Satellites observe highest volcano plume ever

A view of the volcano 100 minutes after eruption began. (Courtesy: Simon Proud / Uni Oxford, RALSpace NCEO / Japan Meteorological Agency)

The eruption of the Hunga Tonga-Hunga Ha’apai volcano in 2022 was the tallest ever recorded, with a volcanic plume that reached almost 58 km in height. According to physicists at Oxford University and RAL Space in the UK, who measured the plume using data from a trio of geostationary satellites, it was also the first to pass through the Earth’s stratosphere and enter the lower mesosphere. The measurements, which are described in Science, shed new light on how volcanic eruptions affect the climate and reveal fresh information about a layer of Earth’s atmosphere that remains poorly understood.

Hunga Tonga-Hunga Ha’apai is situated in the Tongan archipelago in the southern Pacific Ocean. On 15 January 2022, it violently erupted, ejecting an extremely tall column of ash, gases and water into the atmosphere. Until now, however, no one knew exactly how tall it was.

The parallax effect

To measure the plume’s height, physicists led by Simon Proud used parallax, which is the same mechanism our two eyes use to perceive depth. The three weather satellites observed the volcano from different positions, imaging it from multiple angles. These images, which the satellites recorded every 10 minutes during the eruption, enabled the physicists to build a three-dimensional picture of the top of the plume.

“Usually, plume heights are calculated by comparing the plume temperature (measured by a satellite) to the atmospheric temperature (from weather models),” Proud explains. “This eruption went so high that the temperature technique is too inaccurate. Since the parallax method needs nothing more than simple geometry, it is well suited to measuring these types of extreme eruption where other approaches fail.”

Proud adds that parallax-based estimations are only possible with good satellite coverage. They therefore wouldn’t have been possible a decade ago.

An altitude of nearly 58 kilometres

The team’s measurements showed that the plume reached an altitude of nearly 58 kilometres at its highest extent, breaking records set by the 1991 eruption of Mount Pinatubo in the Philippines (40 km) and the 1982 El Chichon eruption in Mexico (31 km). It is also the first plume ever recorded as reaching the mesosphere, which starts at roughly 50 km above the Earth’s surface.

“The mesosphere aspect is important since this part of the atmosphere is usually very dry,” Proud says. “The Hunga-Tonga eruption put water, ash and sulphur dioxide there.”

The amount of water ejected into the mesosphere could have implications for the climate as it might lead to surface warming, he adds. “Because we don’t have much knowledge of the upper atmosphere, the eruption also serves a bit like a natural laboratory, to help us better understand the processes up there.”

Proud and colleagues say they would now like to use their parallax method to study other eruptions. Their goal is to develop a dataset of plume heights that volcanologists and atmospheric scientists can use to model how volcanic ash disperses in the atmosphere. “The technique could be used to measure eruptions (and even severe thunderstorms) as they happen,” Proud tells Physics World.