Significance of the Hunga Tonga-Hunga Haʻapai volcanic eruption and tsunami
15 January 2022 saw the extremely powerful eruption of the Hunga Tonga-Hunga Haʻapai (HTHH) submarine volcano in the remote Kingdom of Tonga in the South Pacific. Producing a volcanic explosivity thought to be equivalent to VEI 5, this eruption was an order of magnitude greater than the 1991 eruption of Pinatubo in The Philippines (Cronin et al. 2022) and represents the biggest eruption of a submarine volcano for nearly one and a half centuries since the cataclysmic destruction of Krakatau in Indonesia in AD 1883. The Tongan eruption is therefore globally significant. Described by Klein (2022) as a one-in-a-thousand year event, Kusky (2022) asks whether a future possible HTHH eruption might even be able to eclipse the devastating 1650 BC eruption of Thera (Santorini) in the eastern Mediterranean. Amongst its various remarkable characteristics, Tonga’s HTHH eruption generated ocean-wide tsunamis, never before recorded in the Pacific instrumental record. Although at least eight known volcanic source mechanisms are recognised for volcanic tsunamigenesis (Paris 2015; Gusman and Roger 2022), it can be said that volcanically generated tsunamis still remain a ‘blind spot’ in our understanding of tsunami hazards. This is particularly relevant for the Pacific Ocean because of its circum-Pacific ring-of-fire, absent in the Atlantic and Indian oceans. The Tongan event thus presents a rare chance to investigate the multiple processes contributing to volcanic tsunamigenesis, that occurred both synchronously and asynchronously with the initiating eruption.
It is appreciated that both the knowledge pool and scientific literature specific to the January 2022 HTHH eruption are rapidly growing. In spite of the fact that field-based studies in Tonga have been hampered by the global Covid pandemic, several research groups have already begun looking into many facets of the event from different perspectives, especially volcanological, atmospheric, tsunamigenic, geospatial, and numerical modelling aspects (for example Amores et al. 2022; Burt 2022; Carvajal et al. 2022; Cronin et al. 2022; Harrison 2022; Tanioka et al. 2022; Yuen et al. 2022; Zuo et al. 2022). This spontaneous surge in scientific interest is also evidenced by the dedicated scientific sessions focusing exclusively on the Tongan eruption and tsunamis scheduled at the 2022 conferences of both the European Geophysical Union (EGU 2022) and the Asia Oceania Geosciences Society (AOGS 2022), in May and August 2022, respectively. Acknowledging this groundswell of interest provides the motivation here. The current aim is to summarise some of the most prominent features of the Tongan volcanic event for science, and at the same time highlight some key gaps that can be identified in field, modelling and theoretical aspects, which are now being addressed by the scientific community.
Setting of the Hunga Tonga-Hunga Haʻapai volcano
Hunga Tonga-Hunga Haʻapai (HTHH) is one of several active volcanoes in the Kingdom of Tonga, an island archipelago nation in the South Pacific. The volcano lies at 20°32.7′S 175°23.6′W, 65 km to the NNW of Tongatapu island and the Tongan capital Nukuʻalofa. HTHH is an active stratovolcano formed as a result of westward subduction of the Pacific tectonic plate beneath the Australian plate along the Tonga-Kermadec (TK) submarine trench, an oceanic trench in the southwestern Pacific stretching between Samoa and New Zealand (Fig. 1). The trench exhibits the fastest convergence rate of tectonic plates globally, at up to 24 cm year−1. All of Tonga’s volcanic islands rise from the Tofua Ridge, which is a typical volcanic frontal island arc with an axis running parallel and some 150 km to the west of the TK trench (Nunn 1998).
The HTHH volcanic complex is almost entirely submarine, marked by two small islands standing on the rim of its drowned caldera (Fig. 2). The volcanic edifice rises some 1800 m from the surrounding seafloor, as seen on bathymetric maps of the region (Chase et al. 1982; GEBCO 2014). The drowned caldera is approximately 5 km across and reaches down to water depths of more than 200 m at its centre.
Recent volcanic activity and eruption
The HTHH volcano has been active over recent decades (Bohnenstiehl et al. 2013) (Fig. 2). Volcanic activity from December 2014 to January 2015 was characterised by a period of constructional growth (Garvin et al. 2018; Hite et al. 2020), and was responsible for a new subaerial cone that joined separate Hunga Tonga and Hunga Haʻapai islets into a single island approximately 5 km wide. The latest phase of eruption from 28 December 2021 to 15 January 2022 is the sixth phase to be recorded since AD 1900, and was appreciably more explosive than its historical precursors (Cronin 2022).
Powerful eruptions on 14 January 2022 were of Surtseyan type. This is an explosive style of volcanism (Thorarinsson 1966) that occurs in the hydro-explosive zone within the top 500 m or so of the ocean surface (Nunn 1994), where hot magma rising rapidly interacts explosively with water (Colombier et al. 2018). Antecedent minor eruptions may have signified the magma system slowly recharging itself in advance of a big event (Brenna et al. 2022). The explosive Surtseyan-type eruptions of 14 January 2022 were likely caused by interaction between very hot (probably andesitic) magma (in the range 900-1100 °C) and seawater at depths of up to 150 m within the drowned caldera. Water pressure at this depth is about 15 bars, which would allow steam instantaneously generated by seawater mixing with fresh magma to boil at around 200 °C, expanding rapidly upwards as an extremely violent uprush of the resulting tephra-steam mixture, evidencing phreatomagmatic activity. Heavy ash fall was observed at the Tongan capital Nuku‘alofa.
