March 20, 2023
Hunga Tonga-Hunga Ha'apai eruption releases highest volcanic plume ever recorded

Hunga Tonga-Hunga Ha’apai eruption releases highest volcanic plume ever recorded

A spectacular and explosive volcanic eruption in January 2022 produced the highest plume of steam and ash in recorded history.

The towering column that emerged from Hunga Tonga-Hunga Ha’apai reached a whopping height of 57 kilometers (35 miles) above sea level.

This height makes it the first volcanic eruption that appears to have completely pierced the stratosphere to breach the mesosphere.

“It’s an extraordinary result, as we’ve never seen a cloud of any type this high before,” says atmospheric scientist Simon Proud of the University of Oxford.

This perhaps shouldn’t come as a surprise: The eruption was one of the largest volcanic eruptions mankind has ever seen. But measuring his plume height accurately took some clever detective work.

The height of a volcanic cloud is usually calculated 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 extend through the troposphere (this is the atmospheric layer closest to Earth, the one we live in), they lose heat, so the temperature of the top of the plume can be used to estimate height.

However, once the plume reaches the stratosphere, at an average altitude of about 12 km, this strategy loses accuracy because the temperature profile of the cloud changes again, this time becoming warmer. So a group of researchers led by Proud took a different approach.

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The researchers still relied on data from satellites, but the measurement was based on parallax. If you’ve ever closed one eye after another and watched objects near 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 the objects we see. 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 km altitude that NASA scientists calculated 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 in altitude.

The much higher height of the Hunga-Tonga plume, however, is a bit puzzling given that the Mount Pinatubo eruption was similar in power: Both eruptions registered as a 6 on the Volcanic Explosivity Index (VEI) scale.

There is an easy answer to this, however. If the Hunga-Tonga plume had been measured using Mount Pinatubo techniques, the maximum height would have been set at about 39 km.

Even if Mount Pinatubo’s crest reached higher than measured, however, we still don’t know what the mechanisms are to reach that altitude. This could be a fun topic to explore.

We also don’t know how a volcanic plume of this height would affect the mesosphere. Since no other volcanic plume has been observed to reach this high, the results were only indirect.

A cloudy substance was observed at the top of the Hunga-Tonga plume. what it is, and how long it will hang there, is unknown.

This means more work needs 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 data set that can be used by volcanologists and atmospheric scientists to model the dispersion of volcanic ash in the atmosphere,” says atmospheric physicist Andrew Prata of the University of Oxford.

“Further scientific questions we would like to understand are: Why did the Tonga plume rise so high? What will be the climatic effects of this eruption? And what exactly was the plume made of?”

The research has been published in Science.

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