One of the largest explosive volcanic eruptions to ever occur on the planet originated from
the Toba supervolcano in Indonesia.
Approximately 74,000 years ago, it ejected 2,800 cubic kilometers of molten rock, nearly
wiping out the human race, and causing a 1,000 year long cooling episode around the globe.
While this supervolcano is well known, Indonesia has in fact a second lesser known supervolcano
which is hiding in plain sight.
Located on the very northeastern edge of the island of Sulawesi, the Tondano caldera is
surrounded by 4 highly active volcanoes such as Soputan.
These volcanoes are all thought to in some manner share a common magma chamber with the
still active supervolcano.
Although Tondano has not erupted in historic times, evidence that it is still active can
be found at several locations within its caldera via a series of active fumaroles.
While the caldera of the Lake Toba supervolcano measures 100 kilometers long and 35 kilometers
wide, Tondano's caldera is similarly massive, albeit a bit smaller.
The caldera of Tondano measures 30 kilometers long and 20 kilometers wide, which places
its original caldera forming eruption in the range of lower end supereruptions.
This supervolcano exists due to the offshore tectonic plate collision of the Birds Head
plate with the Sunda Plate.
The melted crust from this collision then migrates upwards, forming a chain of volcanoes
on the north arm of Sulawesi.
In the same region, a specific large body of magma intruded into the crust between two
major faults, but instead of erupting, it slowly expanded in volume deep in the crust.
This caused the deep magma to become more silica rich, eventually resulting in the eruption
of andesite lava on the surface 2.19 million years ago.
Due to the area being near sea level at the time, there was abundant groundwater, which
caused eruptions to be unusually explosive.
Volcanic activity later transition to forming a series of large lava domes, as the underlying
magma chamber continued to become more silica rich.
By 900,000 years ago, the magma had largely transitioned from andesite to rhyolite, which
was a warning sign of events to come, as large volume rhyolite magma chambers almost always
result in a caldera forming eruption.
After a series of rhyolite lava domes were constructed, the overall volcano became dormant
for several thousand years.
Then, 871,000 years ago, a massive eruption began on the surface.
All of the volatile gas which had built up in the underlying magma chamber was subsequently
released along a north-north east trending fissure, which sent several simultaneous plumes
of ash high into the atmosphere.
As part of the eruption column was too heavy to rise into the atmosphere, it collapsed
downwards, releasing a series of unusually energetic pyroclastic flows.
These flows travelled more than 50 kilometers distant, spreading out in all directions,
and even reaching the modern north end of the island of Sulawesi.
Ash continued to fall over the landscape, piling up more than 100 meters thick in some
locations.
Then, due to the large amount of erupted material, the underlying magma chamber was now partially
empty which caused a 30 kilometer wide section of ground to collapse downwards to fill the
empty space and form a massive caldera.
Although the exact amount of material erupted in this low end supereruption is unknown,
a reasonable estimate is between 600 and 950 cubic kilometers of volcanic rock were ejected.
Layers up to 200 meters thick of a distinguishable rock type known as ignimbrite can be found
throughout the modern landscape, which can be defined by its large crystals and clasts
which are aligned in the same orientation.
Afterwards, residual rhyolite magma erupted onto the surface once again, building several
large lava domes in the massive depression in the ground.
A subsequent stratovolcano which towered to more than 1000 meters in height was subsequently
built over the next 350,000 years and was known as Pangalombian.
Then, in a catastrophic eruption 500,000 years ago, this volcano completely collapsed, forming
a 5 kilometer wide caldera.
Since then, volcanic activity has been largely concentrated within this smaller caldera,
forming a large complex explosion crater known as a "maar" at Lake Linow, where numerous
fumaroles are found today.
Smaller volcanic eruptions also constructed Tampusu, which is a cinder cone with a crater
lake at its summit.
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