The first reconnaissance flights reached Tonga on Monday, but the remnants of a towering ash cloud tens of thousands of feet in the air still made it difficult to assess the damage caused by an undersea volcano that erupted off the coast of the Pacific island nation two days earlier, triggering a tsunami that hit the archipelago’s largest island—Tongatapu.
Geologists are calling the powerful eruption of the Hunga Tonga-Hunga Haʻapai volcano the biggest recorded anywhere in the world in more than three decades. Indeed, the impact of the eruption has been felt around the world, with tsunami waves hitting Australia, Japan, and the west coasts of North and South America.
“Think of them like ripples in a pond when a stone is dropped in—in this case, the eruption was the stone,” explained Brian McNoldy, a senior research associate at the University of Miami Rosenstiel School of Marine and Atmospheric Science. “The ripples in the ocean are bound by land masses surrounding the site, though it looks like there would be some ability to escape between Australia and Antarctica, then enter the southern Indian Ocean, though I don’t know to what extent. When the waves hit a coastline, they bounce back and keep sloshing until the energy is dissipated,” he said.
The eruption was so powerful, it also generated acoustic waves in the atmosphere, with the explosion being heard as far away as Alaska, making it perhaps the loudest sound known. “Eventually it was too weak to be heard, but the wave is still going around the globe at the speed of sound, now on its third trip,” said McNoldy, adding that even Miami might have received a signal from the first wave’s third trip this morning.
He noted that at the speed of sound, it takes about 35 to 36 hours for the acoustic wave to travel around the globe. “Barometers around the world have been detecting these little hiccups in surface pressure, too small to be noticed by people, but easily measured,” he said.
The United Nations Office for the Coordination of Humanitarian Affairs reported “significant infrastructural damage around Tongatapu.” But with telephone and internet lines cut, getting a clear picture of the extent of that damage remains a challenge.
What is clear, however, is that beyond the urgent need for search and rescue, Tonga—which is made up of some 170 islands—faces daunting challenges in the months ahead, according to a University of Miami scientist who conducts extensive research in that region of the world.
“Much infrastructure will have been damaged and will need to be rebuilt,” said Sam Purkis, professor and chair of marine geosciences at the Rosenstiel School of Marine and Atmospheric Science. “More problematic is that atoll islands like Tonga have little land area and limited natural terrestrial resources. For instance, supplies of fresh water are limited to small aquifers. Soils are poorly developed and thin. The tsunami has apparently inundated large portions of the island with seawater, which has the potential to pollute the island’s aquifers and agricultural land.”
Inhabitants of the islands affected by the tsunami may have to rely on desalination—the process of removing salt from seawater—and expensive imports for fresh water for a considerable length of time, Purkis said.
And if coral reefs surrounding the islands were damaged by the tsunami, that could put a further strain on Tonga’s limited natural resources, Purkis added. “Supplies of reef fish will be degraded, and much of the protein for Pacific islanders comes from the reef,” he explained.
The eruption’s impact on climate should also be considered. The volcano spewed a huge plume of ash, gas, and steam into the sky, and more eruptions have occurred since Saturday’s massive discharge.
“Climate models have examined the cooling effect on climate from the sulfates that are interjected into the stratosphere from volcanic eruptions,” said Paquita Zuidema, a professor of atmospheric sciences at the Rosenstiel School. “The plume did reach the stratosphere for this one, so I would expect some stratospheric sulphate loading from this eruption but have not yet seen anything posted about this happening,” she added. “The sulphates cool climate by reflecting sunlight back to space, and climate modelers have used past volcanic eruptions to assess how well their models are doing.”
Falk Amelung, professor of marine geosciences, addresses specific questions about the dynamics of the Hunga Tonga–Hunga Haʻapai eruption and general queries about volcanoes.
The Hunga Tonga–Hunga Haʻapai volcano has erupted regularly over the past few years. But this most recent eruption was massive. What made it so explosive?
The likely trigger was a submarine landslide. Once the overlying rock masses were removed, the volcano could explode. Powerful eruptions frequently start with a landslide. Another example is the 1980 eruption of Mt. St. Helens in Washington State. Emerging island volcanoes are particularly unstable and prone to land-sliding. An interesting fact is that there is geological evidence for a much larger mid-15th century tsunami in Tonga with 30-meter run-up height, likely caused by the collapse of an island volcano.
Shouldn’t the seawater have cooled the magma and prevented such a powerful eruption?
Volcanoes explode because the gases in the magma expand when they ascent. Volcanoes in deep water—a few hundred meters deep—commonly don’t explode because of the weight of the overlying water. The Tonga volcano is in very shallow water, so it was not affected much by the water load. The cooling by seawater is less important but also can play a role. If water cools the magma, the magma turns more viscous and gets stuck. The gas pressure then breaks the solidified magma and gases escape explosively.
What’s the difference between volcanic eruptions and earthquakes? Can they both trigger tsunamis?
This tsunami had three sources: the submarine landslide, the explosion, and possibly the atmospheric pressure change, all displacing the water. Several research groups are currently conducting model calculations to understand the contributions from the different sources. My guess is that there was a massive submarine landslide. The [uninhabited volcanic island of Hunga-Tonga-Hunga-Ha’apai] has nearly disappeared, but it was small. You need massive material movements to create a tsunami of this size. In contrast, at subduction zones, tsunamis are generated when one plate slips under the other, vertically moving the seafloor.
Have you ever seen a volcanic eruption in-person? If so, what was it like?
I have seen only small eruptions of frequently—hourly to daily—erupting volcanoes. Experiencing the power of nature is stunning, and the reason why I got into this field. Particularly memorable is an eruption of Sakurajima volcano in southern Japan that I could watch from my hotel balcony. For the big eruptions, however, I prefer to analyze satellite images on my computer.
