At any point in time, about 80% of all volcanic eruptions occurring on Earth are underwater, says volcanologist Melissa Scruggs, who recently graduated with her Ph.D. from the University of California, Santa Barbara. But because these eruptions are often not explosive and are deep under the sea, they’re out of sight and often out of mind. However, on Jan. 15, 2022, one of the past century’s most violent eruptions issued forth from exactly such a location. A volcanic island poking out of the South Pacific Ocean called Hunga Tonga-Hunga Ha’apai—part of the Kingdom of Tonga—blew its top, with much of the eruption spewing from beneath the waves.
Until about 2015, the volcano’s eruptive crater, or caldera, resided underwater. Two distinct parts of the caldera rim breached the surface as separate islands—Hunga Tonga and Hunga Ha’apai. But then, eruptions beginning in 2014 and continuing into 2015 helped the two seemingly separate mounts coalesce into a single island, Hunga Tonga-Hunga Ha’apai, which persisted until January’s mighty blast.
In a paper published March 2022 in Earthquake Research Advances, mere months after the eruption began—an international team of scientists, led by David Yuen of Columbia University and Scruggs, used multiple datasets, including preliminary seismic, infrasonic, and barometric information collected by SAGE facilities, to narrate the eruption timeline. This event, the authors say, provides a unique opportunity to explore aspects of relatively rare, but terribly violent, volcanic blasts that occur under the ocean. A host of additional papers about the Hunga Tonga-Hunga Ha’apai eruption have been published since March, including several that incorporate data from SAGE facilities into their analyses. In these papers, the authors examine specific aspects of the eruption’s aftermath, such as subsequent tsunamis.
Volcano in the sea
Eruptive activity at Hunga Tonga-Hunga Ha’apai, which rises about two kilometers above the seafloor, was first recorded in 1912. However, because the majority of the volcano is underwater, relatively little is known about its past eruption history.
Hunga Tonga-Hunga Ha’apai shot its first plume of ash into the stratosphere on Dec. 20, 2021 after seven years of quiescence. Following intermittent activity, on Jan. 14, 2022 at 4:20 a.m. local time (Jan. 13, 2022 at 15:20 UTC), another ash plume rocketed into the stratosphere, reaching about 20 kilometers (about 12.5 miles) high. Plumes continued to emerge from the volcano throughout the day, which resulted in the destruction of the middle third of the island. The volcano’s vent was now just below the surface of the ocean, says Scruggs.
The next day, at about 5:02 p.m. local time (Jan. 15 at 04:02 UTC), the volcano exploded again, soon reaching its cataclysmic apex by punching an ash plume upward about 58 kilometers (about 36 miles), well beyond the stratosphere and into the mesosphere. For comparison, the ash plume from Mt. Pinatubo’s 1991 eruption—the second-largest of the 20th century—reached about 40 kilometers (28 miles) into the atmosphere.
The plume eventually spread horizontally, forming an umbrella of ash and gas about 600 kilometers wide—a pattern supported by lightning data. Satellite imagery suggests that the plume remained above 30 kilometers (about 18 miles) for up to about five hours, with eruptive activity continuing between 12 and 13 hours. Based on threading together various data sources, Scruggs explains that the magma chamber was breached that fateful afternoon, letting the bubbly magma move upward through the overlying seawater to the surface. This produces what’s called a surtseyan-style eruption, in which rapidly rising magma interacts with water, producing explosions. The authors posit that the exponential increase in eruption intensity over the next six minutes may have marked the move to a subplinian-style eruption, which would have had a higher eruption column. About six minutes after that, intense explosions began, reflecting the interaction of water with a Plinian-style eruption—a rare phreatoplinian eruption in which the addition of seawater caused this eruption to be even more explosive and powerful than if it were to have occurred above land, says Scruggs. These types of eruptions eject large amounts of ash into the stratosphere or beyond, driven largely by volcanic gasses. In this phase lies the likely source of the peculiar phenomena of this eruption—like Lamb waves and fast tsunamis.
Lamb waves circle the globe
Human ears heard acoustic waves produced by the eruption in places like New Zealand, Hawai’i, and Alaska. As the audible shockwave of the explosion propagated through the atmosphere, so too did infrasound, a signal that’s largely hidden from our cochlear appendages.
All 53 arrays of the International Monitoring System (IMS) operated by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), who are tasked with monitoring nuclear explosions, recorded the infrasound signal, though these data were not publicly available in March. However, Glenn Thompson, a geophysicist at the University of South Florida and coauthor of Yuen and Scruggs’ paper, accessed data via the IRIS Data Management Center (DMC) from 117 infrasound sensors located at Global Seismographic Network (GSN) stations, and found that Hunga Tonga-Hunga Ha’apai’s globally observed infrasound signal was the largest produced in the past 30 years. Using data from 584 barometric (pressure) sensors, also downloaded from the IRIS DMC, Yuen and colleagues note that the low frequency atmospheric Lamb wave, which behaves similar to an earthquake-sourced Rayleigh wave, went all the way around the world in just under 36 hours.
