Within Papua New Guinea on the island of Bouganville is a highly unusual volcano.
Known as Bagana, this volcano may look like a stratovolcano but it is not one.
Unlike stratovolcanoes, Bagana tends to not produce eruptions which are dominated by ashfall
and pyroclastic flows.
Instead, viscous lava lobes rapidly spill out of its main crater in a highly effusive
manner, much like what would be expected at a spatter cone.
This strange type of volcano is known as a "lava cone", which is essentially a stratovolcano
but instead of having overlapping layers of lava and ash, its cone is composed of just
lava.
This cone generally goes against how the majority of volcanoes act as generally speaking, the
higher the amount of silica in the magma, the more viscous it is, and thus the more
explosive the eruptions become.
However, silica content is not the only factor that determines how explosive an eruption
is.
Another important factor is the amount of gas or volatile material within the magma.
In the case of Bagana, it has a high silica content magma, but an unusually low gas content.
This is why its eruptions are only moderately explosively at most, and people live right
next to the volcano while it is actively erupting a mere 4 kilometers downslope.
Lava cones as a whole are the single rarest type of volcano on Earth.
Although volcanoes can have multiple volcanic landforms, the Smithsonian's Global Volcanism
Program compiled a list of all 1,345 active volcanoes on Earth suspected or confirmed
to have erupted in the last 12,000 years.
This dataset happened to list the dominant type of volcano an individual system was,
being something such as a submarine volcano, caldera, or cinder cone.
While there were 665 systems classified as stratovolcanoes, there were only 4 classified
as lava cones.
Of these 4, only 3 are confirmed to have erupted in the last 10,000 years, and due to the geologic
setting of eastern Papua New Guinea the country has two of them.
The Bagana volcano is located in the central portion of the island of Bougainville, where
it is 36 kilometers northwest of the infamous Panguna Mine.
Bagana is considered to be part of a twin volcanic system as the large lake filled caldera
of the Billy Mitchell volcano is located 6 kilometers to the northeast.
In 1580, what used to be a tall volcanic cone collapsed in a powerful explosive eruption,
sending pyroclastic flows more than 25 kilometers distant to the shoreline.
Since this eruption, the Billy Mitchell volcano has been completely silent.
However, not all is what it seems.
Although scientists are not truly sure how old the Bagana volcano is, it is rightfully
the youngest volcanic system in the region.
Based on its current level of lava effusion, the entire volcano may be only 300 years in
age.
Other evidence such as the nature of vegetation growing on its oldest lava flows seems to
support this.
This age approximation is quite interesting, as it means that this volcano began to form
only after Billy Mitchell's caldera forming eruption.
It is thus quite likely in my opinion that Bagana is pulling from a large shared magma
chamber at depth.
In only 300 years, this volcano has grown from land that was beforehand only 700 meters
above sea level to its current height of 1,855 meters.
This has primarily occurred through near continuous large volume lava effusion from its summit
crater, which after overflowing onto the volcano's slopes then travels downhill as a lobe of
material.
Moving at a rate of generally only 3 meters a day, these lava lobes have travelled up
to 4 kilometers from its summit, indicating just how long continuous eruptive activity
has occurred.
This process has repeated dozens if not hundreds of times, forming the modern edifice of Bagana.
Although this volcano's eruptions are either effusive or mildly explosive, they can still
be quite dangerous.
Personnel stationed near the volcano during the 1940s noted that occasional explosions
occurred, partially destroying the summit lava dome.
These explosions sometimes released glowing columns of volcanic gas, which are today referred
to as pyroclastic flows.
While pyroclastic flows are rare here, they have been witnessed travelling up to 2.5 kilometers
from the volcano.
Given my interpretation of the gas content at Bagana, I believe that its pyroclastic
flows could theoretically travel up to 5 kilometers distant.
This is a potential problem as several hundred residents live in the at risk areas.
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