On March 16th, 2014, a volcano north of Taipei in Taiwan produced an unexpected volcanic
eruption.
However, despite the fact that several million people lived adjacent to the volcano, the
eruption went entirely unnoticed.
Despite this, a recent paper proves that a phreatic eruption truly did occur that day,
where it originated from a vent of the Tatun volcanic complex.
Then, 10 months later, a similar eruption also went unnoticed in the same area.
These two eruptions represent Tatun's first verified eruptions since 648 AD, signaling
that the volcanic complex is still very much active.
Although the two volcanic eruptions which occurred were quite small, there is a danger
that a future eruption could be far more powerful.
The Tatun volcanic complex exists on the far northern edge of the nation of Taiwan, where
it is centered 15 kilometers north of Taipei.
Unlike a typical volcano, Tatun does not contain a single central peak.
Rather, it contains an assortment of 20 smaller volcanic edifices which have a rounded shape.
Each of these represent what are referred to as "lava domes".
Lava domes form when viscous magma which erupts onto the surface is unable to travel very
far, so instead of creating a lengthy river of lava like seen in Hawaii, it forms a large
rounded edifice.
Scattered across several of these lava domes are a group of volcanic explosion craters
termed "maars" from which numerous fumaroles are located.
These fumaroles are always emitting sulfur dioxide and carbon dioxide gas due to the
massive amount of molten rock underlying the complex.
At a depth of 20 kilometers below these fumaroles is one of the largest magma chambers on the
planet, which contains 350 cubic kilometers of molten rock.
From this magma chamber, occasionally small quantities of magma intrude shallower into
the crust, creating a series of earthquakes.
However, 9 times out of 10, these intrusions of magma do not reach the surface.
With this being said, magma does not need to reach the surface for an eruption to occur.
Through a lengthy process, magma heats surrounding rock it encounters.
This adjacent rock can subsequently heat nearby groundwater, and if hot enough, can cause
the water to flash to steam.
Over time, a large volume of pressure builds, as steam takes up a greater space then the
same volume of water.
Finally, this material breaks the overlying rock, shooting upwards in what is termed a
phreatic eruption.
The common explosion craters seen around the complex are up to 200 meters wide, and all
formed from phreatic eruptions.
However, while phreatic eruptions generally create a plume of ash, this is not always
the case.
If the volume of steam is small enough, the actual explosion can be completely confined
to a magma conduit, or in this case the pathway which gas takes to the surface and is emitted
via fumaroles.
On March 16, 2014, at 5:10 am in the morning local time, such an explosion occurred.
Although it may have created a small plume of steam, it was completely dark at the time
of the explosion.
So, if no one witnessed the eruption, how do we know it occurred?
The answer is the seismic signal it produced.
At volcanoes around the world, scientific agencies frequently place seismometers and
other equipment to monitor background volcanic activity.
Volcanoes produce a wide range of earthquakes along with an occasional constant tremor referred
to as a "harmonic tremor", which have distinguishable seismic signatures.
Phreatic eruptions, regardless of their size, also have a distinctive signature.
Unlike an earthquake which has a short spike then quickly diminishes, a phreatic eruption
produces a spike in activity which remains high for 15 to 120 seconds.
One such signal was observed on that day, so we know that an eruption occurred.
Alongside a similar eruption on January 5th of 2015, both were centered at the Hsiaoyukeng
fumarole.
Thus, this fumarole is likely to be the site of future phreatic eruptions, which is a potentially
deadly possibility due to how frequently tourists visit that site.
Although we aren't sure what the exact rate of phreatic eruptions produced from this complex
is, recent data suggests that it is more common than initially thought.
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