Hunga Tonga-Hunga Ha'apai

Geography & Climate

Hunga Tonga-Hunga Ha'apai is located in Tonga, within the Tofua Volcanic Arc of the broader Tonga-Kermadec Volcanic Regions. Situated at 20.55° S, 175.38° W in the Southern Hemisphere, the volcano lies within a tropical climate zone. At just 114 meters above sea level, Hunga Tonga-Hunga Ha'apai is a relatively low-elevation volcanic feature. Despite its modest height, it remains an active geological formation with the same underlying magmatic processes as its taller counterparts. The volcanic landform is characterized as a caldera, which describes the physical shape and structure of the volcanic edifice as observed from the surface.

Geological Context

Hunga Tonga-Hunga Ha'apai sits in a subduction zone, where one tectonic plate dives beneath another, creating intense heat and pressure that generates magma. Subduction zones are responsible for many of the world's most explosive volcanoes and deadliest eruptions. For communities in Tonga near Hunga Tonga-Hunga Ha'apai, this tectonic setting means the volcano is capable of producing powerful explosive eruptions, pyroclastic flows, and lahars that can threaten populated areas within tens of kilometers of the summit. The dominant rock type is andesite / basaltic andesite, a dark, fine-grained volcanic rock that forms from rapidly cooling, low-viscosity lava. Basaltic eruptions tend to be less explosive and produce fluid lava flows that can travel long distances. While less immediately dangerous than explosive eruptions, basaltic lava flows can destroy structures and infrastructure in their path, and volcanic gases released during these eruptions can affect air quality over a wide area.

Eruption History Summary

Hunga Tonga-Hunga Ha'apai has 7 recorded eruptions in the geological database, spanning from 1110 CE to 2021 CE. The most powerful recorded event was a paroxysmal eruption with devastating regional consequences in 2021 CE, reaching VEI 5 on the Volcanic Explosivity Index. This eruption frequency indicates a moderately active volcanic system with periodic reawakenings. With eruptive activity as recently as 2021 CE, Hunga Tonga-Hunga Ha'apai remains an actively monitored volcano.

Significance

With a maximum recorded VEI of 5, Hunga Tonga-Hunga Ha'apai has demonstrated the capacity for paroxysmal eruptions comparable to the 1980 eruption of Mount St. Helens. Eruptions of this scale can devastate areas within 20-30 kilometers and produce ash fall that disrupts aviation and agriculture across hundreds of kilometers. With 7 recorded eruptions on file, Hunga Tonga-Hunga Ha'apai has enough documented activity to help scientists identify patterns in its eruptive behavior. Given its recent activity, Hunga Tonga-Hunga Ha'apai is closely monitored by geological survey organizations to provide early warning of future unrest.

GVP Reference Summary

The small andesitic islands of Hunga Tonga and Hunga Ha'apai are part of the western and northern remnants of the rim (~6 km diameter) of a largely submarine caldera located about 30 km SSE of Falcon Island. The topmost sequence of welded and unwelded ignimbrite units from a caldera-forming eruption was 14C dated to 1040-1180 CE (Cronin et al., 2017; Brenna et al. 2022). At least two additional welded pumice-rich ignimbrite units and nonwelded pyroclastic flow deposits, below paleosols and other volcaniclastic deposits, indicated more very large previous eruptions (Cronin et al., 2017; Brenna et al. 2022). Several submarine eruptions have occurred at this caldera system since the first recorded eruption in 1912, including 1937 and S of the islands in 1988. A short eruption in 2009 added land to to Hunga Ha'apai. At that time the two islands were each about 2 km long, displaying inward-facing sea cliffs with lava and tephra layers dipping gently away from the caldera. An eruption during December 2014-January 2015 was centered between the islands, and combined them into one larger structure. Major explosive eruptions in late 2021 initially reshaped the central part of the combined island before stronger activity in mid-January 2022 removed most of the 2014-15 material; an even larger eruption the next day sent an eruption plume high into the stratosphere, triggered shock waves through the atmosphere and tsunami across the Pacific Ocean, and left only small remnants of the islands above the ocean surface.

— Smithsonian Institution, Global Volcanism Program