When most people think of Greece, they tend to think of its rich history rather than its
volcanoes.
Yet, the nation contains 6 active volcanoes, several of which have produced truly catastrophic
eruptions.
For example, the Santorini volcano produced one of the largest eruptions in human history
when it erupted in 1610 BC, causing the majority of the island to sink into the ocean, thus
separating its landmass into several major islands.
Although this eruption was catastrophic, it was not the largest eruption to occur in Greece
in the last 1 million years.
For that eruption, you would need to head towards the island of Nisyros.
There, a massive primarily submarine caldera stretches between the islands of Gyali, Nisyros,
and Pachia.
It measures 12.8 kilometers long and 8.8 kilometers wide.
This caldera originated in a megaeruption at a time in the distant past from a large
volcano to the west of Nisyros.
So, how long ago did this eruption occur?
Why is this apparent volcano no longer visible?
This video will answer these two questions and discuss this caldera forming eruption.
The Kos-Nisyros caldera is located closer to mainland Turkey than to mainland Greece,
and is 150 kilometers east of the island of Santorini.
Just like Santorini and its associated Kolumbo volcano, it is part of an active volcanic
arc formed by the collision to the south of the African tectonic plate with the Aegean
plate.
The melted crust from this collision then migrates upwards, until it erupts onto the
surface, forming a chain of volcanoes.
The best way to understand the true scope of the eruption which formed this caldera
is to look at the island of Gyali.
There, a number of mines are quite prominent, which are all extracting a white material.
These mines are extracting a distinct layer of pumice, which was deposited in a single
caldera forming eruption.
This pumice layer measures up to 200 meters or 656 feet thick in some locations, and was
all deposited in less than 24 hours.
It is even more impressive when you realize that the source vent of the eruption is 5
miles or 8 kilometers away, meaning that even thicker beds of ash and pumice exist!
This caldera and its associated pumice originated approximately 161,000 years from an edifice
which at the time was above sea level and connected three separate islands.
This edifice contained a large stratovolcano which was perhaps 900 meters in height, and
had a whiteish gray appearance.
Before I continue, I want to note that the exact location of this precursor volcano and
its caldera is still up for debate.
Some scientists think it was centered on the island of Gyali, others suggested it was located
between Gyali and Kos, while a third group thought it was located to the west of Nisyros.
I agree with the third group, as the regional geology of the area is a best fit considering
that scenario.
Wherever this stratovolcano was located, its edifice likely also had sections which had
a yellowish brown appearance, as hydrothermal alteration from fumaroles caused its edifice
to become unstable in places.
These fumaroles were fed by a truly massive magma chamber, which had been building in
size since 400,000 years ago.
Over time, it continued to build in size until finally a large eruption began on the surface.
After a large explosion, a column of ash shot 44 kilometers into the atmosphere.
As the day seemingly turned to night due to the quantity of ash which erupted, pyroclastic
flows raced across the ocean from the slowly collapsing volcano, travelling across vast
swaths of the Mediterranean Sea and reaching more than 50 kilometers distant.
In some locations, hundreds of meters thick of volcanic rock were deposited in less than
an hour.
Then, due to the large amount of magma which had erupted, the existing volcanic edifice
collapsed downwards, forming three separate islands.
In total, 110 cubic kilometers of volcanic rock was ejected, and the eruption had a volcanic
explosivity index of a 7.
Although this caldera has not erupted since, more recent activity has occurred at Nisyros
and Gyali.
Nisyros has been particularly active, as the entire island began to form only just after
this megaeruption around 150,000 years ago.
I hope that you enjoyed this video!
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