The creation of the post-caldera domes was the last magmatic activity manifested on Nisyros. None of the eruptions of the volcano recorded in historical sources produced molten rock.
They were all hydrothermal (phreatic) explosions involving the hot steam below the island’s surface.
Sea and rain water penetrate the island’s rocks and accumulate in deep layers. There they are heated by the hot gases escaping from the magma below and by the temperature of the magma itself seeping up through the rocks. Temperatures rise to 400 or 500 Celsius and the water is converted into super-heated steam which exerts tremendous pressure on the layers of rock above. When this pressure exceeds the weight and the cohesion of the covering rocks, it hurls them upwards causing a hydrothermal explosion. The products of these explosions are steam, gases such as hydrogen sulphide, carbon dioxide and methane, accompanied by mud and fragments of old rocks. They result in circular craters, usually of modest size (less than 500 metres in diameter).
Such were all the eruptions recorded on Nisyros during historical times. In the area of Ramos, the southern section of the caldera floor, there are traces of twenty such craters. 10 are well preserved and each has its own name. Most of the crater names currently used were given by Rallis, the entrepreneur who operated sulfur earth quarries in the late 19th century.
The two oldest hydrothermal craters are the Kaminakia (or Kaminia), at the base of the cliff of the caldera below Nikia. Each of them has a diameter of about 150 metres.
The largest and most imposing crater is the one named Stefanos , which monopolizes the interest of tourists, as it is pointed out to them asthe volcano. It is one of the largest and best preserved hydrothermal craters in the world. Ellipsoid in shape, its longest axis is 330 metres in length, its shortest 260 metres, while it reaches a depth of 27 metres. We can’t be sure of its exact age, nor of that of the other craters. Since it is located in soft, friable rock, easily eroded by wind and rain, and is lcoated in the topographically lowest area of the caldera , it cannot be more than 3,000 to 4,000 years old.
Next to Stefanos is a smaller crater, known as little Stefanos (or Andreas).
Most of the hydrothermal craters, however, are accumulated in the area of Lofos, a small post-caldera dome which has literally been destroyed by hydrothermal explosions. Here there are 6 well preserved craters, the creation of three of them recorded in historical sources.
Logothetis is the oldest crater in the area, while Megalos Polyvotis is the largest in the area (diameter 300 by 150 metres), and is more recent than Stephanos. At some point after its formation, this crater became a small lake in which 20 metres of lake sediment were deposited. These are the soft white-gray and yellowish thin layers of rock which can be made out on the walls of the Polyvotis crater which it houses in its centre.
The first extended account of Nisyros to follow the reference in Strabo is to be found in the “Liber Insularum Archipelagi”, the descriptions of the Aegean islands by the Florentine monk Cristoforo Buondelmonti, who visited Nisyros between 1414 and 1420. He has left a wonderful description of the state of the volcano and the intense hydrothermal activity of the time.
«Circa medium mons erigitur altissimus, quo in summitale per subterraneos meatus sulphureus ignis die ac nocte eructat in altum, ut in insula Stronguli apud Liparum habetur. In descensu vero montis ad iactum lapidis, fons calidissimus emanat in imum, et in plano circa lacum profundissimumque obscurum aquae descendunt; Ibique colentes quantitatem maximum sulphuris mercatoribus praeparant. Et quia in tantum viget intensitas caloris, de medio usque verticem nullus est ansus sine sotularibus accedere ligneis.»
«In the centre of the island is a very high mountain which belches out sulphurous flames from underground passages day and night, as it happens in the island of Stromboli, near Lipari. Just a stone’s throw down from the summit there is a hot spring whose waters descend into a deep, dark lake on the plain below. The islanders gather the sulphur in great quantities and sell it to passing traders. From half way up the mountain to the summit the heat is so intense that no one dares ascend without wooden shoes to protect his feet».
In the past Buondelmonti’s descriptions were mistakenly interpreted as accounts of a hydrothermal or even volcanic eruption.
Another account, this time of a hydrothermal eruption in the 15th century, is to be found in Fucks’ “Les volcans et les tremblements de terre” (Volcanoes and earthquakes) (Paris 1876) but does not contain detailed descriptions.
In the past Buondelmonti’s descriptions were mistakenly interpreted as accounts of a hydrothermal or even volcanic eruption.
Another account, this time of a hydrothermal eruption in the 15th century, is to be found in Fucks’ “Les volcans et les tremblements de terre” (Volcanoes and earthquakes) (Paris 1876) but does not contain detailed descriptions.
The German archaeologist Ross Ludwig visited the island on 9-11 August 1841 and described the state of the volcano using graphic terms:
«We found the ground quite dry and, with the exception of a few, similar places, firm under our feet; on the southern rim of the crater, however, along a stretch of some 50 paces, the ground was seething and there were muffled explosions to be heard, repeated every 20 to 30 seconds. There was also a strong smell of sulphur, which the north wind carried over the village of Nikia, and which we could still perceive early this morning, on the southern coast of the island, some one and a half hours walking distance away. After heavy rainfall and especially when the prevailing wind is westerly, the fire apparently becomes much fiercer: A thick steam arises and the explosions become so loud that one can hear them over an hour’s walking distance away».
At the end of November 1871, a large hydrothermal eruption was witnessed and described by the French geologist, H. Gorseix:
“A la suite d’un violent tremblement de terre, les habitants des villages de Nisiros entendirent des detonations comparables a une suite de coups de tonnerre; des flammes rouges et jaunes s’eleverent plus haut que l’ile au-dessus des points ou il existait deja auparavant quelques fumees…; les champs au fond du cratere primitif furent couverts d’une poussiere blanche; la meme nuit, les deux grandes fumerolles s’ouvrirent, et, depius cette epoque, elles n’ont pas cesse de vomir des vapeurs.”
