On the evening of April 10th, 1815, a powerful blast shook one of the tallest volcanoes in
Indonesia.
As the Tambora volcano violently exploded, pyroclastic flows raced across the island,
burying all of the island's settlements.
The volcano subsequently almost completely collapsed, as its center dropped 3 kilometers
in height.
What had just occurred was the largest volcanic eruption on the planet in the last 7,000 years.
In total, 150 cubic kilometers of volcanic rock was ejected alongside 100 million tons
of sulfur dioxide.
This spread around the planet, causing temperatures worldwide to drop and causing famines, leading
to the so-called "year without a summer" as it snowed in New York City during the summer.
While this eruption in Indonesia is well known, it is not the only geologically recent large
eruption to occur in the country.
In fact, there exists a massive caldera which formed in an eruption about twice as large.
What I am referring to is the Lake Maninjau caldera, which measures 20 kilometers long
and 8 kilometers wide.
This massive caldera formed in a megaeruption 52,000 years ago, which covered much of Indonesia
in a thick layer of ash.
Today, evidence that this caldera is still relatively active can be found through occasional
massive die offs of fish in the caldera lake which occur due to hydrogen sulfide released
by underlying fumaroles.
The Lake Maninjau caldera is located in the south central portion of the island of Sumatra
where it is 70 kilometers north-northwest of the city of Padang.
The Maninjau Caldera owes its existence to a lengthy fault line called the great Sumatran
fault.
This fault line stretches across the west coast of the island of Sumatra and represents
a path of least resistance for magma related to the adjacent subduction zone to travel
into, which has lead to the creation of a chain of volcanoes and large calderas such
as Lake Toba.
As it stands right now, the highest point of the jagged peaks which surround the caldera
stands at 1,724 meters or 5,656 feet above sea level.
However, before the caldera formed there used to be a gray topped stratovolcano which towered
to more than 3,100 meters or 10,170 feet above sea level.
Although the exact history of this prior volcano is unclear, it had a complex profile consisting
of a main peak to the north, a secondary peak to the south, and several large rounded rhyolite
lava domes.
These formed via hundreds of thousands of years of volcanic activity which began approximately
880,000 years ago.
This volcano initially started out by erupting andesite lava, which built the volcano via
numerous overlapping lobes of viscous lava.
By 300,000 years ago this volcano was heavily forested on its deeply eroded slopes while
its summit was barren of vegetation and covered in layers of ash.
Over time, the underlying magma leached material from the surrounding crust, causing it to
become more viscous.
Eruptions eventually no longer erupted andesite but instead dacite and then rhyolite.
The appearance of rhyolite marked a dangerous phase of the volcanic complex, signifying
a period of highly explosive eruptions which built lava domes up to a kilometer across.
Right before the caldera forming eruption occurred, the dike swarm of molten rock underlying
the volcano had morphed into a single massive magma chamber which contained several hundred
cubic kilometers of molten rock.
Although the exact trigger of the caldera forming eruption is unknown, it likely occurred
when andesite composition magma mixed with an adjacent relatively isolated rhyolite magma
chamber, which lead a large volume of magma to shoot towards the surface.
As several vents simultaneously erupted lava along a north to south facing trend, a series
of powerful explosions occurred.
These explosions cleared the ground overlying the magma conduits, allowing for millions
of cubic meters of volcanic rock to be released every second.
Part of the eruption column then collapsed downwards, releasing energetic pyroclastic
flows which swept across the landscape, burying everything in a 75 kilometer radius.
Then due to the large amount of erupted material, the underlying magma chamber was nearing on
empty, which caused the ground overlying it to collapse downwards and form a massive caldera.
In total, 250 cubic kilometers of volcanic rock was ejected in an eruption rated on the
volcanic explosivity index as a 7.
Some areas were buried in more than 200 meters thick of ash.
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