Albert McKeon

Jun 15th 2022

Tonga Eruption Offers Opportunity to Study the Power of an Underwater Volcano


An underwater volcano in the Tonga archipelago unleashed a tsunami that pushed towering waves onto the coasts of Australia and Japan and smaller ones toward Alaska and California.

Closer to its crater, the volcano turned the Pacific Island nation of 100,000 people upside down. At least three people died, more than 1,500 are displaced, hundreds of homes were destroyed, telecommunications were severed, and the United Nations has warned of potential food shortages.

While Tonga rebuilds, scientists are studying the factors that caused this natural disaster and are already amazed by certain findings — including how the volcano caused a massive aboveground explosion that typically doesn’t follow an underwater volcano eruption. Their research could offer new clues into volcanic forces and confirm what’s already been theorized.

A Low Simmer or a Big Bang

More than 70% of volcanic eruptions happen underwater. Yet, despite their frequency, much of how underwater volcanoes behave eludes scientists. They are simply too deep in the ocean to observe closely.

All volcanic eruptions, underwater and above ground, follow a foundational blueprint. The molten rock underneath Earth’s surface, also known as magma, will rise from either a terrestrial surface or the ocean floor. The magma’s dissolved gases form bubbles as the pressure of the molten rock wanes during its ascent. When those gases suddenly release, a volcano can release an explosion of hot magma, which technically becomes lava upon eruption.

But, as the U.S. Geological Survey points out, not all volcanic eruptions are explosive. If magma is “thin and runny,” gases escape and flow out of a volcano without destructive fanfare. Lava will thus move slowly enough for people to evacuate. When magma is “thick and sticky,” gases don’t escape, and pressure boils. Big eruptions can blast magma straight into the air. It then falls apart into pieces called tephra, which can be as small as particles of ash or as large as house-sized boulders.

Eruptions of terrestrial volcanoes can be mesmerizing to watch — unless you’re close by. Think of the 1980 eruption of Mount St. Helens in Washington state, when a powerful blast knocked off the mountain’s north face and caused a massive landslide. Or, if you’re a real history buff, visualize the power of the A.D. 79 eruption of Mount Vesuvius, which leveled Pompeii and other Roman towns and left enough evidence for scientists to study to this day — including the recent finding of the skeleton of a man who might have been running away from the blast.

A volcano can produce tsunamis as high as those produced by the largest earthquakes, according to the International Tsunami Information Center (ITIC). The ITIC offers a prominent example of a terrestrial volcano triggering a tsunami: the 1883 eruption of Krakatoa in Indonesia. This volcano pushed lava into the Pacific Ocean and caused 120-foot waves, with the tsunami and eruption itself killing more than 36,000 people.

For an underwater (or submarine) volcano to make an impression above ground, it has to produce a massive blast to overcome the weight of the sea. As Smithsonian explains, magma faces the pressure of tons and tons of ocean water once it reaches the seafloor. Not only that, but seawater also vaporizes when it meets magma at 800 degrees Celsius, expanding into a steam that is powerful enough to tear apart lava. Still, under certain conditions, magma can solidify and create the sort of eruption seen and felt in Tonga.

Capturing Attention Across the World

The two islands that comprise the terrestrial surroundings of the underwater volcano in Tonga — named Hunga-Ha’apai and Hunga-Tonga — peek out roughly 328 feet above sea level. The volcano’s immensity is evident underneath the sea at more than 5,900 feet tall and more than 12 miles wide.

As The Conversation reports, it is not uncommon for an underwater volcano eruption here to escape notice. Hunga-Tonga–Hunga-Ha’apai has blown regularly over the past few decades, with blasts much smaller in scale than the one in January. A few eruptions in the past 13 years released hot jets of steam and magma bursts through waves, but the eruption this year was the type of massive explosion the volcano produces only once every thousand years or so.

“This is a preliminary estimate, but we think the amount of energy released by the eruption was equivalent to somewhere between four to 18 megatons of TNT,” Jim Garvin, chief scientist at NASA’s Goddard Space Flight Center, told the agency’s Earth Observatory. “That number is based on how much was removed, how resistant the rock was, and how high the eruption cloud was blown into the atmosphere at a range of velocities.” As a benchmark comparison, the blast released hundreds of times the equivalent mechanical energy of the Hiroshima nuclear explosion.

Earth Observatory compiled other remarkable scientific observations of the January 15 eruption that caused atmospheric shock waves and tsunami waves to travel around the world:

  • An immense plume of material created what’s known as an umbrella cloud. It had crescent-shaped bow shock waves, a vast number of lightning strikes and extended as much as about 300 miles in diameter.
  • A small amount of ash and gas might have spread upward as high as 24.7 miles, the highest volcanic plume ever measured by NASA’s Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation.
  • The eruption injected about 0.4 teragrams of sulfur dioxide into Earth’s upper atmosphere, but scientists believe it might not pose harm such as cooling global temperatures, because the threshold for climate impacts is about five teragrams.

While many islands in the Tonga archipelago are uninhabited, some of the populated ones were heavily damaged by the eruption and subsequent tsunami. According to NPR, coral reefs around the atolls were also damaged, and ash covered Nomuka Island, which is about 40 miles northeast of the volcano. Not only did waves flood locales in Tonga, but they also went up to three feet high in Hawaii and arrived in small bursts in mainland U.S.

Fifty-three detectors around the world, including as far away as Antarctica, recorded the low-frequency boom from the explosion as it traveled through the atmosphere. It was the loudest event the network of sensors had detected in more than 20 years of operation.

Rebuilding and Learning From the Tonga Volcano

It will probably take years for the people of Tonga to pick up the pieces. The United Nations reports that 84,000 residents — more than 80% of the population — were affected by the eruption, with as many as 70% of households either losing livestock, contending with damaged grazing land or overcoming contaminated water supplies.

Scientists will undoubtedly continue studying the event and how the eruption of submarine volcanoes affect Earth’s atmosphere and overall environment. They also need to determine if this was the volcano’s last big blast in some time or if there are more soon to come.

Hunga-Ha’apai and Hunga-Tonga might find the hope that surfaced after the volcano Tagoro wiped away aquatic life in the Canary Islands. Lava kills marine life, but a study by oceanographers found that within three years of the 2011 eruption of Tagoro, areas 650 away from the crater had a new abundance of tiny fish and other organisms.

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