[1][12] Described by scientists as a "magma hammer", the volcano at its height produced a series of four underwater thrusts, displaced 10 cubic kilometres (2.4 cu mi) of rock, ash and sediment, and generated the largest atmospheric explosion recorded by modern instrumentation.

[8] The eruption caused tsunamis in Tonga, Fiji, American Samoa, Vanuatu, New Zealand, Japan, the United States, the Russian Far East, Chile and Peru.

[18] On 22 and 23 December 2021, 8-to-14-kilometre-high (5.0 to 8.7 mi) plumes containing sulfur dioxide drifted to the north-north-east and spread over the Niuatoputapu, Haʻapai and Vavaʻu island groups.

[52] Preliminary observations showed that the eruption column ejected a large amount of volcanic material into the stratosphere, leading to speculation that it would cause a temporary climate cooling effect.

An April 2022 research paper led by Poli and Shapiro and published by the American Geophysical Union indicates that the eruption is the largest ever observed with modern instrumentation and estimates its VEI to be approximately 6.

[64] Likewise, a study by Diaz and Rigby estimates the energetic output of the eruption to be equivalent to 61 megatons of TNT, making the event more powerful than the largest nuclear bomb ever detonated (Tsar Bomba).

In May 2022, scientists at the National Institute of Water and Atmospheric Research (NIWA) released a bathymetry map indicating a large caldera measuring 4 km (2.5 mi) in width formed from the eruption.

The study analyzed data from 22 tide gauges, eight Deep-ocean Assessment and Reporting of Tsunamis (DART) stations, eight atmospheric pressure time series, spectral analysis and computer simulation.

[71] In mid-May 2024, University of Rhode Island oceanagrapher Roxanne Beinart published a study in the journal Nature Communications Earth and Environment which concluded that the eruption blanketed the surrounding ocean floor with ash for several months, decimating the local marine wildlife.

[75] The column ejected a large quantity of water into the stratosphere, where it disturbed the local temperature balance and caused the formation of anomalous winds.

In contrast, during the Hunga Tonga–Hunga Haʻapai eruption this sulfur was accompanied by large amounts of water vapour, which by acting as a greenhouse gas overrode the aerosol effect and caused a net warming of the climate system.

[77] One study estimated a 7% increase in the probability that global warming will exceed 1.5 °C (2.7 °F) in at least one of the next five years,[78] although greenhouse gas emissions and climate policy to mitigate them remain the major determinant of this risk.

[93] In the wake of the tsunami, King Tupou VI was evacuated from the Royal Palace and traffic jams formed as locals fled inland or to higher ground.

[95] Satellite imagery, digital elevation models and synthetic aperture radar suggest tsunami runups of 45 m (148 ft) at the southern coast of Tofua.

A group of people fishing in Hokianga Harbour had to run for their lives to escape the waves, and reported having to drive through water over 1 m (3 ft 3 in) deep.

[121][123] Significant waves hit the Santa Cruz Harbor,[124] and its parking lot was flooded with about 0.91 m (3 ft 0 in) of water,[123] while Soquel Creek in the neighboring city of Capitola flowed backwards.

[130] Minor tsunamis were measured as far away as the Caribbean Sea and Texas, with National Oceanic and Atmospheric Administration reporting a maximum rise of 12 cm (4.7 in) at the Isla de Mona in Puerto Rico at 16:11 UTC.

Videos and images on social media from the Los Ríos Region showed the tsunami damaging piers, carrying boats and hitting beaches.

[91] Fiji's Attorney General Aiyaz Sayed-Khaiyum urged the public to stay indoors and cover household water tanks in the event of rain due to the risk of fallout of sulfuric acid from the SO2 emitted in the air by the eruption.

[168] Chile also issued a warning for a "minor tsunami" for most of its coastal area, including the island of Rapa Nui; evacuation was declared for twelve other regions.

[182] Tattoo parlour owner Angela Glover, a British resident in Tonga, was among the people missing, swept away by the tsunami when it hit Nukuʻalofa.

[196] Though the extent of the damage in Tonga is still not clear, a blanket of thick ash has contaminated water supplies, cut off communications and prevented surveillance flights, making it difficult for relief efforts to begin.

[207] New Zealand prime minister Jacinda Ardern had earlier stated that an undersea cable serving Tonga was affected, probably due to power cuts, and authorities were urgently attempting to restore communications.

[210] On 4 February, the Associated Press reported that Fonua stated that repair crews would need to replace 87 km (54 mi) of cable, and that he hoped to have it restored the following week.

[219] The shores of Nukuʻalofa had substantial damage as debris and rocks were deposited inland by the tsunami waves, according to an early report from the UN Office for the Coordination of Humanitarian Affairs (OCHA).

Two Boeing P-8A Poseidon maritime patrol aircraft and a Lockheed C-130J Hercules of the Royal Australian Air Force departed on the morning of 17 January 2022 for Tonga to survey damage to roadways, ports and power lines.

[267] On 6 February, The New Zealand Herald and the Fiji Broadcasting Corporation's FBC News reported that SpaceX engineers were working to restore Internet access in Tonga.

New Zealand National Party Member of Parliament Shane Reti had earlier petitioned SpaceX CEO Elon Musk for assistance in providing Starlink satellite technology to the island country.

The Fijian Minister for Communications Aiyaz Sayed-Khaiyum subsequently confirmed that SpaceX engineers would establish and operate a temporary ground station in Fiji to assist with efforts to restore Internet access in Tonga.

[281][282] Tongan Olympian flagbearer Pita Taufatofua, who became widely known during the 2016 Summer Olympics, gathered more than US$330,000 in aid to his native country, after he opened a GoFundMe fundraising website.