“After a violent quake the villagers of Nisyros heard crashes as loud as thunder; red and yellow flames leapt into the air, higher than the island’s loftiest peaks…; the fields at the base of the original crater were covered in white dust. That same night, huge fumaroles opened up and since then have not ceased to vomit forth their fumes.”
In 1872, as tremors and probably also hydrothermal activity continued, rumors were rife in Europe of a great volcanic eruption on Nisyros. The French Academy of Sciences resolved to send H. Gorseix to the island . The articles he published contain his vivid and detailed descriptions of the eruptions and quakes of 1873:
Le 3 juin, apres de fortes secousses de tremlement de terre, ressenties dans toute l’ile, une bouche de 6 a 7 metres de diametre s’ouvrit sur le revers exterieur du cratere adventif et fut le point de depart d’une fente de 50 metres de longueur, derigee N. 22o E. a S. 22o O. Pendant trois heures, il s’en echappa des torrents d’eau chaude salee, accompagnes de projections de pierres et suivis, pendant les trois jours suivants, d’eruptions tres-frequentes d’une boue noiratre tres-fluide. L’eau, s’evaporant, a laisse deposer des couches epaisses de chlorure de sodium et de magnesium, salies souvent par de l’oxyde de fer; elle inonda une grande partie des champs, et, si elle eut coule quelques heures de plus, elle eut trasforme en un vaste lac tout le cratere de l’ancien volcan…. De faibles secousses se faisaient sentir chaque jour; le 11 septembre elle devinrent beaucoup plus violentes. Dans les trois villages de l’ile, les maisons furent presque toutes lezardees; le village de Mandraki, situe au bord de la mer, fut surtout eprouve: les mours des jardins furent en partie renverses; le monastere et l’eglise, situes sur une butte de conglomerat trachytique, furent fortement endomagges. En meme temps, a quelques metres du rivage, la mer devint blanchatre; il s’en echappa des torrents de vapeur d;eau, melee d’hydrogene solfure. L’ ouverture de cette crevasse lineaire coincida avec un fait analogue qui eut lieu sur la falaise de l’ile Hyali, a trois milles en face du village de Mandraki. Les deux crevasses se refermerent quelques secondes apres leur ouverture.”
“On June 3rd, after severe tremors, felt all over the island, an aperture 6 to 7 metres in diameter appeared on the outside edge of the secondary crater and was the starting point for a fissure 50 metres long, running N.22 E to S. 22 W. For three hours torrents of hot, dirty water gushed forth, followed by a hail of stones and then, for 3 days, very frequent eruptions of a blackish, very liquid mud. As the water evaporated it left behind thick layers of sodium and magnesium chloride, often with an admixture of ferrous oxide; the mud had flooded a great part of the fields and, had it continued to flow a few hours more, would have transformed the whole crater of the old volcano into a vast lake…. Feeble tremors were still to be felt every day; on September 11th they became much more violent. In the island’s three villages the houses had almost all seen cracks appear in their walls; the village of Mandraki, by the sea, was worst afflicted: the garden walls were partially demolished; the monastery and the church, situated on an outcrop of trachytic conglomerate, were seriously damaged. Meanwhile, a few yards from the shore, the sea turned a whitish colour; it emitted torrents of steam, mixed with hydrogen sulphide. The appearance of this fissure coincided with a similar phenomenon on the cliff of Yali, three miles away opposite the village of Mandraki. The two fissures closed again within a few seconds of their first appearance.”
Let us summarize the description of the events of 1871-1873: we note that from the beginning of 1871 an increase in the amount of gases being emitted from the fumaroles (points where the hot gases escape from the ground) of Megalos Polyvotis is observed. In either October or the end of November 1871 a violent earthquake sets off the hydrothermal eruptions. Two small craters open up: Polyvotis – in the centre of Megalos Polyvotis, and Alexandros (or Flegethron) on the south cliff of Lofos. The steam which is generated covers the island like a mist. In this first eruption only small quantities of mud and rock are thrown up.
On the 3rd or 8th June 1873 the island is shaken by powerful local tremors, the Alexandros crater widens, a fissure unites it with Polyvotis and eruptions begin, hurling up mud and rocks. Rivers of hot mud pour from Alexandros and cover the area to its south, traveling a distance of 500 metres. The steam and hot water drench the branches of the trees and as they evaporate leave stalagmites of salt (NaCl).
From June to September only small seismic tremors are recorded, while the craters vent a huge quantity of steam, richly imbued with sulphur. On the 11th of September a powerful quake causes damage to the houses of Mandraki. A great underwater fissure opens, commencing on the coast at Mandraki and stretching northwards. Another big fissure cuts across Yali. Eruptions continue for several days within the craters of Polyvotis and Alexandros. The tremors last until December of the same year.
The most recent hydrothermal activity to be recorded at Nisyros dates from 1887, when the crater of Mikros Polyvotis was created. It is described very briefly by the Italian geologist A. Martelli, in his monograph “Il gruppo eruttivo di Nisiro nel mar Egeo”, (The volcanic group of Nisyros in the Aegean sea), published in 1917. “According to the islanders working in the sulphur mines, it was an eruption of mud and altered rocks with a fierce emission of steam.”
