A spectacular and explosive volcanic eruption in January 2022 produced the highest steam and ash plume in recorded history.
The towering column that rose from Hunga Tonga-Hunga Ha’apai reached a tremendous altitude of 57 kilometers (35 miles) above sea level.
That height makes it the first volcanic eruption ever seen to go entirely through the stratosphere to break through the mesosphere.
“This is an extraordinary result, as we have never seen a cloud of any kind this high before.” says atmospheric scientist Simon Proud from the University of Oxford.
This perhaps shouldn’t come as a surprise: the eruption was one of the largest volcanic eruptions humanity has ever seen. But measuring the height of his plume accurately required some clever detective work.
The height of a volcanic column is usually estimated based on the temperature profile measured by satellites taking infrared observations. Since thermal emission, or heat, produces infrared radiation, these satellites can detect volcanic plumes.
As plumes spread through the troposphere (that’s the atmospheric layer closest to Earth, which we live in), they lose heat, so the temperature of the top of the plume can be used to estimate the height.
However, once the plume reaches the stratosphere, at an average altitude of about 12 kilometers, this strategy loses accuracy because the temperature profile of the plume changes again, this time becoming warmer. So a team of researchers led by Proud took a different approach.
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The researchers still relied on satellite data, but the measurement was based on parallax. If you’ve ever closed one eye after another and noticed that objects close to you appear to move from side to side compared to their background, you’ve seen parallax in action.
It is the difference between the apparent position of two objects seen along different lines of sight and is the basis of depth perception in binocular vision. Our brain processes the information from each eye and calculates the distance to objects in sight. 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 this, Proud and his team calculated that the plume reached an altitude of 57 kilometers. Interestingly, this is very close to the 58 kilometer altitude that NASA scientists calculated. back in january using data from two geostationary satellites.
Previously, the tallest volcanic column on record was Mount Pinatubo in the Philippines; its 1991 eruption produced a plume that extended to 40 kilometers in altitude
However, the much greater height of the Hunga-Tonga plume is a bit puzzling, given that Mount Pinatubo’s eruption was similar in strength: both eruptions registered as 6 on the Volcanic Explosivity Index (VEI) scale.
However, there is an easy answer to this one. If the Hunga-Tonga plume had been measured using the Mount Pinatubo techniques, the maximum height would have been set at about 39 km.
Even if the Mount Pinatubo plume reached higher than measured, we still don’t know what the mechanisms are for reaching that height. So that could be a fun topic to explore.
We also don’t know how a volcanic column of that height would affect the mesosphere; since no other volcanic column has been observed to reach that height, the effects have only been indirect.
A cloudy substance was observed in the upper part of the Hunga-Tonga plume; what that is and how long it will stay up there is unknown.
This means there is more 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.” says atmospheric physicist Andrew Prata from 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 made of?”
The research has been published in Sciences.
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