The most violent eruptions, however, commenced the following day around 5:00 pm local time (4:00 am UTC) on 15 January 2022 (NASA 2022b; GVP 2022a; Fig. 3). The largest blast about 15 min after eruption onset was recorded as an extremely shallow earthquake of magnitude Ms 5.8 by the USGS. Thunderclap-like bangs deafened Nuku‘alofa and the thump of the explosions was heard as far away as Anchorage in Alaska on the northern Pacific Rim. Thousands of intense lightning flashes pierced the ash cloud at record-breaking rates, with more lightning created than by any other process previously recorded (Cronin et al. 2022). At least one commentator has speculated that the 15 January eruption may have reached intensity 5 on the Volcanic Explosivity Index (VEI 5 intensity) (RNZ 2022), considerably more explosive than its twentieth century predecessors that did not exceed VEI 2 (GVP 2022b). If so, the 2022 Tongan eruption would be the largest eruption globally for 30 years, since the 1991 eruption of Mt Pinatubo in The Philippines, as well as being the largest submarine explosive eruption since the AD 1883 eruption of Krakatau, between Java and Sumatra in Indonesia.
The new ash plume from this Plinian-style eruption reached well into the mesosphere, before it was distorted by upper level winds. Data from NOAA’s GOES-17 and JAXA’s Himawari-8 satellites were analysed by NASA scientists, who calculated that the plume rose to 58 km, thereby breaking records for the tallest volcanic plume in the satellite record (NASA 2022a; Yuen et al. 2022). The initial umbrella cloud expanded to 500 km in diameter (NASA 2022b), thereafter increasing in area to 12 million km2 by 19 January as it dissipated downwind, reaching as far away as East Africa, a distance of 15,000 km, by 22 January (GVP 2022b).
Cronin et al. (2022) believe that the enormous explosive power of HTHH’s final eruption (so far) cannot be explained by the interaction of magma and water alone, and that a major caldera eruption of fresh magma highly charged with gas was responsible. Their previous age-dating work of old HTHH deposits, chemically matched to volcanic ash deposits on Tongatapu, has shown that substantial caldera eruptions have occurred approximately once every thousand years, the last around AD 1100. For the sequence of eruptions on 15 January 2022, Cronin’s team (Cronin et al. 2022) have developed a hypothetical eruption model that identifies a series of distinct stages in what was clearly a highly complex multi-phase event. Initial seismic signatures are suggestive of an ‘opening trapdoor’ mechanism in the SE sector of the caldera that first decompressed the magma chamber. Satellite imagery shows the initial blast radiated shockwaves most strongly towards the east. Subsequent inward collapse of the caldera then rapidly squeezed out hot, fragmenting magma. This caused the most violent eruptions, destroying most of the spatial extent of Hunga-Tonga and Hunga-Haʻapai islands and cutting down their surface elevations by stripping away some 10-50 m of pyroclastic deposits. Petrology, geochemistry and glassy signatures in the poorly sorted ash are indicative of an extremely fast and complete evacuation of a heterogeneous zoned magma chamber by violent phreatomagmatic activity (Cronin et al. 2022). Perhaps surprisingly, in spite of the large volumes of gas and ash ejected, the approximate SO2 output of 0.4 Tg (4 × 105 tons) from HTHH was modest, and is therefore unlikely to cause temporary global cooling, as otherwise experienced after the violent 1815 Tambora and 1991 Pinatubo volcanic eruptions (Zuo et al. 2022).
Volcanically generated tsunamis
The violent eruption of HTHH on 15 January generated a series of tsunami waves experienced in both proximal and far-field locations. Waves arrived first at the nearby islands of Tonga within 15-30 min of the largest blast. Dramatic videos shot by residents of Nuku‘alofa and elsewhere on Tongatapu were soon posted on social media platforms. These clips captured the initial waves arriving at the coast, rushing on land and sweeping through streets and buildings. The Tongan government reported that the biggest waves up to 15 m high struck the west coast of Tongatapu, as well as ‘Eua and some islands in the Ha‘apai group. Drone footage posted by the Tonga Geological Services (TGS) of Tongatapu’s western beaches similarly indicated that waves reached 15 m, while on Mango Island (75 km to the ENE of HTHH) runup 12 m high washed over the church tower and penetrated 500 m inland (GVP 2022c). Photographs taken during aerial reconnaissance flights by the New Zealand Defence Force revealed the extent of damage, both along affected coastlines and in adjacent low-lying areas inland. Coastal dwellings and infrastructure suffered heavy damage. Three people in Tonga sadly lost their lives to the tsunami waves. Elsewhere, tide gauges installed in ports and harbours of capital cities in surrounding South Pacific Island nations, including Fiji, American Samoa, Cook Islands and Vanuatu, measured wave heights of 0.2-1.2 m (Table 1). For the Hawaiian Islands in the central Pacific, the highest wave heights were recorded on the northern shores of Kauai (Hanalei: 0.82 m) and Maui (Kahului: 0.83 m), with tsunami deposits up to 20 cm thick reported at Kahului Harbour, at an elevation of 2 m above mean low tide and 10 m from the shoreline, overlying the carbonate sand substrate (S. Fisher, field notes).
Perhaps more remarkable was that locations thousands of kilometres distant around the Pacific Rim also experienced these volcanically generated tsunamis many hours later. Over 7000 km away in the NW Pacific, Japan recorded tsunamis along eastern coasts of both its southern and northern islands. Tsunami heights of 1.2 m and 1.1 m were observed in Kagoshima and Iwate prefectures, respectively. Thirty boats were sunk in Kochi Prefecture. Commentators noted that tsunamis arrived unexpectedly, up to 2 h earlier than predicted (Matsumoto 2022). On the opposite side of the Pacific Rim, more than 10,000 km distant from the Tongan eruption, coasts of Peru and Chile in the SE Pacific experienced 2 m tsunamis, causing two further fatalities.