Β. Creation of Nisyros

Nisyros began to be formed on a base approximately 300 metres deep which consists of limestone. This was created 150 million years ago from the activity and accumulation of marine organisms (corals and crustaceans) in shallow water,

and is located here after the great Alpine mountain-forming processes. The same rocks, on which the volcano of Nisyros began to rise, are now seen on north-western Kos, the Turkish coast (Knidos peninsula), and on the island of Kandeleousa (or Andeleousa or Fanari), 25 kilometers south-west of Nisyros.

• The volcano emerged above the sea and began to accumulate a subaerial cone approximately 150,000 years ago, just after the major Kos eruption. During the years that followed, violent, explosive as well as moderate volcanic activity occured in alternating sequence. Eruptions blasted the molten rock into the air, which, as it fell back to the ground was crushed and cooled, creating thin or thicker volcanic tephra beds. At times, slow rivers of molten rock would spurt from faults to fill valley areas and create consolidated, thick lava flows. Thus, for the next 80,000 years, a classic stratovolcano developed, in essence a large volcanic cone above sea level from these alternations of tephra, lava flows and dome layers. Its diameter is estimated to be approximately seven (7) kilometers and its peak height is 700 meters; a main crater atop was possibly filled by the water of a small lake. An island barely different from contemporary Nisyros in terms of size and shape.

• 70,000 years ago, volcanic activity occured at the southern and eastern slopes of the volcano, manifesting itself through viscous rhyolitic and dacitic lava spurting out; thus the Argos lava formations were created to the south, as well as the Pahia Ammos formation to the east.

• Over time, the Nisyros cone continued to develop and more viscous and gas-rich magma started to accumulate at shallower depths, just a few kilometers below ground surface. Eventually the vast sums of energy accumulated could only escape in one way: vast explosions. The first series of vast eruptions occured approximately 60,000 years ago. Entrapped magmatic gases blasted the molten rock; it then became pulverized and produced millions of tons of pumice stone. Factors decisively influencing the energy balance are sea water, water from ground water tables and water in small lakes in the volcano’s craters: mixed with magma, it turned into superheated steam, thus multiplying the potency of these eruptions, characterized as hydro-volcanic by volcanologists. Volcanic ash was ejected several kilometers high, often carrying away the fragments of older rocks, even of limestone located at significant depths below the volcano. The several gray tephra layers now mainly covering the eastern slopes of the island have been deposited by an equivalent number of explosions over a period of several thousands of years, sometime between 60,000 and 55,000 years ago. Tephra from these eruptions was carried several kilometers away by the wind; it is now found in Pyrgousa, Pahia and the northern plains of Tilos. These deposits created a geological formation now called ‘Kyra’, named after the monastery situated at the eastern slopes of the island.

• Following these eruptions, new andesitic magma ignited more volcanic activity, both as regards hydro-volcanic eruptions when in contact with water, and lava flows covering the NE and SW parts of the island, extending its area mainly to the NE and creating the cape of Katsouni.

• These eruptions were followed by the moderate extrusion of large amounts of viscous dacitic magma which gave rise to domes on the northern part of the island. Some of these domes collapsed and overlaid superimposed layers of hot block and ash flows on the northern and northwestern slopes of the volcano. Emporio is built on such lavas; these were therefore named the Emporio Lavas.

• After volcanic activity had stopped for a period of several thousands of years, approximately 45,000 years ago, the first devastating Nisyros volcano eruption occured. In a minimal period, only a few days, 8 billion tons of molten rock ejected into the air, producing over 20 billion cu. m. of pumice and ash. These were blown to heights exceeding 12 kilometers, creating a huge cloud and covering the entire island with tephra layers, up to 15 meters thick. Thus, the ‘lower pumice’ geological formation was laid. The volcano top collapsed in the 3 cu. km void created under the island, following the ejection of magma; thus, the first Nisyros caldera was created.

• Following a period of rest, the thick molten rock, low in gas and energy, started to spurt sluggishly. It erected large lava domes close to the eastern edges of the first caldera; when overflowing above the edges, it ran off to cover the southeastern slopes of the volcano with very thick lava flows, the so-called Nikia lavas. .

• The subsequent period of rest, lasting a few thousand years, was interrupted by the second devastating Nisyros eruption, approximately 35,000 years ago. Its characteristics are identical to the previous major eruption mentioned above. Nisyros was once more covered with twenty billion cu. m. of pumice and ash, falling as rain from the sky or as dense flaming clouds of gas and ash sweeping the slopes of the volcano. The resulting void generated below the island after blasting of magma was filled after the collapse of the middle part of the volcano, thus creating the Nisyros caldera as it can be seen today. Blasting activity only lasted a few days; the deposited tephra formed the ‘upper pumice’ geological formation, i.e. white pumice and ash now occupying the northern part of the island.

• In the ensuing period of rest, the Upper Pumice of Yali settled over Nisyros. The viscous magma with low gas concentrations remaining after the eruption in the lower layers of the magma chamber blended with newer magma, which was slowly coming in, started to spurt slowly over a period of several thousand years, creating the ‘post-calderic domes’, i.e. the hills of Mporiatiko, Nifio, Profitis Ilias and Trapezina, thus filling approximately 2/3 of the caldera. Moreover, the Karaviotis dome was created out of the caldera. Thus Nisyros obtained its current form approximately 25 to 20,000 years ago. The creation of the central domes is the latest magma activity that occured in Nisyros. None of the historic volcanic eruptions produced molten rock. They are all hydrothermal eruptions resulting from the presence of superheated steam under the surface of the island.