Indonesia as a country for many years has contained more actively erupting volcanoes
than any other nation.
As of June 11th of 2023, this number was 7 volcanoes which included the following.
Mount Merapi and Mount Semeru on the island of Java.
The island volcano of Krakatau.
The Ibu and Dukono volcanoes on Halmahera.
Lewotolok on Lembata island, and Karangetang on Siau island.
Then, on June 13th, an 8th volcano in the country began erupting, this time on the unusually
shaped island of Sulawesi.
The new eruption occurred at an interesting twin peaked stratovolcano known as Lokon.
I want to note that this name choice is somewhat unusual as Lokon has not produced an eruption
in recorded history while Mount Empung to the north produced its last eruption in the
18th century.
Thus, all other recent eruptions of this volcano have instead occurred at a 650 meter or 2130
foot wide complex explosion crater partially carved into the edge of Mount Lokon's northern
flank.
Known as Tompaluan Crater, it in the last 100 years has consistently erupted an average
of once every four years.
This crater was also the site of the newest eruption, which began with a bang as an explosion
of material originating on the crater floor occurred, causing a plume of ash to rise to
a height of 470 meters or 1,542 feet above the crater rim edge.
More smaller explosions occurred thereafter but something was seemingly missing; lava.
As, the latest eruption was not magmatic but rather, phreatic.
It was triggered indirectly by magma or heated rock at depth, which caused intense heat to
rise into a body of underlying groundwater.
This groundwater eventually gained so much heat that it flashed to steam.
Since the same mass of steam takes up far more volume than the water it once was, it
causes an immense amount of pressure to build.
This pressure eventually causes rock to crack via a path of least resistance which is almost
always upwards, causing ash and volcanic gasses to be expelled in a phreatic eruption.
Intermittent weaker phreatic explosions have been ongoing since the first one on June 13th,
creating more several hundred meter high ash plumes.
As a result of the hazards associated with this volcanic activity as it could produce
additional explosions with little or no warning, a 1500 meter exclusion zone was implemented
aroun Tompaluan Crater in all directions.
You might note that Tompaluan Crater is elongated to the north-northeast, seemingly aligning
with two craters on Mount Lokon, a lava flow, and the summit of Mount Empung.
I believe this to not be mere coincidence but rather repesents what once was or still
is a dike body of magma which feeds the overall volcanic complex.
This dike parallels a tectonic plate boundary to the east, which is why the Lokon volcano
exists in the first place.
This plate boundary is somewhat unusual as the tectonic plate is moving towards the east
and the west at the same time at different sections of it, which in the case of the island
of Sulawesi collided with the Sunda Plate.
As a tectonic plate was subducted underneath the Sunda Plate, parts of it melted and rose
into the crust as magma, eventually erupting onto the surface and forming a chain of volcanoes.
We do not know exactly how long this double peaked volcano has been erupting for, but
I think it is safe to say that Mount Lokon is older than Mount Empung.
The summit region of Lokon appears rather smoothed over, and with a lack of fresh looking
ash or lava flow deposits, this peak has probably not erupted iny my opinion for more than 5,000
years.
On the other hand, Mount Empung has a considerable number of young looking albeit forested lava
flows on its slopes of a likely andesite to dacite composition.
These viscous lava flows have travelled up to a distance of 4.1 kilometer away.
In the last century, the only active vent aka Tompaluan Crater has produced numerous
small to moderately sized explosive eruptions, a select few of which required evacuations.
However, these larger explosions were for the most part linked to lava dome growth related
explosions and since no lava dome is actively growing, it appears unlikely for now that
any very large explosions will occur, just smaller ones in my opinion.
This eruption of Tompaluan Crater could eventually progress to a magmatic eruption, but clear
cut warning signs of this are not yet present.
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