Calderas with their steep walls and large size are found at a wide range of volcanoes,
having largely formed in very large explosive volcanic eruptions that eject 2 cubic kilometers
of more of tephra.
Such features can be found at a select group of stratovolcanoes, such as the Crater Lake
caldera in Oregon.
Yet, some calderas do not form explosively but rather effusively as shown via the 2018
widening and deepening of Kilauea's caldera, which formed due to a large volume flank eruption.
Yet, such effusively formed caldera are almost always found only at shield volcanoes.
This is what makes the following volcano all the more unusual.
As in 2000, a stratovolcano off the coast of mainland Japan known as Miyakejima erupted,
which over the course of approximately a month and a half formed a brand new caldera.
This new caldera measured 1.6 kilometers wide and 330 meters or 1082 feet deep, giving the
volume of the caldera and thus the approximate amount of magma which seemingly vanished underneath
it that caused the collapse to be around 0.64 cubic kilometers.
And here lies the problem.
Although the 3 month long eruption of Miyakejima did erupt explosively with a fairly sizable
ash plume that occurred alongside pyroclastic flows, the amount of tephra the volcano ejected
was a full order of magnitude lower than it should have been to form such a large caldera.
As, despite this being incredibly rare, the caldera at Miyakejima did not primarily form
due to an explosive eruption, but rather due to magma draining away from the volcano and
going elsewhere in a process almost exclusive to some shield volcanoes.
Before I explain why this unusual VEI 3 eruption occurred, I will discuss some basic facts
about the volcano.
Miyakejima is located approximately 180 kilometers south of Tokyo where the volcano makes up
the entirety of an island which is part of the Izu island chain.
Having an area of about 55 square kilometers and being 9.4 kilometers long, approximately
2,500 people live on its landmass.
Like other volcanoes in the Izu island chain, Miyakejima exists due to a tectonic plate
collision to the east.
There, the Pacific Plate is subducting underneath the Philippine Sea Plate.
The melted material from this collision then migrates upwards until it reaches the surface
and erupts, forming a chain of volcanoes.
For much of the last 500 years, Miyakejima produced a moderate to highly explosive eruption
about once every 50 years.
That was until June 26th of 2000 when an earthquake swarm underneath the summit of the Miyakejima
volcano occurred.
Over the next few hours, these quakes slowly migrated away from the summit until they began
occurring further and further away from the volcano in the ocean to the west of the island.
Then, on June 27th a swarm of earthquakes involving 21 magnitude 4 quakes struck a few
kilometers west of the island.
Clearly something unusual was occurring, and we now know that these quakes marked the movement
of magma, which triggered a fairly brief submarine eruption that created patches of discolored
water on the same day.
That submarine eruption ended but after a week seismic activity began increasing again,
with a large cluster once again occurring progressively further west of the island,
continuing to move in that direction.
Then, beginning in mid July, the summit region of the volcano began subsiding, indicating
that a new caldera was starting to form.
We now know that this caldera collapse was triggered not by the submarine eruption which
occurred but rather by at least 2/3rds of a cubic kilometer of magma draining away from
the volcano due to a formation of a 33 kilometer long dike.
As magma moved underneath the ocean surface and did not erupt, additional earthquakes
were created, generating the abundant observed earthquakes.
As shown by the number of large maar explosion craters around Miyakejima which measure up
to 1200 meters wide, this volcano has produced 6 VEI 4 eruptions in the last 10,000 years.
However, these explosive eruptions are generally thought of as too small to have formed the
two calderas known to have formed on this volcanic island before the 2000 caldera in
the last 100,000 years.
Thus, it is quite possible that although they may have formed in explosive eruptions, a
similar subsurface dike like what was witnessed in 2000 may have instead formed these older
calderas.
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