On January 15th, 2022, the evening in the nation of Tonga was interrupted by the sound
of a powerful explosion.
To the northwest of the island of Tongatapu, the Hunga Tonga Hunga Ha'apai volcano violently
exploded with a force equivalent in energy to the simultaneous detonation of 23 million
tons of TNT.
Subsequently, the world's tallest eruption plume ever measured shot to a height of 58
kilometers or 190,000 feet.
Part of this eruption column then collapsed downwards, creating energetic pyroclastic
flows which traveled across more than 20 kilometers of ocean, generating a series of highly destructive
tsunamis which were measured at up to 20 meters or 66 feet in height.
These tsunamis would go on to cause damage around the entire Pacific Ocean, and the sound
of the blast was heard as far away as the city of Fairbanks and sections of Yukon in
Canada.
What had occurred was clearly historical in size, representing one of the planet's largest
recent explosive eruptions since the industrial age began.
However, due to a number of strange features and peculiarities of the eruption, many people,
myself included, incorrectly underestimated the size of the eruptive sequence.
It is now 5 months later, and a team of scientists has thoroughly mapped the seafloor surrounding
the volcano.
Of the area mapped, fresh ash and sediment was found over an area of more than 8,000
square kilometers, with a volume of 7 cubic kilometers.
This displaced new material had a volume of 7 cubic kilometers.
However, this total included sediment and was not all ash, and of what was ash, some
of the material was remobilized older ash from the edifice of the volcano.
When I asked researcher Shane Cronin, he stated that at a minimum 85% of the fresh material
on the seafloor was new ash.
In other words, the eruption ejected at least 6 cubic kilometers of tephra, indicating a
volcanic explosivity index of a 5, meaning this was the planet's largest explosive eruption
since 1991.
But wait, there is even more unexpected new information!
Beforehand, the caldera of Hunga Tonga Hunga Ha'apai had been thoroughly mapped, measuring
4 kilometers wide and having a maximum depth of 150 meters.
It was initially expected that this eruption only deepened the existing caldera on the
order of 10 to 30 meters.
Yet, when the caldera floor was measured, it came back that the entire caldera floor
was now at least 700 meters deeper.
Assuming that the caldera floor was filled in by at least 50 meters of ash and sediment,
this indicates that the caldera lost a whopping 750 meters or 2,460 feet in height!
Taking this into account, we can get our best estimate of how large the eruption truly was.
By taking the area of the caldera and multiplying this by the depth lost, we get a figure known
as the dense rock equivalent or DRE.
The DRE was measured to be between 6 and 6.5 cubic kilometers.
This can be a bit confusing, but this figure is not the amount of material which erupted.
1 cubic meter of andesite composition magma tends to create between 2.2 and 2.5 cubic
meters of ash, as the ash is less dense than the original magma.
Taking this into account, the total volume of erupted material was between 13.2 and 16.25
cubic kilometers of volcanic rock.
This means the eruption was had a volcanic explosivity index of a 6, and was the largest
to occur on the planet since Novarupta erupted in 1912.
Yes, it was even larger than the 1991 eruption of Pinatubo.
Another unexpected piece of evidence revealed that in all likelihood, this volcano is still
erupting, just to a far lesser extent than seen on January 15th.
Within the newly deepened caldera, especially on the northwest side, a series of vents are
actively emitting dense columns of ash observed to be 50 to 60 meters in height.
One factor which initially threw off many scientists as to the true size of this phreatoplinian
eruption was the meager amount of sulfur dioxide released, totaling about 650,000 tons.
Although the eruption was sulfur dioxide poor, much of the apparently missing gas instead
is assumed to have dissolved in the water, becoming compounds such as sulfuric acid.
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