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Nea Kameni is a small uninhabited Greek island of volcanic origin located within the flooded caldera of Santorini. Nea Kameni and the neighbouring small island Palea Kameni (the new and old 'burnt islands') have formed over the past two millennia by repeated eruptions of dacite lava and ash. Major eruptions over the past 300 years took place in 1707-1712, 1866-1870, 1925-1928, and 1939-1941. The last small eruption happened in 1950, and involved lava dome extrusion.
Nea Kameni is nearly round and has a diameter of approximately 2 km and an area of 3.4 km2. Nea Kameni is monitored closely by scientists from the Institute for the Study and Monitoring of the Santorini Volcano (ISMOSAV), and is a protected scientific site. The nearly barren island is visited daily by dozens of tourist boats throughout the summer. The visitors take a well maintained gravel path to the 130-meter-high volcanic crater, from which wisps of a sulphurous steam rise, transforming the environment in places into a "yellow wasteland".
This provides a perfect location to see a particular kind of volcanic rock - dacite.
As the general name implies, volcanic rocks originate are a type of igneous rock that results from volcanic processes within the earth’s crust. Volcanic rocks are classified both in terms of texture and chemistry.
Texture
Volcanic rocks are usually fine-grained or aphanitic to glassy in texture due to rapid cooling. They often contain clasts of other rocks and phenocrysts. Phenocrysts are crystals that are larger than the matrix and are identifiable with the unaided eye. Rhomb porphyry is an example with large rhomb shaped phenocrysts embedded in a very fine grained matrix.
Volcanic rocks often have a vesicular texture, which is the result voids left by volatiles (gas) escaping from the molten lava. Pumice is a rock, which is an example of explosive volcanic eruption. It is so vesicular that it floats in water.
Naming
Volcanic rocks are named according to both their chemical composition and texture. Basalt is a very common volcanic rock with low silica content. Rhyolite is a volcanic rock with high silica content. Rhyolite has silica content similar to that of granite while basalt is compositionally equal to gabbro. Intermediate volcanic rocks include andesite, dacite, trachyte and latite.
Pyroclastic rocks are the product of explosive volcanism. They are often felsic (high in silica). Pyroclastic rocks are often the result of volcanic debris, such as ash, bombs and and tephra, and other volcanic ejecta.
Dacite
Dacite is of intermediate compositions betwee andesite and rhyolite. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram. Dacite is also defined by silica and alkali contents in the TAS classification.
The word dacite comes from Dacia, a province of the Roman Empire which lay between the Danube River and Carpathian Mountains (now modern Romania) where the rock was first described.
On hand specimen many of the hornblende and biotite dacites are grey or pale brown and yellow rocks with white feldspars, and black crystals of biotite and hornblende. Other dacites, especially pyroxene bearing dacites, are darker colored.
In thin section, dacites may have an aphanitic to porphyritic texture.
Porphyritic dacites contain blocky highly zoned plagioclase phenocrysts and/or rounded corroded quartz phenocrysts. Subhedral hornblende and elongated biotite grains are present. Sanidine phenocrysts and augite (or enstatite) are found in some samples. The groundmass of these rocks is often aphanitic microcrystalline, with a web of minute feldspars mixed with interstitial grains of quartz or tridymite; but in many dacites it is largely vitreous, while in others it is felsitic or cryptocrystalline.
Dacite usually forms as an intrusive rock such as a dike or sill. Examples of this type of dacite outcrop are found in northwestern Montana and northeastern Bulgaria. Nevertheless, because of the moderate silica content, dacitic magma is quite viscous (Whittington, 2009) and therefore prone to explosive eruption. A notorious example of this is Mount St. Helens in which dacite domes formed from previous eruptions.
Dacitic magma is formed by the subduction of young oceanic crust under a thick felsic continental plate. Oceanic crust is hydrothermally altered causing addition of quartz and sodium (DeVore, 1983). As the young, hot oceanic plate is subducted under continental crust, the subducted slab partially melts and interacts with the upper mantle through convection and dehydration reactions (Drummond, 1990). The process of subduction creates metamorphism in the subducting slab. When this slab reaches the mantle and initiates the dehydration reactions, minerals such as talc, serpentine, mica and amphiboles break down generating a more sodic melt (Fyfe, 1975). The magma then continues to migrate upwards causing differentiation and becomes even more sodic and silicic as it rises. Once at the cold surface, the sodium rich magma crystallizes plagioclase, quartz and hornblende (Defant, 1991). Accessory elements like pyroxenes provide insight to the history of the magma.
The process by which dacite forms has been used to explain the generation of continental crust during the Archean eon. At that time, the fabrication of dacitic magma was more ubiquitous due to the availability of young hot oceanic crust. Today, the colder oceanic crust that subducts under most plates is not capable of melting before the dehydration reactions therefore inhibiting the process (Atherton, 1993).
The formation of dacite provides a great deal of information about the connection between oceanic crust and continental crust. It provides a model for the generation of felsic, buoyant, perennial rock from a mafic, dense, short-lived protolith.
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