Hunga Tonga–Hunga Haʻapai () is a submarine volcano in the South Pacific located about south of the submarine volcano of Fonuafoou and north of Tongatapu, Tonga's main island. It is part of the highly active Kermadec-Tonga subduction zone and its associated volcanic arc, which extends from New Zealand north-northeast to Fiji, and is formed by the subduction of the Pacific Plate under the Indo-Australian Plate. It lies about above an active seismic zone. The volcano rises around 2,000 m from the seafloor and has a caldera which on the eve of the 2022 eruption was roughly below sea level and at its widest extent. The only major above-water part of the volcano are the twin uninhabited islands of Hunga Tonga and Hunga Haʻapai, which are respectively part of the northern and western rim of the caldera. As a result of the volcano's eruptive history, the islands existed as a single landmass from 2015 to 2022: they were merged by a volcanic cone in a volcanic eruption in 2014–2015, and were separated again by a more explosive eruption in 2022, which also reduced the islands in size. The Hunga Tonga–Hunga Haʻapai volcano has seven historical recorded eruptions.
The most recent eruption, in January 2022, triggered a tsunami that reached the coasts of Japan and the Americas, along with a volcanic plume that soared 58 km (36 miles) into the mesosphere. It was the largest volcanic eruption since the 1991 eruption of Mount Pinatubo and the biggest explosion recorded in the atmosphere by modern instrumentation, far surpassing any 20th-century volcanic event or nuclear bomb test. NASA determined that the eruption was "hundreds of times more powerful" than the atomic bomb dropped on Hiroshima. It is believed that the 1883 eruption of Krakatoa is the only eruption in recent centuries that rivaled the atmospheric disturbance it produced. The January 2022 eruption is the largest volcanic eruption in the 21st century.
Volcano and caldera
Hunga Tonga–Hunga Haʻapai is a submarine volcano in the Kermadec-Tonga Ridge in South Pacific, a ridge formed by the convergent boundary where the Pacific Plate is subducted by the Indo-Australian Plate, forming a long volcanic and island chain. Hunga Tonga–Hunga Haʻapai volcano lies almost completely underwater, with the exception of two small volcanic islands, Hunga Tonga and Hunga Haapai. They are, respectively, the remnants of the northern and western rim of the volcano's caldera.
The volcano's base at the seafloor is approximately 20 km in diameter, rising roughly 2,000 m towards the sea surface. Before the 2022 eruption, the volcano's caldera was roughly 150 m below sea-level, and had a size of 4 × 2 km. Its northern and southern portions were filled by volcanic deposits from previous eruptions. The caldera is believed to have been formed by this eruption. Before the 2015 eruption, the two subaerial islands were about apart, and were each about long. They are both composed largely of andesite and layered tephra deposits,Gupta, Alok K. Igneous Rocks. Mumbai, India: Allied Publishers, 1998. Ewart, A.; Bryan, W.B.; and Gill, J.B. "Mineralogy and Geochemistry of the Younger Volcanic Islands of Tonga, S.W. Pacific." Journal of Petrology. 14:3 (1973). with steep rocky cliffs. This andesite tends to be of the basaltic type. The 2009 eruption arose from two vents located to the south and northwest of Hunga Haʻapai. The tephra deposited around each vent became connected to the island and nearly tripled its size, but such deposits disappeared with erosion in the following months. Bathymetric data indicate the growth of lava domes along the western caldera rim and mass movements on the northwestern rim between 2008 and 2016. The volcano is also frequently recorded by hydrophones when it generates acoustic noise underwater.
Geography Islands
Hunga Tonga–Hunga Haʻapai are the only subaerial parts of the volcano. Hunga Tonga is the eastern island, while Hunga Haʻapai is the western one. They are part of Tonga's Haʻapai group of islands, an island arc formed at the convergent boundary where the Pacific Plate subducts under the Indo-Australian Plate.Ewart, A. "A Petrological Study of the Younger Tongan Andesites and Dacites, and the Olivine Tholeiites of Niua Fo'ou Island, S. W. Pacific." Contributions to Mineralogy and Petrology. 58:1 (January 1976) Hawkesworth, C.J.; Turner, S.P.; McDermott, F.; Peate, D.W.; and van Calsteren, P. "U-Th Isotopes in Arc Magmas: Implications for Element Transfer from the Subducted Crust." Science. 276:5312 (25 April 1997).
Before the 2014–15 eruption, which connected them into a single island, the islands were separated by about of ocean water. Before the 2022 eruption, the highest point in the former Hunga Tonga reached an elevation of , while Hunga Haapai was only above sea level. Neither island was large; before they were connected in 2015, each island was about long, with Hunga Tonga being roughly and Hunga Haapai being in size. They are much smaller after the 2022 eruption. Neither island was developed due to a lack of an acceptable anchorage, although there were large guano deposits on each island.
After the 2015 eruption, the smaller Hunga Tonga island, approximately to the northeast of Hunga Haapai, became attached to the crater via a -wide tombolo, and further sandy deposits had built up at the southern end of the crater's connection with Hunga Haapai. The caldera itself has eroded rapidly in the southeast, originally allowing an opening that flooded the crater with seawater to form a bay. This bay later became separated from the open ocean by a shallow sandbar, forming a lagoon. Initially it was believed that the entire island would be eroded rapidly, but by 2017, scientists believed that the process could take decades.
