NISYROS of the Twelve Islands (Greece)

Nisyros, view into the caldera with Stefanos Crater. (© stupiduglyfool, via mapcarta.com)

How many of the millions of sunshine-hungry European travellers to Greece would have known that there is the attraction of a fuming active volcano in their favorite holiday destination? And I am not talking famous Santorini here, but a little island with only a few hundred residents in the far south-east of the country, near the Turkish main land.

Aerial view of the islands Yali (front) and Nisyros from a plane approaching Kos KGS airport. (© Tobias Schorr, via Nature Discovery Tours)

Nisyros (Greek: Νίσυρος), or Nissiros, is an entirely volcanic island in the south-eastern Aegean Sea. More specifically, it is part of the Dodecanese Islands group (literally: Twelve Islands). It is located 17 km from both the Turkish coast (to the E) and the larger island of Kos (to the N). It’s almost round shape is at no point wider than 8 km.

Poseidon crushing Polybotis under a huge rock; black-figure amphora from the 5th c. BC. (Yorgos Fafalis, © Athens, Museum of Cycladic Art, inv. no. SP 98)

If we believe the myth, Nisyros is the rock thrown in battle at the Titan Polivotis by the mighty God Poseidon. Now Polivotis lies beneath it, and every time he struggles to escape, his breath penetrates the ground and the earth trembles…

Greece presently has four volcanic centers considered to have erupted in the last ~2000 years: Santorini (last in 1950), Nisyros (in 1888), Milos (in 140), and Methana (258 BC). These form the backbone of the Aegean Active Volcanic Arc. A blast from Santorini some 3600 years ago is believed to have wiped out the Minoan civilization.

Google Earth image showing the active volcanic centers of the Hellenic Arc. Click!

From Sea to Desert to Sea again

The Aegeis is a vast landmass that emerged from the old Tethys Ocean, covering the area from the modern-day Ionian Sea to Asia Minor. The area of the modern Mediterranean Sea was also once a part of the Tethys. For several hundred thousand years in between, though, the entire basin was an arid desert, which was eventually flooded with water from the Atlantic.

Gradually, as a result of tectonic shifts and ruptures, the landmass of Aegeis fragmented and a large part of it was submerged beneath the waves again. This is how the Aegean Sea and islands were formed. Geological monuments attest to these changes, such as the Petrified Forest on Lesvos, which was formed 18 million years ago and preserved under a layer of volcanic matter.

Tectonic map of the Aegean active volcanic island arc (or Hellenic arc). (© Mikenorton, via Wikipedia)

Nisyros is located at the eastern end of the quaternary calc-alkaline Aegean active volcanic island arc (also called Hellenic volcanic arc). This island arc is the result of the NE-directed subduction of the East-Mediterranean oceanic lithosphere (at the northern part of the African plate) below the Turkish–Aegean micro plate (part of the Eurasian continental plate). The southern Aegean is one of the most rapidly deforming regions of the Himalayan-Alpine mountain belt.

The Kos-Nisyros volcano-tectonic complex

The neotectonic structure of the Kos-Nisyros-Tilos area is a complex tectonic graben between the Kos tectonic horst in the north and the Tilos tectonic horst in the south. Large volumes of recent volcanic rocks divide the basins on both sides of Nisyros and continue up to the Knidos Peninsula in the coastal area of Minor Asia. (P. Nomikou et al., 2010)

Nisyros is found in the southern part of a larger and geologically more complex area, the Kos-Nisyros volcano-tectonic complex. This whole submarine area including northern Nisyros is considered to be a large collapsed caldera, created by the phreato-magmatic Kos-plateau tuff eruption (“Kos ignimbrite”) before 160 ka. North of Nisyros, towards Kos, there is another even much smaller island, Yali, which is also a volcano. Its latest pumice layers cover soils containing pottery and Neolithic obsidian artifacts, so, it also has erupted “fairly recently”.

 

THE VOLCANO

Google Maps – Terrain of Nisyros

Nisyros was built up during the last 150 ka; it is much younger than its neighbor island Kos. After an early basaltic submarine stage, it developed as a stratovolcano. The central caldera was formed in multiple stages, beginning at 44 ka. Following the early paroxysmal events, the style of volcanism on Nisyros underwent a gradual change from the effusive and mildly explosive activity: the emplacement of (rhyo)dacitic domes lead to larger eruptions. At least two large Plinian pumice eruptions were involved in the caldera collapse. The last magmatic activity of Nisyros is of unknown age and no such eruptions occurred during historical times.

Geological map of Nisyros. The stratigraphy has been divided into two suites: LPA low-porphyricity andesites, and HPRD high-porphyricity [rhyo]dacites. (Klaver et al., 2017, after Volentik et al., 2005b)

That doesn’t mean there were no eruptions at all – in the 19th century several hydrothermal explosions rocked the island. With the high seismic unrest, thermal water and fumarolic gas discharges a continuous activity is obvious on Nisyros.

