a volcanologist with the USGS who monitored the mountain from 2011-16, said sheer size, mineral composition and heat presented logistical difficulties for scientists and public officials hoping to predict its movements. “Mauna Loa is a beast,” he said.
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.
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 1 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 such as 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 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, Orcutt said, it will reliably produce volcanic activity. “Few things on Earth are so predictable,” he added.
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 such as 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,” 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,” 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,” 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 seafloor, but most result from rising magma, which presses up incessantly, fracturing rocks, creating new melts and forming paths of less resistance.
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 Solomon noted, “on geological time scales, 38 years is pretty short.”
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.”