On July 27th, 2022 the towering Raung stratovolcano in Indonesia rather unexpectedly erupted at
5:19 pm local time, sending a plume of ash to a height of 4,832 meters or 15,853 feet
above sea level.
Since Raung is already a very tall volcano, this represented a plume which rose approximately
2,000 meters above the vent on the caldera floor.
What had just occurred was a phreatic explosion, the volcano's first eruption since April of
2021.
Phreatic eruptions are caused by the interaction of shallow magma with the underlying water
table.
The magma heated the overlying water, causing it to flash to steam.
Since the same mass of steam takes up more volume than the water it once was, it causes
an immense amount of pressure to build, eventually resulting in a series of upward explosions.
Over the next 9 minutes, three more phreatic explosions occurred, which each lasted between
71 and 173 seconds in length.
Afterwards, sporadic to semi continuous ash emissions continued for a period of several
hours.
During this time, the ash from the eruption largely drifted to the west and northwest,
causing fine particles of material to fall on the city of Jember.
Due to the risk of further eruptive events, the alert level of Raung was raised to 2 or
yellow and a 3 kilometer exclusion zone was placed around the volcano.
Although this eruption might seem to have occurred without warning to an outside observer,
there was actually numerous warning signs beforehand.
Since early May of 2022, the Raung volcano had produced an increasing abundance of sulfur
dioxide gas which was detected from satellite.
Sulfur dioxide is produced from volcanoes when magma is relatively close to the surface.
Then, in early June, instruments began detecting magmatic uplift within Raung's caldera.
This was later attributed to the intrusion of 1.7 million cubic meters of magma at a
depth of 2.9 kilometers.
A month later, the shallow intrusion of magma began heating Raung's fumaroles, creating
thermal signals which were detectable from satellite.
Also in early July, intermittent harmonic tremor signals, indicating the underground
movement of magma and venting of volcanic gasses began being registered by seismometers.
These signals greatly spiked on July 23rd, and remained elevated until the phreatic eruption
occurred a few days later.
For context, the Raung volcano is located on the eastern section of the island of Java
within Indonesia where it is 40 kilometers east of the city of Jember.
Raung, like the nearby Ijen volcano to the east which erupts blue lava, is situated on
the edge of a massive ancient caldera.
This ancient caldera formed approximately 50,000 years ago, when a supermassive eruption
created a 20 kilometer wide depression in the ground.
Since then, volcanic activity has largely occurred on the exterior ring fractures created
by the collapse, constructing a series of stratovolcanoes.
Mount Raung is one of these stratovolcanoes, and has been intermittently been erupting
since at least 38,000 years ago.
One of these eruptions and its subsequent events caused an entire secondary peak of
the volcano to collapse, generating a massive landslide which travelled up to 79 kilometers
distant.
Another recent eruption formed Raung's 2 kilometer wide and 500 meter deep caldera.
The vast majority of eruptions in the last 500 years have been completely confined to
within this caldera and were rather small, although a large plinian eruption did occur
in 1593.
These smaller eruptions, just like what occurred on July 27th originated from a small volcanic
cone on the caldera floor.
While phreatic eruptions are relatively common at Raung, its eruptions typically produce
loud strombolian activity, ejecting bombs of lava over the adjacent slopes of the volcano.
Due to the natural shape of the caldera, it causes the sound of frequent strombolian to
vulcanian eruptions to be greatly amplified, acting like a sort of natural amphitheater.
This is why Raung roughly translates to "roaring mountain".
In the next several days to weeks, it is quite likely that more phreatic eruptions will occur,
being largely confined to Raung's caldera rim.
It is also probable that a strombolian to vulcanian magmatic eruption will eventually
occur due to the shallow intrusion of magma, but it will likely be several weeks to months
before that batch of material reaches the summit.
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