The long history of Greece contains records of numerous volcanic eruptions. For example,
the nation's most famous volcano, Santorini, produced one of the largest volcanic eruptions of
the last 10,000 years, thus forming a caldera, and partially destroying a bronze age civilization.
However, Santorini, which last erupted in 1950, is not the only volcano which is likely to erupt
in the next 100 years. 155 kilometers to the east of Santorini is the island of Nisyros. This island
is also completely volcanic in origin, and has erupted four times since the island was settled.
The most recent of these eruptions occurred quite recently in 1888. As we zoom in, you will note
that a large caldera exists on the island, hinting at far more explosive volcanic eruptions in the
past. This video will discuss this volcano, and mention what its future hazards are.
Like the other active volcanoes in Greece, Nisyros exists due to the collision of two tectonic plates
200 kilometers to the south of the island. There, the African tectonic plate is being
subducted underneath the Aegean plate. The subducted crust from this collision then melts
and travels upwards, until it erupts onto the surface, producing a chain of volcanoes. (pause
The Nisyros volcano began forming 150,000 years ago, when a volume of magma broke through the
Mediterranean Sea. Initially, this volcano produced hundreds of eruptions, which created an
unusual type of lava referred to as pillow lava. However, as the volcano continued to erupt, lavas
became more silicous, causing the composition of lava to change from basaltic andesite to andesite.
By 125,000 years ago, this stratovolcano had breached the surface of the Mediterranean
Sea. (pause 2 seconds) Over the next 90,000 years,
this volcanic complex grew to a height of 900 meters. By 35,000 years ago,
Nisyros looked like a 6.5 kilometer wide island covered with gray volcanic rock, which had a
central peak and several smaller secondary peaks. However, at depth, the type of magma continued to
change, as more silica leached into the magma chamber. This caused the composition of lava
to change from andesite to dacite, and then to rhyolite. Eventually, Nisyros stopped erupting.
However, at depth, pressure continued to build as a large magma chamber filled with magma. Then,
34,000 years ago, this pressure was released via a massive eruption. As pyroclastic flows raced
down the entire island, large volumes of ash began raining on islands as far away as Kos. Eventually,
the volume of lava erupted was so great that rock collapsed downwards to fill the empty space
in the underlying magma chamber. This formed the first layer of the modern caldera on the island.
Over the next several thousand years, a large rhyolite lava dome erupted on the southeast of
the island. Although the eruption started near the caldera rim, it eventually flowed outwards
into the ocean, thus expanding the island. Then, 24,000 years ago, a second massive eruption
occurred. This eruption quickly destroyed the portion of the lava dome within the caldera rim.
As a plume of ash rose 35 kilometers into the atmosphere, a second and final caldera
collapse occurred, creating the modern edifice. Over the next 12,000 years, a series of viscous
lava domes erupted, forming a series of prominent hills on the western and northern section of the
caldera. Interestingly, many churches on the island are built on top of these lava domes.
Although lava has not erupted on Nisyros in the last 10,000 years, volcanic eruptions have
continued. Since the volcano's magma chamber is only 3 kilometers beneath the surface,
it occasionally heats up large patches of groundwater. This groundwater then rapidly
flashes to steam, and generates an explosion on the surface called a phreatic eruption. The
largest of these explosions formed a 285 meter wide and 28 meter deep crater called Stefanos.
Since 1451, 4 additional explosions have occurred, which occurred at the sites shown on screen. The
last of these eruptions occurred in 1888. Although Nisyors has not erupted in more
than 125 years, it will erupt again. Geologic records indicate that these explosions occur
about once every 200 years. By measuring the temperature of hot springs and fumaroles in
the central caldera, you can determine if magma is suddenly rising to the surface.
Any rapid increase in temperature should result in the temporary closure of the caldera.
When the next explosion occurs, it could eject rock which could fall anywhere on the island.
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