In fact, according to a detailed analysis published in Science led by Robin Matoza, a volcanologist at the University of California, Santa Barbara, he and his colleagues observed that over the course of six days, the Lamb wave encircled the globe four times. The passage of the Lamb wave caused what’s called air-to-ground coupling.
“To a first order, a positive pressure change at the ground surface pushes down on the ground, resulting in a downward vertical displacement,” explains Matoza. A pressure decrease at the surface, he says, would result in an upward movement. Seismic stations are sensitive to pressure changes because of the associated up-and-down ground motion. Thus, Matoza and colleagues used seismometers around the world, including GSN instruments, to trace the passage of the Lamb wave.
Rare tsunamis
Eruptions that produce tsunamis are relatively rare, especially when compared with those caused by underwater earthquakes. The mechanisms of how eruptions produce tsunamis are also distinct. For instance, when Anak Krakatau was erupting in December of 2018, a flank collapse into the ocean displaced enough water to cause a devastating tsunami in Indonesia’s Sunda Strait.
In the case of Hunga Tonga-Hunga Ha’apai, the tsunami moved much faster than expected and propagated into the Caribbean and Mediterranean—basins without direct ocean routes, as the Matoza paper notes.
Even after about 100 hours, or four days, “data from western North America tidal gauges suggests the ocean was sloshing around,” says Scruggs.
In a Science paper led by Tatsuya Kubota, a Research Fellow at Japan’s National Research Institute for Earth Science and Disaster Resilience, the authors used information from a variety of sources, including barograph data from the GSN, to investigate how the tsunami could have arrived more than two hours earlier than expected. They found that the fast-moving atmospheric Lamb waves drove the increase in sea height for the initial wave. Much like the above-mentioned air-to-ground coupling that helped land-based seismic sensors see the Lamb waves, air-to-sea coupling appears to be the culprit.
Moreover, bathymetric variations in the Pacific Ocean—changes in the topography of the sea floor—contributed to the long-lasting sloshing. Kubota and colleagues note that because the tsunamis had multiple sources related to both the Lamb waves and bathymetry, the result was a much more complicated and longer-lasting tsunami when compared to an earthquake-induced event.
Seismicity and size
Seismic stations around the globe recorded the seismic waves induced by the eruption, with the USGS estimating a magnitude-5.8 event that coincides with the eruption’s change to a towering, Plinian-style plume. Based on these data, the VEI (Volcanic Explosivity Index, which scales from 1 to 8, with 8 being apocalyptically explosive), was likely a 6—one of the largest ever recorded, according to Scruggs. When they looked at the Lamb wave data, the VEI estimate came in between 5 and 6, supporting the seismically-sourced conclusion. Matoza and colleagues also found that the Lamb wave data compared well with another famous VEI 6 eruption—that of Krakatau in 1883.
In another paper published in Shock Waves by physicist Jorge Diaz and academic Sam Rigby of the University of Sheffield, they used data from several sources, including the IRIS DMC. They calculated that this eruption released the equivalent amount of energy as about 61 megatons of TNT, says Rigby. According to these calculations, this eruption released more energy than the 1980 eruption of Mount St. Helens and slightly more energy than the detonation of Tsar Bomba, the largest human-made explosion in history.
This eruption showcases the complex nature of cascading natural hazards, especially concerning shallow, explosive underwater volcanic eruptions in which water and magma inherently mix, according to Scruggs. In particular, tsunami warning systems aren’t designed to find this kind of tsunami; they largely rely on an understanding of earthquake-induced ocean waves, though Scruggs notes that they’ve been updated to include landslide mechanisms. Disentangling how eruptions like Hunga Tonga-Hunga Ha’apai produce tsunamis—whether from collapse or the interaction between the surface and the atmosphere or the explosive power of the eruption itself, she says, will help monitoring agencies keep an eye and ear out for tsunamis produced by underwater eruptions.
Citations:
Yuen, D. A., Scruggs, M. A., Spera, F. J., Zheng, Y., Hu, H., McNutt, S. R., … & Tanioka, Y. (2022). Under the surface: Pressure-induced planetary-scale waves, volcanic lightning, and gaseous clouds caused by the submarine eruption of Hunga Tonga-Hunga Ha’apai volcano. Earthquake Research Advances, 100134.
Díaz, J. S., & Rigby, S. E. (2022). Energetic output of the 2022 Hunga Tonga–Hunga Haʻapai volcanic eruption from pressure measurements. Shock Waves, 32(6), 553-561.
Kubota, T., Saito, T., & Nishida, K. (2022). Global fast-traveling tsunamis driven by atmospheric Lamb waves on the 2022 Tonga eruption. Science, eabo4364.