In the last 100,000 years, the nation of Indonesia has consistently been one of the most frequent
sites of large magnitude explosive eruptions.
These range from the Toba supereruption 74,000 years ago, to two caldera forming eruptions
of Krakatau, to the 1815 eruption of Mount Tambora, which caused the so called year without
a summer.
While these three volcanoes are well known, there are also a number of lesser known similarly
explosive counterparts.
Several of these volcanoes can be spotted by the massive depressions relevant eruptions
formed in the ground known as calderas.
One such lesser known caldera is the Ranau caldera, which once produced an eruption at
least 3 times larger than Krakatau's famous eruption in 1883.
This ancient eruption created the modern Lake Ranau, which loosely outlines the caldera
formed by its catastrophic eruption.
Although this specific volcano does not have a single known eruption in recorded times,
it may still have produced volcanic activity.
At least twice in the last century, a strong sulfur odor was detected originating from
the lake, and this coincided with a mass die off fish within the lake.
This does not mean an eruption occurred, but rather that fumaroles showed an increase in
activity within the lake, suggesting that there is still an at depth magma chamber.
The Ranau caldera is located in southern Sumatra, where it is 350 kilometers northwest of the
city of Jakarta.
From satellite, Ranau's most prominent feature is its large caldera, measuring 16.5 kilometers
long and 12 kilometers wide.
This caldera is quite interesting, as it is located along a lengthy strike slip fault
line known as the Great Sumatran fault.
The area where the volcano is and its caldera are bounded by two parallel strike slip faults,
although one of these is no longer active.
This caldera represents what is known as a stepover or pull apart basin.
A stepover forms when strain at the end of one strike slip fault transfers to a parallel
fault, causing the area to spread apart over time.
Thus, any volcanism along this fault can be assumed to occur as the area represents a
path of least resistance, allowing for any molten rock intruding into the crust to erupt
into this weak point.
The exact geologic history of this complex remains unclear, but here is what likely occurred.
Stepovers typically are low points in the terrain, meaning that before this volcano
produced its catastrophic eruption, it likely did not have much of a prominence.
Instead, it likely consisted of a group of distant lava domes, most likely of an andesite
to dacite composition.
Remnants of two such lava domes and viscous flows can be found on the western side of
the caldera.
These formed through highly explosive eruptions which generated frequent pyroclastic flows.
At some point, a massive highly silica rich magma chamber began building underneath Ranau,
involving a volume of more than 100 cubic kilometers of material.
Eventually, this material became primed for a major eruption.
Although we cannot be sure of this, the eventual eruption was likely caused by a large magnitude
earthquake on the Great Sumatran fault.
I am basing this assertion on the fact that the now more highly active Suoh volcano seems
to produce highly explosive eruptions in the weeks following a large magnitude earthquake,
such as in 1933.
Regardless of how the eruption was triggered, a massive eruption began at Ranau 33,640 years
ago.
As a plume of ash shot more than 45 kilometers into the atmosphere, part of the voluminous
eruption column collapsed downwards, producing long reaching pyroclastic flows.
These pyroclastic flows were largely controlled by topography, scorching the landscape and
wiping out all life up to 85 kilometers distant.
For example, the city of Muaradau which is 40 kilometers distant was buried by 4 to 5
meters of ash.
Then, due to the large amount of erupted material, the underlying magma chamber had been severely
drained.
This caused a large section of overlying rock to collapse downwards to fill its empty space,
forming Ranau's modern caldera.
In total, at least 150 cubic kilometers of volcanic rock was ejected, meaning the eruption
had a volcanic explosivity index of a 7.
In the aftermath of this eruption, the Seminung stratovolcano formed in the eastern section
of the caldera, with its most recent activity likely occurring only 5,000 to 10,000 years
ago.
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