On January 15th, 2022 the Hunga-Honga Hunga Ha'apai volcano in the nation of Tonga was
primed to blow.
The previous day, a landslide had occurred on its ash covered plateau, cutting the island
in two and causing seawater to seep into the underlying magma conduit.
This water was subsequently heated by the adjacent magma, causing it to flash to steam.
Since steam takes up a larger space then the same volume of water, it caused an immense
amount of pressure to build.
Then, at 4:28 pm local time, the volcano catastrophically exploded in what was equivalent in energy
to a 6 megaton blast of TNT.
What had just occurred was the single largest volcanic eruption on the planet since 2011.
Since it is now a month later, us scientists know a bit more about the eruption which occurred
that day, which at first presented a number of unresolved questions.
For example, we initially believed that the eruption plume only reached 35 kilometers
or 115,000 feet in height.
However, NASA later released a statement which showed that the eruption column had reached
even further to an impressive height of 58 kilometers or 190,000 feet in height, which
is more than halfway into space.
This means that the eruption plume surpassed a theoretical limit of 50 kilometers in height
which no prior eruption column had done, passing into the mesosphere.
Typically, the lower boundary of this atmospheric layer largely kills any upwards mobility a
plume has, causing it to disperse and maxing out the cloud's height.
Thus, it was quite a surprise to see the plume temporarily reach a height of 58 kilometers,
as this even surpasses the eruption plume height of the last Toba supereruption.
You heard that correctly, the eruption column was taller than one produced by a supervolcano.
So, why was this plume height able to rise into the mesosphere, defying all expectations?
The answer to this involves several factors.
First, the atmosphere is not distributed evenly around the planet, as at the poles it is thinnest
and at the equator the atmosphere is thickest.
Since the eruption occurred at 20 degrees south, it had the capability to travel beyond
50 kilometers in height.
The second reason involves the depth which the explosion occurred at.
If this same explosion had occurred on land, say high in the Andes Mountain range, it would
have involved far less water and only created a 30 kilometer high eruption plume.
Or, if the explosion had occurred any deeper in the water, the seawater pressure would
have reduced the size of the explosion.
Thus, the eruption occurred in the perfect location and perfect depth, which was amplified
in size by surrounding seawater.
Third, the explosion began at depth and cascaded upwards in a straight vertical line, being
funneled upwards by the adjacent magma conduit.
Since the eruption occurred, more thorough analysis has been made of Hunga Tonga's older
eruptions.
New analysis shows that the volcano produced similar major explosive eruptions which potentially
formed calderas in both 200 and 1100 AD.
The major eruption which occurred in 2022 seemingly follows this 900 year long pattern,
which in each case culminated in a single or series of VEI 4 to 5 eruptions.
Speaking of calderas, it is quite possible that the caldera deepened during Hunga Tonga's
eruption.
Looking at its two remaining islands, satellite data suggests that they are uniformly 10 meters
shorter than they were before.
This amount of subsidence would perfectly align with expected values based on half a
cubic kilometer of magma being ejected that day and the caldera's total area.
However, these figures are potentially misleading.
It is also possible that the entire caldera rim partially slid to the south giving off
the appearance of subsidence occurring.
Or, the explosion may have been more uneven than we first thought, conveniently blowing
off the top 10 meters of the islands while leaving the rest intact.
As for the still ongoing earthquakes adjacent to Hunga Tonga, these have been confirmed
as caused by magma refilling its underlying magma chamber which was partially emptied
by January's eruption.
While this could culminate in another powerful explosion, the refilling process is likely
to take several years to decades, so another imminent eruption seems unlikely.
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