How the Ocean Works
How did the ocean remain so quiet during Tonga’s eruption?
At 4:30am, Donna Willoya couldn’t sleep.
It was January 15, 2022, and the resident of Wasilla, Alaska had been having bouts of insomnia. She hopped out of bed, made tea, and stepped out onto her back deck for some fresh air. It was a beautiful, quiet night, she recalls, until suddenly, a thunderous explosion echoed through the dimly-lit sky.
“When we hear noises like that, it’s typically gunshots since there’s a lot of hunting in the area,” Willoya said. “But I’m from a family of hunters and very familiar with gunshots. This sound was clearly different—it was like unlike any I had ever heard.”
She had no idea what the boom was, so she shrugged it off and went back to bed. Later the next day, she saw the news report on TV: a massive submarine volcano 10,000 miles away in the South Pacific had erupted and sent audible shockwaves all the way to Alaska.
“I never thought something from Tonga would be heard on my deck,” she said.
The volcano, known as the Hunga Tonga–Hunga Haʻapai, had been restless for about a month before a perfect storm of events occurred: First, the volcano’s 150-meter-deep caldera collapsed. Then, seawater percolated through its exposed cracks and faults. Finally, magma rose up from the depths of the volcano and collided with the seawater at more than 1,000 degrees Celsius.
This resulted in one of the most intense volcanic eruption ever recorded, with some estimates suggesting that the explosion was hundreds of times stronger than Hiroshima. It not only sent atmospheric waves that circled the globe several times, but thrust more than a trillion grams of water vapor into the stratosphere and sent an ash cloud 35 miles into the mesosphere. Hurricane-force winds blew at the edge of space, while atmospheric pressure waves injected so much energy into the ocean below that meteotsunamis formed around the world.
For volcanologists, the Tonga eruption was a case study beyond their wildest dreams. But for scientists studying how sound travels through the ocean, it was downright baffling. It turns out that while the explosion was loud enough to reach Alaska, things stayed pretty quiet in the ocean.
“We detected very weak acoustic signals in the hydrophone data, which was really surprising given the intensity of the eruption,” said Gil Averbuch, a postdoctoral scholar at WHOI who studies how acoustic waves propagate in the ocean. “We were expecting to see much more prominent signals.”
Submarine volcanic eruptions, like the Hunga-Tonga, cause earthquakes that transmit seismic energy below the seafloor. Once those seismic waves interact with the water column via geological/bathymetric features in the ocean, they turn into acoustic waves (aka, sound waves).
A global network of underwater microphones, known as hydrophones, record and measure these waves, giving scientists like Averbuch a read on the ruckus they cause in the ocean. The network is run by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) for the purpose of detecting nuclear explosions around the world, but they have no trouble picking up soundwaves from other sources.
The question of why the ocean remained so quiet during the Tonga eruption is a complicated one. Averbuch says it's one of “many open questions” that he and more than 70 other scientists around the world have been grappling with since the “once in a lifetime event,” as he refers to it.
Averbuch believes that underwater geology may have played a role in blocking sound waves as they traveled through the ocean. The Lau Basin, where the volcano sits, is a relatively shallow, yet geologically-complex environment of steep underwater cliffs and seamounts.
“Since the volcano is surrounded by underwater features, the acoustic waves coupled directly from the volcano were blocked, and did not travel far.” said Averbuch.
WHOI assistant scientist Wenbo Wu, who has not been involved in studying this particular volcano, reinforces the idea that these types of features can interfere with sound travel in the ocean.
“Shallow bathymetry is not conducive to long-distance propagation of acoustic waves, as they can be attenuated or blocked by interactions with the seafloor,” said Wu.
Another factor that may have quelled the ocean while hell broke loose in the atmosphere was the relative amount of energy the explosion transferred into the water column.
“Clearly, most of the energy went into the atmosphere, which is why the atmospheric Lamb waves propagated around the world for days,” said Averbuch. “But in looking at the weak signals in the hydrophone data, we were surprised by the relative lack of acoustic energy in the ocean.”
Moving forward, Averbuch hopes to better understand how energy from the Tonga eruption was distributed among the subsurface, ocean, and atmosphere. That may help shed more light on why there was such an imbalance between the acoustic observations recorded above and below the ocean’s surface.
“There are still so many open questions about this eruption,” said Averbuch. “Fortunately, there’s unprecedented data from all the monitoring instruments that were in place—so much so that they will keep scientists busy for many years to come.”
MEET THE AUTHOR Evan Lubofsky
Evan Lubofsky is a science writer and editor at Woods Hole Oceanographic Institution. Read more
