Hunga Tonga–Hunga Haʻapai in the context of "Little Boy"

A caldera (/kɔːlˈdɛrə, kæl-/ kawl-DERR-ə, kal-) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcanic eruption. The ejection of large volumes of magma in a short time can upset the integrity of a magma chamber's structure by in effect removing much of the chamber's filling material. The walls and ceiling of a chamber may now not be able to support its own weight and any substrate or rock resting above. The ground surface then collapses into the emptied or partially emptied magma chamber, leaving a large depression at the surface that may have a diameter of dozens of kilometers. Although sometimes described as a crater, the feature is actually a type of sinkhole, as it is formed through subsidence and collapse rather than an explosion or impact. Compared to the thousands of volcanic eruptions that occur over the course of a century, the formation of a caldera is a rare event, occurring only a few times within a given window of 100 years. Only nine caldera-forming collapses are known to have occurred between 1911 and 2022, with the caldera collapses at Kīlauea, Hawaii, in 2018 and Hunga Tonga–Hunga Haʻapai in 2022 being the most recent. Volcanoes that have formed a caldera are sometimes described as "caldera volcanoes".

An eruption column or eruption plume is a cloud of super-heated ash and tephra suspended in gases emitted during an explosive volcanic eruption. The volcanic materials form a vertical column or plume that may rise many kilometers into the air above the vent of the volcano. In the most explosive eruptions, the eruption column may rise over 40 km (25 mi), penetrating the stratosphere. Injection of aerosols into the stratosphere by volcanoes is a major cause of short-term climate change.

A common occurrence in explosive eruptions is column collapse when the eruption column is or becomes too dense to be lifted high into the sky by air convection, and instead falls down the slopes of the volcano to form pyroclastic flows or surges (although the latter is less dense). On some occasions, if the material is not dense enough to fall, it may create pyrocumulonimbus clouds.

In December 2021, an eruption began on Hunga Tonga–Hunga Haʻapai, a submarine volcano in the Tongan archipelago in the southern Pacific Ocean. The eruption reached a very large and powerful climax nearly four weeks later, on 15 January 2022. Hunga Tonga–Hunga Haʻapai is 65 kilometres (40 mi) north of Tongatapu, the country's main island, and is part of the highly active Tonga–Kermadec Islands volcanic arc, a subduction zone extending from New Zealand to Fiji. On the Volcanic Explosivity Index scale, the eruption was rated at least a VEI-5. 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.

The eruption produced a volcanic tsunami that affected Tonga, Fiji, American Samoa, Samoa, Vanuatu, New Zealand, Japan, the United States, the Russian Far East, Chile and Peru. At least four people were killed, some were injured, and some remain possibly missing in Tonga from tsunami waves up to 20 m (66 ft) high. Tsunami waves with run-up heights up to 45 m (148 ft) struck the uninhabited island of Tofua. Two people drowned in Peru when 2 m (6 ft 7 in) waves struck the coast, while another died of indirect causes in Fiji. It was the largest volcanic eruption since the 1991 eruption of Mount Pinatubo. NASA determined that the eruption was "hundreds of times more powerful" than Little Boy. The eruption was the largest explosion recorded in the atmosphere by modern instrumentation, far larger than any 20th-century volcanic event or nuclear bomb test. It is thought that in recent centuries, only the Krakatoa eruption of 1883 rivalled the atmospheric disturbance produced.