Hunga Tonga-Hunga Ha’apai eruption created highest volcanic plume ever recorded: ScienceAlert

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

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

That height makes it the first-ever volcanic eruption to be completely ejected through the stratosphere to breach the mesosphere.

“It’s an extraordinary result, because we’ve never seen such a high cloud,” says atmospheric scientist Simon Proud from the University of Oxford.

This should perhaps come as no surprise: The eruption was… one of the largest volcanic eruptions mankind has ever seen. But accurately measuring the plume’s height took some clever detective work.

The height of a volcanic plume is usually estimated from the temperature profile measured by satellites that conduct infrared observations. Because thermal emission or heat produces infrared radiation, these satellites can detect volcanic plumes.

As plumes extend through the troposphere (that’s 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 its height.

However, once the plume reaches the stratosphere, at an average height of about 12 kilometers, this strategy loses accuracy as the plume’s temperature profile changes again, this time warmer. So a team 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 seen objects close to you that seem to shift 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 it 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 and captured images every 10 minutes.

From this, Proud and his team calculated that the plume reached a height of 57 kilometers. Interestingly, this is very close to the 58-kilometer altitude that NASA scientists have calculated back in January using data from two geostationary satellites.

Previously, the highest volcanic plume ever was Mount Pinatubo in the Philippines; to be 1991 eruption produced a plume that extended to 40 kilometers at height.

However, the much higher elevation of the Hunga-Tonga plume is a bit baffling, as the Pinatubo eruption was similar in strength: Both eruptions were recorded as a 6 on the Volcanic Explosivity Index (VEI) scale.

However, there is a simple answer to this. If the Hunga-Tonga plume had been measured using the Mount Pinatubo techniques, the maximum height would have been set at about 39 kilometers.

Even if Mount Pinatubo’s plume is higher than measured, we still don’t know the mechanisms for reaching that height. So that could be a fun topic to explore.

We also don’t know how a volcanic plume from that height would affect the mesosphere; since no other volcanic plume this high has been observed, the effects have only been indirect.

A hazy dust was observed at the top of the Hunga-Tonga plume; what that is and how long it lingers up there is unknown.

This means that there is more work to be done to help us understand this fascinating and devastating event.

“We also want 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 distribution of volcanic ash in the atmosphere,” says atmospheric physicist Andrew Prata from the University of Oxford.

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

The research was published in Science.