Around the globe, a number of very large depressions in the ground termed "calderas" can be observed.
These features form during the planet's largest volcanic eruptions where so much magma is
expelled that the rock overlying the partially empty magma chamber collapses downwards, forming
a large depression.
Supervolcanoes tend to produce the world's largest calderas, which often measures tens
of kilometers in length.
For example, the Taupo supervolcano in New Zealand formed a 35 kilometer wide caldera
in an eruption 27,000 years ago.
While this is quite large, it does not compare to the 100 kilometer wide caldera of the Lake
Toba supervolcano, which for the longest time was considered to be the world's largest caldera.
This was, until 2019.
That year, an even larger caldera was found, measuring 150 kilometers or 93 miles in length.
Even more interesting is that this caldera, called the Apolaki caldera, formed deep in
the ocean at depths of more than 3000 meters below sea level.
Despite its appearances, the Apolaki caldera does not actually represent a supervolcano,
although it is volcanic in origin.
The Apolaki caldera is located off the eastern coast of the northern Philippines where it
is 400 kilometers east northeast of Manila.
This caldera is situated on a large oceanic plateau which is referred to as the Benham
Rise.
This plateau is truly vast, as it stretches across 110,000 square kilometers which is
essentially the size of the nation of Cuba.
It formed during one of the most large scale episodes of volcanic activity on the planet,
in what is termed a flood basalt.
Flood basalts are very rare geologic events often associated with mass extinction events
where large sections of the planet are coated with several kilometers thick of lava in a
geologically brief timespan.
This typically occurs through lengthy fissure eruptions which stretch for more than 200
kilometers, where each point of the fissure ejects large volumes of molten rock.
During a period of a few million years, hundreds of thousands to several million cubic kilometers
of lava erupt onto the surface.
As a whole, flood basalts are almost universally caused by the "head" of a mushroom shaped
mantle plume reaching the surface.
We have seen examples of this with the Yellowstone hotspot which created the Columbia River Flood
Basalts in Oregon.
The Deccan Traps flood basalts in India which were also created by a hotspot which is today
underneath Reunion Island.
The Benham Rise was also inferred to have been created by a hotspot, which may very
well still exist today.
Remember, the crust overlying hotspots moves while the hotspot itself does not.
Approximately 47.9 million years ago, the area which is now the Apolaki Caldera was
much closer to the equator and exists where the Caroline Islands do today.
These modern day islands happen to exist due to an underlying hotspot, which I hypothesize
is the same one which created the Apolaki Caldera.
Approximately 47.9 million years ago, a lengthy fissure opened deep on the ocean floor of
the Pacific Ocean.
Over the span of several years, this fissure output more than 500 cubic kilometers of lava,
building a small plateau out of viscous black volcanic rock known as basalt.
Every few thousand years, a similarly large eruption occurred, as this process continued
for 5 million years.
By 45 million years ago, the vast majority of the Benham rise had been constructed, whose
eruptions were fed by a group of underlying large magma chambers which had a roughly circular
shape.
Then, the divergent plate boundary on the eastern edge of the caldera suddenly shifted
40 kilometers to the east.
This caused the dynamics of the entire volcanic system to change, as the magma from the group
of magma chambers to the west were funneled eastwards towards the new rift zone which
represented a path of least resistance.
This slowly drained these magma chambers, leading to the partial collapse of the ground
overlying them.
Caldera formation took place over the span of years to decades due to this massive flank
eruption, which covered a wide swath of land in 13,000 cubic kilometers of fresh lava,
forming what is known as the Loro Spur.
In other words, the world's largest caldera was formed in a similar manner to the caldera
at Kilauea in 2018; not through an explosive eruption but through a distant flank eruption
which drained its underlying magma chamber.
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