THERE’S A HOLE IN THE BOTTOM OF THE SEA: A VIDEO ABOUT TWO ATTEMPTS TO MAP THE HUNGA TONGA-HUNGA HA’APAI CALDERA AFTER THE JAN. 15 2022 ERUPTION

Eruption of the Hunga Tonga – Hunga Ha’apai (HH-HT) volcano in the Tonga island arc was one of the most amazing natural events of our time. Scientists knew that the eruption changed the volcano significantly and suspected that the submarine caldera changed most. Fortunately we knew the “before” because a 2015 bathymetric survey revealed flat caldera seafloor at 160mbsl, but we needed to know the “after”. Resurvey of the caldera was needed as soon as it was safe. Marine geoscientific cruises are normally scheduled years in advance, so another approach was needed. Two efforts were independently launched: a co-operative US-NZ effort, involving the first and second authors and one led by the third author. Because the water depth in the caldera was unknown but could be very shallow, both used small ships. In the first case the ship was the 28m-long R/V Kaharoa. In the second case, the 19m MV Pacific Horizon was used. Neither ship had a multibeam sonar system needed to survey the caldera so these had to be added before getting underway. The Kaharoa left Wellington NZ on April 5 to begin its 2400 km voyage north to the volcano. The crew tested the sonar a few days into the trip and found that it wasn’t working so they turned back to New Zealand where divers inspected the hull, discovering the sonar had disappeared. Apparently, the ship had sailed over a log which removed the sonar; this effort was disappointingly stumped. Meanwhile, Cronin was 65 km from HH-HT in Nuku’alofa where he and his team had tried twice to find the bottom of the caldera, first with a 9m fishing boat and sonar that could image to 300m depth, then with a Tongan Navy patrol vessel with a 800 m-range single beam sonar that still couldn’t see the caldera bottom. Finally, in early May, Cronin’s team installed a sonar system on the 19m MV Pacific Horizon and mapped the caldera floor during 3 days in mid-May, finding that it was now 860 mbsl, also finding sonar evidence for hydrothermal plumes along its northern margin, suggesting new lava is flowing into the caldera. A drop of 700m in the caldera floor during one event is remarkable, indicating that about 6.5 km^3, of rock and magma from beneath the caldera floor was removed. These and other results were reported at the EGU meeting in late May, 2022 by Cronin and his team. This video will share some of the excitement and disappointment of these mapping efforts.