When people think of recent very large volcanic eruptions, a few places come to mind.
Mount Vesuvius in Italy, Tambora in Indonesia, and Mount Saint Helens in the United States.
However, the series of volcanoes within the nation of Papua New Guinea are often forgotten
about, despite the fact that this country has produced some of the planet's largest
volcanic eruptions in the last 5,000 years.
In this country, vast calderas which sometimes measure more than 6 miles or 10 kilometers
across are quite common in the landscape.
Although some of these calderas exist on separate islands, a dense assortment of young calderas
are centered on the island of New Britain.
Two such young calderas on this island include Dakataua and Rabaul.
However, there is another large caldera adjacent to the large Rabaul volcanic system.
What you are looking at is not an error in the often noisy ocean topography data.
Rather, this large depression in the ground is another massive caldera which is more than
750 meters deep in some locations, representing a separate volcanic system known as Tavui.
Although not much is known about this volcano, it is just as dangerous to the nearby population
centers as Rabaul.
For example, one of Tavui's eruptions in 4946 BC smothered much of the landscape in more
than a meter of ash, sending pyroclastic flows across the ocean to several population centers.
The reason why the island of New Britain contains so many large calderas relates to is geologic
setting.
To the south of the island, a major tectonic plate collision is ongoing as the Solomon
Sea Plate is subducting underneath the South Bismarch Plate.
The melted material from this collision then migrates upwards until it erupts onto the
surface, forming a chain of volcanoes.
As a whole, the Tavui caldera measures 12 kilometers long and 9 kilometers wide.
The caldera walls are mostly a steep sheer cliff, which is more than a kilometer tall
in some locations and is a popular diving spot.
Part of the caldera walls are located on land, and several adjacent villages to these walls
are how the volcano got its name.
Within the deep caldera are a series of two volcanic cones which formed after the presumably
larger of the volcano's 2 known eruptions.
The first recorded eruption from this volcano occurred 79,000 years ago, when what was before
presumably a submarine volcanic cone on the shallow ocean floor violently exploded, sending
a tall plume of ash above the water and high into the atmosphere.
This major eruption caused several meters thick of ash to fall over a vast area.
Part of the eruption column then collapsed downwards, resulting in energetic pyroclastic
flows which raced across the ocean in all directions.
Then, due to the large amount of erupted material, a large section of ground overlying the magma
chamber collapsed downwards to form a truly massive caldera.
The exact size of this eruption which deposited a layer of volcanic rock known as the Tokudukudu
Ignimbrite is unknown, but at a minimum was similar in size to the 1883 eruption of Krakatoa.
Although the ages of the subsequently formed submarine volcanic cones are unknown, they
were primarily constructed out of basaltic and rhyolitic magma.
It is subsequently presumed that this volcano did not erupt for several tens of thousands
of years.
Then, in 4946 BC, a second major explosive eruption occurred.
This time, the water logged eruption column only barely broke the ocean's surface as the
vast majority of it was too heavy.
This caused it to collapse downwards, sending highly energetic pyroclastic flows more than
15 kilometers across the ocean to the nearby island.
Then, the existing caldera subsided downwards on the order of about a dozen meters, deepening
the existing edifice.
In total, 5 cubic kilometers of volcanic rock was ejected meaning the eruption had a volcanic
explosivity index rating of a 5.
No subsequent eruptions have occurred, and attempts to locate a magma chamber underneath
this volcano have been unsuccessful, thus leaving two possibilities.
Either Tavui is now truly an extinct volcano, or a very small but unusually deep magma chamber
exists which was missed by sensors.
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