#nisyros

Nisyros is a collapsed stratovolcano. It originally formed in this site from a long series of volcanic eruptions triggered by subduction – a bit of oceanic crust linked to the African plate is being pushed down beneath Europe in the Aegean, leading to a set of volcanic islands including Nisyros and nearby Kos island. Nisyros and Kos are both part of a gigantic caldera complex, which formed in a massive eruption about 150,000 years ago; Kos island is part of the rim of this caldera, while Nisyros is a new volcanic cone that grew on the fractures formed by the caldera. The large Kos caldera is about 20 kilometers wide, and Nisyros itself hosts a much smaller caldera that is 3-4 kilometers across. It was produced by a collapse of the smaller stratovolcano that formed Nisyros island about 2000 years ago. The island is still volcanically active. This crater, known officially as Stefanos crater (but apparently just called “the volcano” by some of the visitors) is a remnant of a set of hydrothermal explosions in the latter half of the 19th century. There is still molten rock available at depths of about 5 kilometers beneath this island, and to form this crater enough of it moved up to intersect groundwater, flash to steam, and cause an explosion that cleared out the crater. Stefanos is one of the best-preserved hydrothermal explosion craters on Earth, and you can see the layered rocks altered by sulfur on the walls. Just to make sure we cover the scale – this crater, a few hundred meters wide, is just one part of a 3-kilometer-wide caldera making up the island, and that island is one of several that are part of a 20 kilometer caldera. As the size of the crater/caldera increases, so does the size of the eruption that produced it. The full Kos caldera is about 2x the size of the Santorini caldera, remnant of the eruption of the Thera volcano and likely cause of the collapse of the Minoan civilization. In 1996, there was a period of increased activity on this island, including earthquakes, tremor suggesting magma motion, and changes in the composition of the volcanic gases released at these craters. Although no eruption took place, that reminder clearly demonstrates that Nisyros is an active, hazardous volcanic system, and it is currently monitored by EU authorities for future activity. -JBB Image credit: Claire Tresse https://flic.kr/p/rAsQDp Read more: https://www.volcanodiscovery.com/nisyros.html https://www.greece-is.com/nisyros-the-sleeping-giant-of-the-dodecanese/ https://link.springer.com/article/10.1007/s00445-004-0381-7 http://volcano.oregonstate.edu/vwdocs/volc_images/europe_west_asia/nisyros.html