Tonga eruption’s volcanic plume reached above the stratosphere twice

The plume ejected by the Hunga Tonga-Hunga Haʻapai volcano in January entered the mesosphere, the layer of atmosphere above the stratosphere, twice during the eruption

By Madeleine Cuff

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The giant plume of ash, gases and water ejected by the Hunga Tonga-Hunga Haʻapai volcano during its eruption in January was so tall it entered the mesosphere, satellite data has confirmed.

The volcanic plume was 57.5 kilometres high at its peak, the highest ever recorded for a volcano. It entered the mesosphere – which sits above the stratosphere – twice during the eruption, once at 4:30 UTC on 15 January, and again at 4:50 UTC the same day.

The volcano is mainly underwater in a remote part of the Pacific Ocean. It is marked above the water surface by two small, uninhabited islands. Even so, the eruption killed six people. Loud booms were heard as far away as New Zealand and Australia.

“We were very lucky that the eruption happened in a remote place,” says Simon Proud at the University of Oxford. “If this had happened somewhere more populated it would have been incredibly devastating.”

A view of the Tonga eruption, 10 minutes after it started, taken by Japan’s Himawari-8 satellite

Simon Proud / Uni Oxford, RALSpace NCEO / Japan Meteorological Agency

Usually, scientists rely on the temperature of a volcanic cloud – and the surrounding clouds – to calculate how high a volcanic plume reaches. But with the Hunga Tonga-Hunga Haʻapai eruption the plume was too high for this method to work.

Instead, Proud and his colleagues used satellite data and the parallax effect, where the exact location of objects can shift according to where they are viewed from, to help calculate the exact height of the plume.

“We’re using images from a few different weather satellites that all look at a different angle at this volcano,” he says. “If you have different estimates from different angles, you can figure out the height very exactly.”

The findings confirm a NASA analysis published in February, which used only two satellites and suggested the eruption was 58 kilometres high at its peak. Proud says the latest assessment is more robust.

Volcanic eruptions that reach the mesosphere are incredibly rare, and this is the first time scientists have been able to confirm an occurrence. The last eruption on this scale is likely to have been the 1883 eruption of Krakatoa.

Proud says this new method of measuring volcano plume height could also be useful to assess the height of storm clouds as they gather steam, and so estimate their severity.