The islands figure in Tongan mythology as one of the few islands which were not overfished, and hence thrown down from heaven to land on earth. Tongans called them the islands which "jump back and forth" (i.e. suffer earthquakes). The first Europeans to see the islands were those with the Dutch explorers Willem Schouten and Jacob Le Maire in 1616. The British explorer Captain James Cook visited them several times in 1777 and learned their Tongan names.
Scientific study
In June 2017, French explorers Cécile Sabau and Damien Grouille landed on the island from their sailing boat COLIBRI. Aside from taking some of the very few pictures of the island before it was decimated by the 2022 eruption, they collected a total of 16 rock samples, documented with GPS plotting and 3D pictures. This material was studied by scientists at NASA Goddard Space Flight Center, led by Dr James B. Garvin. They studied Hunga Tonga–Hunga Haapai, using it as a model for volcanic shapes on Mars, and concluded that it eroded in ways that are remarkably similar to the erosion patterns seen on similar landforms on Mars. This suggested Mars was once flooded briefly by water, but that the water receded fairly quickly. They said that further study of the similarities between Hunga Tonga–Hunga Haapai and Martian volcanic landforms was needed.
Another analysis of the samples showed that the volcanic ash that forms much of Hunga Tonga–Hunga Haapai reacted with the warm oceanic water around it. This chemical reaction turned the ash into much harder rock, and volcanologists believed the island would last for several decades rather than be eroded. This made Hunga Tonga–Hunga Haapai only the third volcanic island in the last 150 years to survive more than a few months.
In October 2018, scientists visited the island and discovered that its surface was covered with gravel, sticky mud, and vegetation. The island was also populated by a variety of birds. They also found that the island seemed to be eroding more quickly than previously thought due to rainfall.New Tonga island 'now home to flowers and owls' , bbc.co.uk/news, accessed 7 February 2019 The survey also found that coral reefs around the volcano quickly recovered from the eruption.
Ecosystem
Surveys in 2019 found ferns, guava trees, mosses, sedges, silky jackbean and Asteraceae species growing on the island formed in 2014 and on Hunga Tonga–Hunga Haʻapai . Cockroaches, hermit crabs, moths, rats and spiders lived on the islands. Marine animals included humpback whales, reef sharks, sea snakes, whale sharks and numerous fish species.
Before it was destroyed by the 2022 eruption, Hunga Tonga–Hunga Haʻapai were covered with grasslands, coconuts and bush. They are a breeding ground for seabirds; bird species identified include barn owls, brown boobies, brown noddies, great frigatebird, lesser frigatebird, Pacific golden plover, Polynesian starling, red-footed boobies, red tailed tropicbirds, ruddy turnstone, sooty terns and white tailed tropicbirds. Marine debris occurs on the islands.
The dominant coral genera around Hunga Tonga–Hunga Haʻapai are Acropora, Pocillopora and Porites. Volcanic activity has impacted coral reefs around the islands, but impacts are locally variable and there is robust recruitment of new corals at numerous sites where they were wiped out. Birds quickly resettle after volcanic eruptions as well.
Eruptions
Pre-21st century Hunga Tonga–Hunga Haʻapai erupted underwater at its rocky shoal in 1912 and 1937, and south of this shoal in 1988. During the 1988 eruption, three vents formed a long alignment and produced ash, steam and tephra. One pyroclastic flow was dated to 1040–1180 CE, correlating to ash deposits found on Tongatapu, and to an unknown tropical eruption in 1108 CE that had produced 1 °C of global cooling.
2009 eruption
On 16 March 2009, a submarine eruption near Hunga Tonga–Hunga Haapai began spewing steam, smoke, pumice, and ash thousands of feet into the sky. The eruption devastated Hunga Haapai, covering it in black ash and stripping it of vegetation and fauna. Between 18–20 March, a number of Surtseyan eruptions sent ash plumes as high as to into the atmosphere, with prevailing winds pushing the ash cloud about east-northeast of the eruption site and widespread and significant haze reported at Vavau away. Steam plumes on 20 March were measured at above sea level. By 21 March, Tonga's chief geologist, Kelepi Mafi, reported lava and ash issuing from two vents – one on the uninhabited island Hunga Haapai and another about offshore. The eruption had filled the gap between the two vents, creating new land surface that measured hundreds of square metres.
Two Air New Zealand airline flights into Tonga were delayed due to safety concerns caused by the volcanic ash, but flight schedules returned to normal shortly thereafter."Night Flights Postponed As Volcanic Activity Continues." Matangi Tonga. 23 March 2009 ; "Flights Back to Normal But Tongan Volcanoes Still Erupting." Radio New Zealand International. 24 March 2009. Tongan officials also expressed concern that the eruption could significantly harm the country's fishing industry. The volcanic eruption drew worldwide attention. The volcano was featured in a segment of the television program Angry Planet in 2009.Angry Planet, "Entertainment", Cape Argus, 20 November 2009
2014–2015 eruption
A new eruption began at Hunga Tonga–Hunga Haapai on 19 December 2014. Local fishermen reported a tall white steam plume rising from the ocean over the undersea volcanic mount. Satellite images taken on December 29 showed the eruption continuing, with a smoke and ash plume rising from the site, and discolored seawater (possibly caused by smoke and ash released below the surface, or by disturbance of the seabed).; The eruption continued into 2015, with a tall ash cloud rising into the sky on 6 January 2015.