Hunga Tonga–Hunga Haʻapai

Hunga Tonga–Hunga Haʻapai is a submarine volcano in the South Pacific located about 30 km south of the submarine volcano of Fonuafoou and 65 km 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 100 km above a very 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 150 m below sea level and 4 km 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 VEI 2 volcanic eruption in 2014–2015, and were separated again by a more explosive eruption in 2022, which also reduced the islands in size.

Its most recent eruption in January 2022 generated a tsunami that reached as far as the coasts of Japan and of the Americas and a volcanic plume that reached 58 km, into the mesosphere. As of February 2024 the eruption is the largest volcanic eruption in the 21st century. Hunga Tonga–Hunga Haʻapai likely had a previous major explosive eruption in the late 11th or early 12th century (possibly in 1108). Several known historical eruptions occurred in 1912, 1937, 1988, 2009, 2014–15 and 2021–22.

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. Before the 2015 eruption, the two subaerial islands were about 1.6 km apart, and were each about 2 km long. They are both composed largely of andesite and layered tephra deposits, with steep rocky cliffs. This andesite tends to be of the basaltic type.

Samples from the islands suggest a long eruptive history. 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. The caldera is believed to have been formed by this eruption. Submarine eruptions at a rocky shoal – about 3.2 km southeast of Hunga Haapai and 3 km south of Hunga Tonga – were reported in 1912 and 1937; both eruptions had a Volcanic Explosivity Index (VEI) of 2. Another eruption in Hunga occurred from a fissure 1 km south-southeast of Hunga Haapai island in 1988; this eruption had a VEI of 0. 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.

Islands
Hunga Tonga and 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.

Before the 2014–15 eruption, which connected them into a single island, the islands were separated by about 1.6 km of ocean water. Before the 2022 eruption, the highest point in the former Hunga Tonga reached an elevation of 149 m, while Hunga Haapai was only 128 m above sea level. Neither island was large: before they were connected in 2015, each island was about 2 km long, with Hunga Tonga being roughly 390000 m2 and Hunga Haapai 650000 m2 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 1.5 km to the northeast of Hunga Haapai, became attached to the crater via a 380 m-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 has later become 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.

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. 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 100 m offshore. The eruption had filled the gap between the two vents, creating new land surface that measured hundreds of square metres. The eruption devastated Hunga Haapai, covering it in black ash and stripping it of vegetation and fauna.

The volcanic eruption drew worldwide attention. The volcano was featured in a segment of the television program Angry Planet in 2009.

2014–2015 eruption
From November to December 2014, volcanic plumes and a series of earthquakes at volcanoes occurred north of Tonga for several weeks, indicating resumed volcanic activity in the area.

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 3 km into the sky on 6 January 2015.

The eruption entered a new stage on 11 January 2015, when the volcano began sending ash plumes as high as 9 km into the sky. An Air New Zealand flight on 12 January had to be diverted to Samoa, while a number of other flights between New Zealand and Tonga were cancelled. An ash plume reached 4.5 km on January 13. Officials identified two vents, one on Hunga Haapai and another about 100 m offshore and underwater. Large rocks and wet, dense ash were being ejected up to 400 m into the air. By 16 January, a new island had been formed by the explosion. Tongan officials estimated the new island to be 1 km wide, 2 km long, and 100 m high, although geologists said the new island would probably exist only a few months until ocean waves wore it down. Ash and acid rain were falling in an area about 10 km from the new island, and Hunga Tonga and Hunga Haapai had both been denuded of vegetation.

Despite the volcano's eruption, which was spewing a steam cloud 1 km into the air, international flights to Tonga resumed on 16 January, as volcano and aviation experts deemed the eruption no longer a threat to airliners.

Geologists from Tonga and New Zealand who visited the volcano on January 19 said the eruption had quieted in the last 24 hours. They noted that nearly all the eruption was now coming from the vent on the new island, with steam clouds rising to a height of 7 to(-), and ash and rock being thrown to a height of about 200 to(-). Emission of ash was limited, with magma rocks hitting the ocean causing some steam explosions. The team found no floating volcanic debris, such as pumice rafts, and the smell of volcanic gases was intermittent. Tongan officials established a zone 20 km in diameter around the island to protect visitors from rock, ash, and acid rain.