This is from the description of historic activity given by “Discovery Team” on their Geocache website: “It is suspected that the volcano erupted in 1422. In 1871, an eruption was accompanied by earthquakes, explosions, and red and yellow flames. Ash and rocks spewed out and covered the floor of Rammos, destroying the fruit gardens there. During a three-day-long eruption in 1873, a 20-25 foot (6-7 m) diameter crater formed and ash and blackish mud was ejected. The bottom of Lakki and Ramos was transformed into a lake by hot saline water that overflowed the crater. The most recent eruption was in 1888. This strong eruption threw out a cylindrical pipe of volcanic material at least 80 feet (25 m) in diameter. Mud, rocks and steam were also ejected. In 1956, fumaroles were observed along the west and south sides of Rammos.” (slightly corrected)

Nisyros, central caldera with Stefanos Crater. (© no author given, via oddviser.com)

The center of the island is dominated by the modern caldera with a diameter of 3.8 km. The caldera rim is generally over 250 m high, the highest point is 588 m. The west and northwest of the caldera is occupied by five tightly packed lava domes, of which the Profitis Ilias dome (698 m) represents the highest point on the island. In the southeastern caldera, the floor of the Stefanos crater (diameter 300 m) has a geothermal area with mud pools and fumaroles.

Mandraki from the sea, with the impressive Monastery of the Panagia Spiliani perching on basaltic pillow lava rocks. (© Melanopetra, via melanopetra.gr)

The oldest lavas are found in the NW, near the island’s main town Mandraki. These are formerly submerged basalt pillow lavas. They have been raised by tectonic activity so much that they now appear above sea level. The volcano has been built on a submarine base of Mesozoic limestone. Some of these old rocks are metamorphosed to skarn* by contact with the rising magma. The volcanic products of Nisyros are basaltic-andesite to rhyo-dacite tephras and lavas, with abundant mafic inclusions.

Polyvotis, another of the many craters phreatic eruptions have left on Nisyros. (© lenuleka, via esys.org)

The last hydrothermal eruptions occurred in the southern part of the Lakki plain. They created the craters of Polyvotis and Phlegethon (1871–1873) and the Polyvotis Micros crater (1887), and they partially destroyed the small Lophos dome. A large fumarolic field is now present in this area, which is affected by fracturing along the main active fault systems.

Skarn from Mt. Monzoni, Northern Italy, by Siim Sepp (Sandatlas). It is composed of calcite (blue), pyroxene augite (green), and garnet grossular (orange).

*Skarns or tactites are calcium-bearing calc–silicate rocks. Skarns are most often formed at the contact zone between intrusions of magma bodies and carbonate sedimentary rocks such as limestone and dolomite. Hot fluids derived from magma are usually rich in Si, Fe, Al and Mg. These fluids mix in the contact zone, dissolve calcium-rich carbonate rocks, and convert the host carbonate rock to skarn deposits. This is a metamorphic process called metasomatism. Skarns are often mineralized and can form significant ore deposits for a variety of metals. (Siim Sepp, Sandatlas)

 

PRESENT ACTIVITY

As the Titan Polivotis hasn’t managed to wriggle free from under Nisyros, the island is still rocking and heaving at times. Nisyros lies in an area of intense tectonic activity. The presently active part of the Kos-Nisyros volcano-tectonic complex is located at the NW coast of Nisyros island.

Nisyros: hydrothermal area on the floor of Stefanos Crater (© no author given, via oddviser.com)

A significant increase in seismicity and ground deformation there in the period 1995-1998 caused considerable concern – not much was known about possible precursors to eruptions in this volcano, or about how an eruption would develop. Since then quite a lot of papers have been published.

Cross section schematic of Nysiros volcano (Source)

It turns out that presently no activity is related to rising magma. However, a pluton under Nisyros has been emplaced in the past and the lava domes need watching as well as the hydrothermal activity.

In January 2003, the crater of Nisyros was declared off-limits to visitors. Temperatures in the hydrothermal system had increased from 210°C to 315°C, there was continuous microseismic activity as well as changes on the surface. The ban on visitors was prompted by a crack on the volcano that almost tripled in length over one year (but was apparently not of volcanic origin).

Hydrothermal activity
Hydrothermal explosive activity was mainly located in the eastern half of the caldera floor. There it produced numerous hydrothermal craters, 10 of which are well-preserved. Fumarolic activity and the craters are now related to the main NW- and NE-trending active fault systems. The outlet temperature of the fumaroles is close to 100°C, while temperatures at the bottom of two geothermal boreholes range from 300-340°C at 1550-1800 m depth. The absence of strongly acid gases (i.e. SO2 , HCl, HF) and the relatively high CH4/CO ratios indicate that fumarolic effluents are generated from a boiling hydrothermal liquid rather than from direct magma degassing. Also, the chemical composition of fumarolic gases from Nisyros indicate that both arc-type magmatic water and local seawater feed the hydrothermal system. The largest inputs of magmatic water seem to occur below the area of the Stephanos–Polyvotis Micros craters.

