The island of Nisyros is a stratovolcano at the eastern end of the Hellenic island arc. This arc of volcanoes is related to the northward subduction of the African plate beneath the Aegean microplate. Santorini, a better known volcano, is west of Nisyros.
The base of the island is made of hyaloclastite, lava flows, and breccias, mostly of andesite composition. These rocks are capped by pyroclastic deposits and volcanic domes of dacite composition. The pyroclastic deposits are related to two explosive phases of the volcano. Each phase was Plinian in character, producing tall column of ash as high as 9-12 miles (15-20 km) above the volcano. The collapse of the tall columns of ash made the pyroclastic deposits. The volume of magma erupted was great enough to cause the summit of the volcano to collapse, making a caldera. The two explosive phases were probably several thousand years apart and occurred roughly 25,000 years ago. After the caldera formed, eruptions produced the lava domes.
A great depression, Rammos and Lakki (shown in the above photo), is located just east of the center of the island. The west rim of this depression is formed by St. Elias, the highest point on the island. St. John mountain is on the east side of the depression. The depression is steep sided and its bottom is about 300 feet (100 m) above sea level. Stefanos, a small explosion sink, is at the far end of the valley. This view is from the north.
Stefanos (shown above) is one of five small explosion sinks along the south part of the depression. Stefanos is about 1,000 feet in diameter (300 m) and 80 feet (25 m) deep. Alexandros, an explosive vent, is just beyond Stefanos in the photo. Two dacite domes, St. Elias (left) and Nifios (right), form the west wall of the depression.
This photograph shows the wall of the Stefanos explosion sink.
Close up of Alexandros, an explosive vent on the southeast edge of the St. Elias dome.
It is suspected that the volcano erupted in 1422. In 1871, an eruption was accompanied by earthquakes, detonations, and red and yellow flames. Ash and lapilli were erupted 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, lapilli, and steam were also ejected. In 1956, fumaroles were observed along the west and south sides of Rammos .
Click here here for a Space Shuttle photo of Nisyros.
All photographs generously supplied by Manny Konstantinidis.
Sources of Information:
Georgalas, G.C., 1962, Catalogue of the active volcanoes of the world including solfatara fields; Part XII Greece: International Association of Volcanology, Rome, Italy, 40 p.
Limburg, E.M., Varekamp, J.C., 1991, Young pumice deposits on Nisyros, Greece: Bulletin of Volcanology, v. 54, p. 68-77.
Martelli, A., 1917, Il gruppo eruttivo di Nisiro nel Mare Egeo: Mem. Soc. Ital. dlle Scienze (detta dei XL), Ser. 3a, t. XX Roma.
Nicholls, I.A., 1971, Santorini Volcano, Greece - tectonic and petrochemical relationships with volcanoes of the Aegean region: Tectonophysics, v. 11, p. 377-385.
Simkin, T., and Siebert, L., 1994, Volcanoes of the world: Geoscience Press, Tucson, Arizona, 349 p.