On the afternoon of December 4th, at 3:20 pm local time, the Semeru volcano in Indonesia
produced a large explosive eruption.
A lava dome on this volcano collapsed to the southeast, causing an immense amount of superheated
gas to be released via what is termed a pyroclastic flow.
These pyroclastic flows raced downhill and spread out in all directions although much
of this material remained confined to a large river channel.
By the time these flows stopped, they had charred a length of 11 kilometers.
As a vast amount of land surrounding the volcano was covered by more than 30 centimeters of
loose ash, subsequent rainfall swept much of this material away, generating a hazard
known as a "lahar".
As they traveled downslope, they overflowed river channels and flooded buildings with
in some locations depositing several meters of rock.
Since the recently deposited ash was still superheated from the eruption, the subsequent
lahars were not only actively boiling when they flowed downhill but also highly acidic.
One of these lahars led to the destruction of the Gladak Perak bridge 13 kilometers to
the southeast of Semeru's summit.
The following two days, more smaller volume pyroclastic flows occurred, which traveled
up to 4 kilometers southeast from the volcano.
What had just occurred was a large "pelean" style eruption.
It was the same type of eruption which took the life of 2 famous volcanologists in 1991
when a large pyroclastic flow was released from Mount Unzen.
By their very nature, Pelean style eruptions are inherently difficult to predict.
They occur when a flow of lava with a high viscosity, that is more similar to honey than
that of water, builds on top of an active vent.
Eventually, it becomes so large and thick that gasses are unable to escape the vent,
causing an immense amount of pressure to build.
As the lava dome continues to grow, it eventually becomes oversteepened until finally part or
all of it collapses, which allows the gasses to escape and generate pyroclastic flows.
In the case of the December 4th eruption, this final collapse was directly triggered
by several days of heavy rainfall, which heavily eroded the summit lava dome.
In other words, the lava dome at Semeru was already oversteeped, and was going to produce
a large eruption at some point in the next several weeks regardless of the weather.
However, a period of heavy rainfall triggered the eruption a few days to weeks early by
eroding just enough of the lava dome's base for it to finally collapse.
Because this collapse was triggered by such a weak force, it suggests a method which could
be used to prevent additional large eruptions.
Theoretically, an already weak lava dome could be artificially triggered to collapse a few
days or weeks early.
If this could be accomplished earlier rather than later, the resulting eruption would be
far smaller and cause significantly less damage.
Collapsing part of a dome could be accomplished by precision perfect placement of mining explosives
on the edge of its already weak structure.
After all residents in an at risk area are evacuated, the explosives would be detonated,
hopefully causing an eruption.
One issue with pelean style eruptions is that it causes civilians in harms way to wait for
weeks or even months for an eruption to occur, during which time many will return, and once
again be in harm's way.
If scientists could instead say for example "we will trigger an eruption on September
21st, pack your valuables" it would vastly increase the effectiveness of an evacuation
order as people like certainty.
Of course, several factors must all be present for this methodology to work.
First, it can only be attempted on a volcano which erupts viscous lava, and has a growing
lava dome as not many volcanoes on the planet produce viscous lava flows.
Second, the lava dome in question would need to be relatively small, as this method would
quickly lose its effectiveness on much larger masses of rock such as this several kilometer
wide lava dome.
Several hundred meters across at a maximum works best.
Third, the lava dome would already need to be close to collapsing.
Determining whether this is the case is truly difficult, but a level of warning is possible.
If all of these factors are in place, then scientists could attempt to trigger an eruption.
Thanks for watching.
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