One of the most amazing natural events of our time happened on January 15, 2022, when a submarine volcano in the southwest Pacific Ocean called Hunga Ha'apai -Hunga Tonga violently erupted.  This was the most powerful volcanic eruption in the past ~30 years, and the first of its kind recorded on a wide range of modern scientific instruments.  The explosion shot a plume of ash high into the atmosphere, higher than any eruption since 2006 when satellites capable of measuring eruption column heights were in orbit.  It caused a kind of sonic boom  that had not been recorded since atmospheric testing of nuclear weapons ended in the early 1960s and also caused a strong tsunami that did significant damage to residents of the nearby Kingdom of Tonga.   Our grant supported renting a a multibeam sonar and installing this on the hull of a New Zealand research vesssel  for GNS scientist Dr. Cornel de Ronde to take to the volcano and see how the eruption had changed it and also to make a video about this for the public.  On 5 April, 2022 Cornel and a small team sailed on the R/V Kaharoa from Wellington, New Zealand, northwards towards the volcano.  The voyage began well enough but a few days out of port, a serious problem appeared because the ship sailed over a tree log which sheared off the entire multibeam unit. With the loss of the sonar, this attempt to map the new Hunga Ha?apai -Hunga Tonga caldera was stymied.

            Fortunately for the video effort, another New Zealand geoscientist ? Prof. Shane Cronin of U. Auckland was in the Kingdom of Tonga, just 65 kilometers from the volcano and was also making plans to map the new caldera.  On April 9 he used a 9m fishing boat that was equipped with a sonar that could see down to 300m depth and went over the caldera but couldn?t see the bottom of it.  Ten days later he used a Tongan Navy patrol boat with a 800 depth m-range single beam sonar and still couldn?t see the bottom! Finally, he arranged for a bigger boat ? the 19m Pac Horizon, borrowed an 80kHz sonar that could see deeper and spent several days adapting it to fit on the Pac Horizon.  Once installed, a survey team ? consisting of Shane and four geoscientists from Tongan Geological Services - set sail at dawn on May 17^th  and headed for the volcano, reaching it in a few hours.  They swathmapped the caldera floor in 3 days. The survey revealed that the caldera floor had dropped about 700 meters. The Jan. 15 eruption had removed 6 and a half cubic kilometers of rock and magma but caused minor changes to the caldera diameter. This is a stupendous change in volume, equivalent to removing about 6500 Empire State Buildings at once.  We don?t have many before-and-after eruption comparisons of caldera but this is definitely one of the big ones, greater than Kraktoa 1883 but smaller than Tambora 1815 eruptions!  It's a good thing that the caldera was surveyed as soon as it could be because it might be filling with new lava.  The sonar survey discovered some reflective plumes on the caldera floor, perhaps due to ongoing submarine eruptions. This volcano is not done yet! More work is needed to see what is happening now on the caldera floor. After 3 days of work, the Pac Horizon retuned to Nukualofa.  This story was told in a 10.5 min. video < https://www.youtube.com/watch?v=i38DkgU0f7s> that was released in Oct. 2022 and has to date been viewed by >131,000 people.

Last Modified: 04/18/2023
Modified by: Robert J Stern