A spectacular and explosive volcanic eruption in January 2022 produced the highest plume of steam and ash in recorded history.
The towering column that surged from Hunga Tonga-Hunga Ha’apai reached a whopping 57 kilometers (35 miles) above sea level.
This height makes it the very first volcanic eruption to have completely crossed the stratosphere to pierce the mesosphere.
“It’s an extraordinary result, because we’ve never seen a cloud of this size before,” says atmospheric scientist Simon Proud from the University of Oxford.
Perhaps this should come as no surprise: the eruption was one of the largest volcanic eruptions humanity has ever seen. But measuring the height of its plume with precision required some astute detective work.
The height of a volcanic plume is usually estimated based on the temperature profile measured by satellites making infrared observations. Since thermal emission, or heat, produces infrared radiation, these satellites can detect volcanic plumes.
As the plumes expand through the troposphere (this is the atmospheric layer closest to Earth, the one we live in), they lose heat, so the top temperature of the plume can be used to estimate height.
However, once the plume reaches the stratosphere, at an average altitude of about 12 kilometers, this strategy loses accuracy as the plume’s temperature profile changes again, this time warming up. So a team of researchers led by Proud took a different approach.
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Researchers have always relied on satellite data, but the measurement was based on parallax. If you’ve ever closed one eye after another and watched objects close to you appear to move from side to side relative to their background, you’ve seen parallax in action.
It is the difference between the apparent position of two objects seen from different lines of sight, and is the basis of depth perception in binocular vision. Our brain processes information from each eye and calculates the distance to objects in view. We can use parallax to calculate all kinds of distances.
To obtain parallax measurements of the Hunga Tonga-Hunga Ha’apai eruption, the researchers used data from three geostationary weather satellites that observed the event from different positions in low Earth orbit, taking images every 10 minutes.
From there, Proud and his team calculated that the plume reached an altitude of 57 kilometers. Interestingly, this is very close to the altitude of 58 kilometers calculated by NASA scientists in January using data from two geostationary satellites.
Previously, the highest volcanic plume on record was Mount Pinatubo in the Philippines; its 1991 eruption produced a plume that extended up to 40 kilometers a.s.l.
The much larger height of the Hunga-Tonga plume, however, is a bit confusing, given that Mount Pinatubo’s eruption was of similar strength: both eruptions were recorded as a 6 on the index scale. explosiveness (VEI).
There is an easy answer to this one, however. If the Hunga-Tonga plume had been measured using Mount Pinatubo techniques, the maximum height would have been set at around 39 kilometers.
Even though the Mount Pinatubo plume reached higher than measured, we still don’t know what the mechanisms are to reach this altitude. So that could be a fun topic to explore.
We also don’t know how a volcanic plume of this height would affect the mesosphere; as no other volcanic plumes were observed reaching this level, the effects were only indirect.
A turbid substance was observed at the top of the Hunga-Tonga plume; what it is, and how long it will stay up there, are unknown.
This means that there is still work to be done to help us understand this fascinating and devastating event.
“We would also like to apply this technique to other eruptions and develop a plume height dataset that can be used by volcanologists and atmospheric scientists to model the dispersion of volcanic ash in the atmosphere,” explains atmospheric physicist Andrew Prata of the University of Oxford.
“Other scientific questions we would like to understand are: why did the Tonga plume rise so high? What will be the climate impacts of this eruption? And what exactly was the plume composed of?”
The research has been published in Science.
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