First Results from Survey of Underwater Volcano Near TongaChanges in seas around volcano and current condition of ecosystems
A press conference was held on May 23 at New Zealand’s National Institute of Water and Atmospheric Research (NIWA) in Wellington to announce the findings of the first part of a joint research project being carried out by The Nippon Foundation and NIWA to survey the area around the Hunga Tonga-Hunga-Haʻapai underwater volcano, which erupted in January 2022, and surrounding regions off the coast of Tonga.
The first part of the project was carried out from April 9 to May 6 using a NIWA manned research vessel to survey the features of the eruption, changes in the topography of the volcano and surrounding sea floor, the scope and accumulation of volcanic ash, and the current status of ecosystems in the surrounding seas. The vessel mapped the sides of the volcano and surrounding sea floor using multibeam sonar, conducted geological surveys that involved taking samples and analyzing sediment, seawater and rocks, and took still photographs and videos of the volcano, surrounding seas, and sea life using sophisticated underwater cameras.
The second part of the project will be carried out from mid-June to mid-July using an unmanned surface vehicle (USV) to survey the area around the crater, and the data from the two surveys will then be analyzed together. Understanding the effects of the eruption will help with recovery in Tonga, for which the ocean is a valuable resource, and is also expected to lead to improved disaster preparedness in many countries including Japan.
The main findings of the manned survey were as follows.
Features of the eruption
- The shape of the erosion scars on the upper slopes of the volcano supports the occurrence of pyroclastic flows (extremely hot mixtures of ash, pumice, and gas flowing at high velocities). Large deposits more than 20 kilometers from the volcano show that the pyroclastic flows reached distances far from the volcano’s peak.
Changes to the topography of the underwater volcano and surrounding seas
- Given the scale of the eruption, major changes to the shape of the volcano had been expected, but except for one portion near the crater that was blown off by the eruption, the shape was mostly unchanged. On the other hand, there were significant changes in the surrounding area.
- The survey collected topographical data for a 22,000 square kilometer area around the volcano. Comparing this with data taken after the previous eruption (in 2016) showed that the topography had changed in roughly 8,000 square kilometers of the area surveyed (Figure 1).
A total of roughly 7 cubic kilometers of new sediment was confirmed on the sea floor of the surveyed area (red portion in Figure 1), of which roughly 2-3 cubic kilometers is believed to have been lost from the sea floor (blue portion in Figure 1), with the remaining 4 cubic kilometers believed to have been lost from areas around the crater. (A better understanding of the topography around the crater is expected after the unmanned survey.)
Effect on ecosystems
- No living organisms were observed on the volcano’s sides or in the deep slopes extending to the southwest and northwest corresponding to the pyroclastic flows (Photo 1 and marked with an “x” in Figure 2).
- However, diverse and abundant fish and other populations were seen around the summits of underwater peaks about 15 kilometers from the area directly affected by the eruption, and corals and other invertebrates were observed on the slopes of those peaks (Photo 2). The researchers believe the fish may be migrating fish that have returned, but the slow rate of growth of the invertebrates means that they could not have grown to the level observed in the time since the eruption, leading to the conclusion that they survived the eruption. In addition, a group of mussels living near hydrothermal vents on a peak in the southernmost portion of the survey area appeared healthy despite being covered by some ash, and was in the exact same location as observed in a previous survey in 2007 (Photo 3). These results show the adaptability of deep-sea life to their environments, and will be useful in forecasting the recovery of ecosystems going forward.
Post-eruption volcanic activity
- A roughly 15-meter layer of volcanic ash was observed extending downward from a depth of 200 meters. The density of the ash was highest near the north side of the crater (Photo 3), and decreased as it got further from the volcano. In addition, the fact that thermal anomalies were detected in high-density areas indicates that extremely small amounts of volcanic material continue to seep from the volcano. Nevertheless, the risk of another major eruption is seen being low for some time.
Plans going forward
- Unmanned survey of area around the crater using an unmanned surface vehicle (USV)
- Period: June–July 2022 (30 days during this period)
- Location: Crater and surrounding area (in permitted areas only)
- Contents: Seafloor topology mapping
- Analysis of data from research vessel and USV and announcement of results
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