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Nisyros is composed entirely of volcanic rock. Its type and formation is directly dependent on the composition of the magma, which feeds the action of its creation.
In the case of Nisyros, the original magma which triggers the volcanic activity is what geologists call calc-alkaline basalt: a fluid made up of silica dioxide, constituting 50% of its weight, aluminium trioxide (19%), magnesium and ferrous oxide (8%), calcium oxide (12%), sodium oxide (2.5%) and less than 1% of titanium dioxide and potassium oxide. This is the primary magma always found in volcanoes emerging in zones where one lithospheric plate is subducting under another. When this magma rises to the surface quickly, soon after its formation in the earth’s crust, it produces basaltic lava. However, if it remains within the crust for a long time, it gradually changes, acquiring more silica, potassium and sodium. The basalt thus gradually evolves intoandesite (silica dioxide 56-63%), dacite (silica dioxide 63-70%), and finally rhyolite (silica dioxide >70%). The texture of volcanic rock (amorphous glass and mineral crystals) depends both on the composition of the magma and on the speed of cooling. Thus the amount of volcanic glass – a material with a chaotic structure produced by the sudden cooling of molten rock – depends chiefly on the speed at which the magma cools; it is encountered in greater quantities in the rhyolites than in the basalts. The opposite is the case with the mineral crystals, usually found in greater quantities in the basalts rather than in the rhyolithes. The mineral crystals found in the rocks of Nisyros are:
The basalts and andesites are mainly composed of feldspars, pyroxenes and olivines. The dacites and rhyolites are composed of feldspars, pyroxenes, hornblendes and glass. The colour of the rock is also a result of its chemical composition and rate of cooling. When the cooling rate is low, the colour varies from dark grey in the basalts to an ash-white in the rhyolithes. If the cooling rate is high, even the rhyolites are black. A characteristic example is obsidian, a rock of black rhyolitic volcanic glass with very few crystals. The gases hosted by the magma also play a decisive role in the final form of the volcanic rocks. A fluid basalt or andesitic magma which contains many gases produces rocks with a spongy form and a characteristic black or dark red colour which we call scoriae. A viscous dacitic or rhyolithic magma rich in gases produces the pumice stone, a white rock consisting of ‘blown’ glass with few crystals. The air bubbles often make up more than 70% of the volume of the stone, enabling it to float in water. All the above forms and kinds of rock are to be found on Nisyros. The basalts and andesites could mainly be found prior to the catastrophic eruptions, as ashen grey or ash-coloured flows of lava were forming the first volcanic cone of the island. Between these flows we often encounter volcanic ash of red and black scoriae of the same composition. The dacites and rhyolites mainly date from the end of the construction of the subaerial volcanic cone: like the ash-white and ash-grey thick flows of the Argos lava, like the tephra of the Kyraeruptions, like the domes and flows of lava of Emporio. They are also prevalent in the catastrophic eruptions with the rhyolitic lower and upper pumice, the thick rhyolitic flows of lava and the domes of Nikia, the dacitic post-caldera domes from Boriatiko to Karaviotis. Quite often, dacites and rhyolites present andesite enclaves: these constitute new basic magma entering the magma chamber and blending with the already existing acid magma. The only rock mentioned above, which cannot be found on Nisyros is obsidian. Its absence is made up for in Yali in the north-eastern part of the island, which is composed of obsidian and perlite (hydrated volcanic glass). A special class of rocks on Nisyros are the altered rocks. They are found mainly in the area of Ramos, as it is here that the most intense hydrothermal activity occurred; and this activity was responsible for their creation. For thousands of years the area has witnessed the escape of gases and dozens of hydrothermal eruptions. The volcanic rocks in the area are in constant contact with the high-temperature fluids of the fumaroles, which are essentially a diluted form of sulphuric acid. Thus their original composition is modified and in their place remain kaoline (a white mineral consisting of silica and aluminium), and opal (amorphous silica). At the same time significant quantities of sulphur and gypsum crystals are deposited. Finally, a strange and quite rare type of rock, which geologists call Skarn, is also to be found among the rocks thrown up from the depths by the eruptions of Kyra and the two last catastrophic eruptions. Its color ranges from pale green to a dark oily hue, or from pinkish-red to black, according to the crystals it contains. These are mainly pyroxenes, olivines, epidotes (minerals of iron, magnesium and aluminium, in crystals whose colors range from dark yellow to black) and garnets (minerals of silica, calcium and aluminium with crystals whose colors range from red to black). The crystals are often large and well-formed, so much so that samples may even be collected. Skarn is created when the molten rock penetrates limestone or marble. The heating and the mixture of limestone (CaCOs) and magma trigger a number of reactions which result in depositing of the minerals and creation of the skarn around the walls of the magma chamber. Subsequently, the eruptions break off fragments of the walls and force them together with the magma. Thus we find them on the surface in the layers of volcanic tephra. The rocks of Nisyros provide the islanders with great building materials and they support their economic activity. The “Paleokastro”, was constructed using only basaltic andesite, one of the hardest rocks in the world. Even today, we are impressed by the ability of the craftsmen of the time to carve the stone so precisely as to achieve a perfect alignment of the sides of the huge blocks. Beautiful little houses built with hewn slabs of andesite and dacite are found scattered all over the island countryside, most of them now abandoned. Many of the island’s churches and houses are decorated with lava carvings, skillful enough to be described as works of art. Nisyros has been known for its celebrated millstones since ancient times , and the handmills, made of basalt and andesite, are widely exported. Strabo refers to them in his account of the island: «(Nisyros) is round, mountainous and rocky, and from these rocks the islanders make millstones. Hence the profusion of millstones in the islands all around» . Pumice is the rock which has seen the most commercial exploitation in the area and has made an important contribution to the island’s economy over the last decades. The little island of Yali has huge resources of the highest quality pumice which covers a significant part of local and even global demand. The pumice of Nisyros is of lower quality. A small quarry operated for a few years on the northern slopes of the island, exploiting the ‘upper pumice’ formation. The sulphur found in the altered rocks of the Ramos area, especially near the hydrothermal craters, has also been the object of commercial exploitation at times. Buondelmonti was the first to mention the production of sulphur on Nisyros. In 1879, systematic exploitation was initiated by the entrepreneur A. Rallis, who built an enrichment plant in the bay of Agia Irini and exported the sulphur mainly to Middle Eastern countries. After 1885, activity declined and was eventually abandoned, resulting in the enrichment plant being destroyed. The derelict remains of the installations can still be seen today in the bay of Agia Irini. A second round of intense mining activity was seen in 1944-45, its main destinations being Samos, Crete and the Peloponnese. Today the issue of mining activity in the region has to be seen from a different perspective: the perception of Nisyros as a place of outstanding natural beauty, an open-air geological museum, one of the few remaining geo-ecosystems in equilibrium, whose protection and preservation must be an overriding priority.
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