Hunga Tonga–Hunga Haʻapai was an uninhabited volcanic island that existed in the South Pacific from 2009 to 2022, located about 30 km (19 mi) south of the submarine volcano of Fonuafoʻou and 65 km (40 mi) north of Tongatapu, Tonga's main island. The previously twin islands of Hunga Tonga and Hunga Haʻapai were merged by a volcanic eruption in 2009, and a more explosive eruption separated the islands again and reduced them in size in 2022. The erupting volcano is part of the highly active Tonga–Kermadec Islands volcanic arc, a subduction zone extending from New Zealand north-northeast to Fiji. It lies about 100 km (62 mi) above a very active seismic zone. The island arc is formed at the convergent boundary where the Pacific Plate subducts under the Indo-Australian Plate.
The volcano itself is a submarine volcano that breached sea level in 2009 due to a volcanic eruption and lies underwater between two islands, Hunga Tonga and Hunga Haʻapai, which are respectively the remnants of the western and northern rim of the volcano's caldera. The caldera is roughly 150 m below sea-level, and rises 2000 m from the sea-floor. The two islands (part of the Haʻapai group) are about 1.6 km (0.99 mi) apart, and each is about 2 km (1.2 mi) long and composed largely of andesite. This andesite tends to be of the basaltic type. Hunga Tonga reaches an elevation of 149 m (489 ft), while Hunga Haʻapai comes to only 128 m (420 ft) above sea level. Neither island is large: Hunga Tonga was roughly 390,000 m2 (0.15 sq mi) and Hunga Haʻapai 650,000 m2 (0.25 sq mi) in size before they were connected. 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.