Nisyros Island in the book “Liber Insularum Archipelagi”, c. 1420, C. Buondelmonti. The map shows an eruption of gases in the central part of the volcanic island; Buondelmonti mentions in his description the commercial exploitation of sulphur and millstones.

Nisyros: Stefanos Crater, the “must see” place for all guided tours on the island. (© Lifeinapixel-Fotolia)

 

JUDGE FOR YOURSELF AND BEWARE…

This 2013 YT video by Ulf Kletzsch shows the wonderful Stefanos crater in detail. But let me tell you: no matter if access to the crater had been approved, no amount of scientifically founded reassurement would make me go down into that crater! People are walking on the thin ground, that is hollowed out like a Swiss cheese, without even looking were they tread. Also, the entire bowl must be full of invisible poisonous gases. It’s really calling out loud for an accident to happen!

As on any active volcano, several types of hazard are possible on Nisyros: seismic activity from regional tectonics, seismic activity associated with magmatic and hydrothermal unrest, hydrothermal eruptions, magmatic eruptions, landslides, and tsunami. The more surprising I found the outcome of a 2006 survey by B. A. Carrett:

– Most of the residents of Nisyros did not think that their volcano could be a threat to them;
– none (!) of them thought that a local tsunami could ever happen;
– most were not aware that an emergency plan for an eruption did NOT exist at all.
– Also, in general, they were not interested in receiving more information on the state of the volcano.

Tourists were somewhat better informed about the possible hazards, but most believed that there, surely, was an emergency plan… I wonder if things have changed in the last twelve years. In a quick search I did not find any official directions – at least for tourists – if and how evacuation etc. would be handled in case of an eruption.

Polyvotis Crater walls of pumice on Nisyros. (© Tedmek, via wikimedia)

The authors of a geochemical study conclude in their paper: “The historical hydrothermal eruptions, the present chemical changes of the fumaroles and the physical phenomena affecting Nisyros could represent long-term precursors of a new period of volcanic unrest, which might culminate in a magmatic eruptive phase. Similar phenomena actually preceded recent magmatic eruptions such as at Montserrat, Lesser Antilles and Guagua Pichincha, Ecuador.” (G. Chiodini et al., 2002)

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BESIDES…

Besides all the geological points of interest, Nisyros also has a long, rich and well-documented history of human occupation. Beginning with artifacts from settlements in Neolithic times and historical records from the Hellenistic epoch, evidence can be found through Roman, Byzantine, Venetian and Ottoman eras, up into modern times. There are the remains of ancient caverns and saunas, old forts, churches and monasteries with hagiographic frescoes; figurines, carvings and other historic pieces of craftmanship.

Ruins of the Acropolis of Nisyros, Paleokastro, on the hill above Mandraki. (© BarryH, via barrysramblings.com)

Old building slabs saved for reconstruction work. (© BarryH, via barrysramblings.com)

One of the awe-inspiring historical ruins is the Paleokastro (ancient castle), built from the 6th to the 4th century BC on the hill overlooking Mandraki. It is an example of a classical Hellenistic Acropolis, and a marvel of architecture and engineering. It is constructed with heavy basaltic andesite lava blocks, tightly fitted, to withstand millenia. The weight of some of the slabs that form the walls reach 3 to 4 tons. Six towers are still standing on the South side along with a 230m-long wall, and more on the other side.

Nisyros, Paleokastro. (© Mike1979Russia, via Wikipedia)

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Street in Mandraki, the main town of Nisyros. (© Shutterstock)

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Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.

 

Enjoy! – GRANYIA

 

SOURCES & FURTHER READING
– GVP, Nisyros
– Geochemical Indicators of Possible Ongoing Volcanic Unrest at Nisyros (2002, PDF)
– Geochemical evidence for mixing of magmatic fluids with seawater, Nisyros (2003, paywalled)
– Seismological and SAR signature of unrest at Nisyros caldera (2001, PDF)
– Morpho-Tectonic Structure of Kos-Nisyros-Tilos Volcanic Area (2010, PDF)
– […] perspective to magma differentiation at Nisyros (2017, paywalled)
– Geophysical Hazards and Hazard Awareness on Nisyros Volcano (2006, PDF)
– Epilogue Nisyros Island, the Inaccessible Outpost Between Orient and Occident, Home of a Restless Giant (2018, paywalled)
– Barry’s Ramblings (Blog)
– Nisyros-Seite v. Andreas Brüderlin (German)

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