In the later summer of 1257, a powerful blast shook one of the tallest volcanoes in Indonesia.
As the 4,200 meter tall Rinjani volcano violently exploded, pyroclastic flows raced across the
island, burying numerous settlements as far away as 45 kilometers distant such as the
town of Pamatan.
Then, due to the large amount of ash which had erupted, the towering volcano almost completely
collapsed as its center dropped 2.5 kilometers in height.
What had just occurred was one of the planet's deadliest eruptions, rating on the volcanic
explosivity index as a 7, as 120 cubic kilometers of tephra was ejected.
What remains of the volcano today is a deep caldera lake surrounded by steep walls.
While this eruption was quite large, there was another even larger geologically recent
eruption in the vicinity.
To the west on the island of Bali is a tall volcano which grew out of a recent collapse
scar.
What I am referring to is Mount Batur, which produced two powerful eruptions in the last
35,000 years, one of which was 16% larger than the 1257 eruption at Rinjani.
The modern Mount Batur and its basalt to basaltic andesite lava flows are situated within two
calderas that happen to contain a crater lake.
The smaller caldera closely defines the recent extent of Batur's lava flows measuring 7.5
kilometers long, while the larger caldera which steeply cuts into the adjacent Mount
Abang is 13.5 kilometers in length.
Like other regional volcanoes, Mount Batur exists due to an ongoing tectonic plate collision
south of the island.
There, the Indo Australian plate is subducting underneath the Eurasian Plate.
The melted material from this collision then migrates upwards, before erupting on the surface,
thus forming a chain of volcanoes.
In this section of Indonesia, large calderas are surprisingly common, hinting at a process
which repeats at various volcanoes.
For example, Batur is located 25 kilometers east of the similarly massive Bratan Caldera.
Zooming outwards from the island of Bali, the Ijen, Rinjani, and Tambora calderas are
all in relative close proximity to one another.
Each one of these calderas produced a VEI 7 eruption, and had similar compositions.
Typically, large caldera forming eruptions eject dacite to rhyolitic magmas.
Yet, for the most part these calderas formed in basalt to andesitic eruptions.
Instead of the aforementioned massive eruptions being triggered by the build up of pressure
due to viscous magma, they occurred due to to the formation of a trap for gas and volatile
materials in magma.
The taller a volcano becomes, generally speaking the better of a trap it creates.
Thus, once a volcano reaches a certain height in this geologic setting, it becomes far more
likely to produce a major explosive eruption.
In this case, the height of the volcanoes before they collapsed was between 3,500 and
4,300 meters tall.
The Mount Batur stratovolcano had been intermittently erupting since 510,000 years ago, building
a large volcanic cone out of basaltic andesite and andesite lavas interspersed with layers
of ash.
Eventually, a satellite vent formed to the southeast, creating the tall Mount Abang stratovolcano.
By 35,000 years ago, the twin peaked stratovolcano had a summit height of 3,800 meters or 12,500
feet.
Underneath its edifice, a large natural trap had formed as massive quantities of gas and
volatile materials which originated from the underlying magma chamber continued to build.
This culminated in a powerful explosive eruption 34,565 years ago, which sent a plume of ash
more than 40 kilometers into the atmosphere.
Part of the eruption column subsequently collapsed downwards, resulting in lengthy pyroclastic
flows which travelled across much of Bali.
Then, due to the large amount of erupted material, almost the entirety of Mount Batur collapsed
downwards, forming a deep 13.5 kilometer wide caldera.
In total, 140 cubic kilometers of tephra was ejected in this eruption and layers of ash
more than 120 meters thick were deposited.
Over the next 10,000 years an andesite to dacite lava dome complex grew out of the caldera.
However, since all of the underlying volatile content had not been relieved, it resulted
in a second caldera forming eruption 24,380 years ago.
This destroyed the existing lava dome complex, creating a 7.5 kilometer wide caldera.
In total, about 30 cubic kilometers of tephra was ejected.
The modern Mount Batur which we see today has entirely formed since then, and has never
since produced an eruption close in size to its two caldera forming eruptions.
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