The recent highly explosive eruption of Hunga Tonga Hunga Ha'apai has brought additional
attention to other volcanoes in the nation of Tonga.
One of these other volcanoes is a massive ring shaped island covered in recent lava
flows.
I am not referring to the similar looking Tofua volcano, but rather the Niuafo'ou volcano.
Unlike Tofua, this volcano is not a stratovolcano but rather a shield volcano.
Thus, many of its eruptions are relatively peaceful, although some have impacted the
island's settlements.
One such eruption was filmed in 1943, which sent lava towards the western edge of the
island.
The Niuafoʻou shield volcano is located in central western Tonga, where it is 205 kilometers
west of the city of Hihifo.
The most prominent feature of the island is its 5 kilometer or 3 mile wide caldera, which
may have formed through two massive eruptions.
Before I go into how this island formed, I want to note something interesting I uncovered
while looking at historical documents regarding Niuafo'ou.
At the moment, we do not know when its central caldera formed.
However, a map drawn of the island in 1646, where it is labeled as "Island of Good Hope"
does not show its central caldera which would be very obvious to any map maker.
While maps of that area were notoriously inaccurate, the mere possibility that a large eruption
occurred sometime between 1616 and 1814 when thorough record keeping of the island began
needs to be further explored.
The Niuafo'ou volcano has a unique geologic setting among the volcanoes within Tonga.
To the east and north of the island, the Pacific plate is colliding with the Indo Australian
Plate.
The melted crust from this collision then migrates upwards, forming a chain of volcanoes
including Tofua and Hunga Tonga Hunga Ha'apai.
Since some of this material made a weak point in the overlying crust then snapped backwards,
it formed a thin zone in the plate.
This formed what is termed a "back arc basin" where the land is rifting apart.
Niuafo'ou is located in the middle of this rift zone.
Although we aren't sure exactly how old this volcano is, we do know that it is younger
than 80,000 years old.
It began forming when a volume of low silica basaltic lava erupted from a fissure onto
the ocean floor at a depth of around 1300 meters.
Over the span of tens of thousands of years, this fissure solidified into a single point,
which began building a tall volcanic cone.
This cone erupted significant quantities of a type of lava termed "pillow lava" due to
its rounded clasts looking like pillows.
Eventually, its lavas broke the surface, and the volcano began building a broad volcanic
shield.
As the island continued to expand, a prominent intrusion of magma expanded around its summit.
This region eventually collapsed downwards, forming a small caldera with an occasional
lava lake in its center.
Continued rifting of the island created a series of northwest trending faults, from
which lava erupted, expanding the overall island.
As for what occurred next in this island's geologic history, it remains unclear.
Possibility one is that these fissure eruptions drained additional magma from its magma chamber,
causing the caldera to slowly expand piece by piece until its modern appearance.
Possibility two is that this caldera formed through two massive plinian eruptions, as
two separate caldera rims are visible, which are especially notable in the northern section
of the crater lake.
Since 1814, this volcano has erupted 14 times.
Several of these eruptions occurred from distinct vents within the crater lake.
One such eruption in 1886 was about a fourth the size of Hunga Tonga's recent eruption,
and lasted for a 19 day period via a vent in the northeast section of the caldera.
However, the vast majority of eruptions have been confined to a series of fissures on the
southwest of the island.
These have created spatter cones which output low volume flows of lava that slowly travel
towards the ocean.
This means that only one of the island's settlements is typically at risk during eruptions.
With this being said, remnants of volcanic explosion craters termed maars have been found
to the south of the main settlement.
Thus, it is not safe to live on the island whenever an eruption with any explosive component
to it is suspected to begin or ongoing.
Using the US Geological Service method of rating volcanoes, Niuafoʻou would be classified
as a high threat volcano.
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