Tongan officials declared the eruption at an end on 26 January, after observing no new gas, ash, or rock emerging from the island vent. By this time, the island was 1 to 2 km wide, 2 km long, and 120 m high. The new island had joined with Hunga Haapai, and was about 200 m away from joining with Hunga Tonga. Locals visiting the island said seabirds were nesting.

In June 2015, entrepreneur Ian Argus Stuart became the first person to overnight on this new island formation. Spending 11 nights on the island, Stuart survived eating nothing but seagull eggs and squid. Stuart went to Hunga Tonga with the help of the Spanish explorer Álvaro Cerezo, who provides castaway experiences to remote desert islands around the planet.

Post–2015 scientific study
In June 2017, French explorers Cécile Sabau and Damien Grouille landed 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. In an article published in late 2017, the scientists concluded that Hunga Tonga–Hunga Haapai eroded in ways that are remarkably similar to the erosion patterns seen on similar landforms on Mars. The scientists noted that 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 bird life. They also found that the island seemed to be eroding more quickly than previously thought, due to rainfall. The survey also found that coral reefs around the volcano quickly recovered from the eruption.

December 2021–January 2022 eruption
On 20 December 2021 the volcano erupted, causing a large plume that was visible from Nukuʻalofa. The Volcanic Ash Advisory Centre Wellington issued an advisory to airlines. Explosions could be heard up to 170 km away. The initial eruption continued until 2 am on 21 December. Activity continued, and on 25 December, satellite imagery showed that the island had increased in size.

As activity on the volcano decreased, it was declared dormant on 11 January before restarting on 14 January after the volcano sent an ash cloud 20 km. The Tongan government subsequently issued a tsunami warning. On the next day, the volcano violently erupted again, about seven times more powerfully than the eruption on 20 December 2021. The initial volcanic plume rose to 58 km into the mesosphere, the greatest height ever reported for a vapor plume. There were numerous reports of loud booms across Tonga and other countries, such as Fiji and as far away as New Zealand and Australia. A boom was heard in Alaska, 10,000 km from the source seven hours after eruption. The Met Office in the UK also detected shockwaves from the eruption. The eruption set off a massive atmospheric shock-wave travelling at about 300 m per second. The energy released by the eruption has been calculated using various methods and thus have led to a range of values. While one study estimated a release equivalent to 61 megatons of TNT, more powerful than the largest nuclear bomb ever detonated, another study proposed at least five blasts, two of relatively small size around 4:00 UTC (<0.1 Mt) and three increasingly larger ones at 4:06 (0.5 Mt), 4:18 (4 Mt), and 4:56 (15 Mt). Near the eruption, the explosion damaged property, including shattered windows. A tsunami warning was issued just after 5:30 p.m. by the Tonga Meteorological Services and the tsunami flooded coastal areas in Tonga.

Two people were killed in Peru and two fishermen were injured in San Gregorio, California. Four deaths were confirmed in Tonga, including a British woman whose body was found after she went missing when the tsunami struck.

According to a report in the journal Geophysical Research Letters, while comparable to other volcanic eruptions on some measures, the 2022 Hunga Tonga–Hunga Haʻapai eruption sent unprecedented amounts of water vapor into the stratosphere.

On 15 January a satellite radar survey showed that most of the island had been destroyed, and only small parts remained. These included remnants of Hunga Tonga and Hunga Haʻapai.

A survey of the caldera by a remote operated vehicle in August 2022 found continuing signs of volcanic activity.

In August 2022, a NASA report on the January 2022 eruption of Hunga Tonga–Hunga Haʻapai stated, "The huge amount of water vapor hurled into the atmosphere, as detected by NASA’s Microwave Limb Sounder, The excess water vapor injected by the Tonga volcano... could remain in the stratosphere for several years... may have a small, temporary warming effect... would not be enough to noticeably exacerbate climate change effects." Later it was found that the excess water vapor from the 2022 eruption would remain in the stratosphere for about 8 years, and help making the 2023 ozone hole one of the largest and most persistent in history. In addition, research suggests that the water vapor could influence winter weather across the globe for several years.