We never did get a chance to talk about the Hunga Tonga-Hunga Ha‘apai eruption on Jan. 15, 2022. Tonga is in one of the most seismically active areas of the world, due to high rates of convergence between the Australia and Pacific plates – up to something like 150 to 240 mm/yr along the Tonga trench. The Kermadec-Tonga subduction zone generates large earthquakes on the interface between the descending Pacific and overriding Australia plates, within the two plates themselves and, less frequently, near the outer rise of the Pacific plate east of the trench.
Since 1900, forty (40) M7.5+ earthquakes have been recorded in this region, mostly north of 30°S (the middle line in the figure – the significant uptick in earthquakes is quite visible.) You can determine which plate is overlapping which by looking at the depth of the foci. One of the largest normal fault earthquakes ever recorded, an M8.1, occurred here on September 29, 2009, south of Samoa and 40 km east of the Tonga trench. It generated a tsunami that killed at least 180 people.
Hunga Tonga (HT) and Hunga Ha’apai (HH) are located about 30 km SSE of Falcon Island. They are part of the western and northern remnants of the rim (~6 km diameter) of a largely submarine caldera from an eruption 14C dated to 1040-1180 CE .


There had been a series of small eruptions in December, 2021 and some relatively powerful blasts shook Hunga Tonga-Hunga Ha‘apai on January 13, but it was an even more intense series of explosions early on January 15 that captured global attention, generating atmospheric shock waves, sonic booms, and tsunami waves that traveled the world. The data from this extraordinary eruption was still being collected when classes started and we never got a chance to go back and look at it, so I thought I would pull together some links for you to find out more.
According to estimates by NASA scientists, the blast was equivalent to around 4–18 megatons of TNT and produced atmospheric waves that rippled through the ionosphere and made the entire atmosphere ring like a bell. Unfortunately, there were no active seismometers in Tonga, so the quakes that happened around Hunga Tonga–Hunga Ha‘apai were not tracked in detail, though the existing suggested that the quakes following the January 15th eruption were generated by fresh magma rising into the crust to refill the reservoir. Hopefully something has been deployed by now.
While the eruption threw ash high into the atmosphere, to a height of at least 30 kilometers thus into the stratosphere, the volcano didn’t emit enough sulfur dioxide to change global climate, as some other major eruptions have.

Credits: NASA Earth Observatory image by Joshua Stevens using GOES imagery courtesy of NOAA and NESDIS”

Author: Tim Schmit, NOAA/NESDIS/ASPB

All in all, this eruption was a bit of a game changer and will be the subject of intense study for years to come. It should also lead to some rethinking about the hazards presented by these understudied volcanoes and how our monitoring resources are deployed.
References / Additional Resources
Tonga Geological Services, Government of Tonga Facebook page https://www.facebook.com/tongageologicalservice
Hunga Tonga-Hunga Ha‘apai Erupts https://earthobservatory.nasa.gov/images/149347/hunga-tonga-hunga-haapai-erupts
Hunga Tonga-Hunga Ha‘apai listing on Smithsonian Global Volcanism Program website https://volcano.si.edu/volcano.cfm?vn=243040
Tonga Eruption Made Waves in Earth’s Ionosphere https://eos.org/research-spotlights/tonga-eruption-made-waves-in-earths-ionosphere
Tonga Volcano Plume Reached the Mesosphere https://earthobservatory.nasa.gov/images/149474/tonga-volcano-plume-reached-the-mesosphere
Dramatic Changes at Hunga Tonga-Hunga Ha‘apai https://earthobservatory.nasa.gov/images/149367/dramatic-changes-at-hunga-tonga-hunga-haapai
NASA Mission Finds Tonga Volcanic Eruption Effects Reached Space https://www.nasa.gov/feature/goddard/2022/sun/nasa-mission-finds-tonga-volcanic-eruption-effects-reached-space
Scientists Unravel How the Tonga Volcano Caused Worldwide Tsunamis https://www.quantamagazine.org/scientists-unravel-how-the-tonga-volcano-caused-worldwide-tsunamis-20220413/
Explosive Tonga volcano ‘surprisingly intact’ https://www.bbc.com/news/science-environment-61529290
Nature News – Why the Tongan eruption will go down in the history of volcanology https://www.nature.com/articles/d41586-022-00394-y
Tonga volcano eruption created puzzling ripples in Earth’s atmosphere https://www.nature.com/articles/d41586-022-00127-1