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28 avril - 3 mai 2018: Le Joyau Égéen caché: l'Île de Milos - Île de Milos, Grèce
5-13 mai 2018: Montagnes de Feu - du Stromboli à l'Etna - Eoliennes + Etna
12-20 mai 2018: Montagnes de Feu - du Stromboli à l'Etna - Eoliennes + Etna
19-27 mai 2018: Montagnes de Feu - du Stromboli à l'Etna - Eoliennes + Etna
9-17 juin 2018: Montagnes de Feu - du Stromboli à l'Etna - Eoliennes + Etna
24 juin - 1 juil. 2018: Le Grand Lac de Lave du Nyiragongo - République démocratique du Congo et Rwanda
12-14 juil. 2018: Spécial Volcan Krakatau - Le Krakatau, détroit de la Sonde
15-31 juil. 2018: Volcans et Épices - Mer des Moluques (Sulawesi, Indonésie)
22-27 juil. 2018: Expédition au Volcan Yasur - Vanuatu (Mer du Sud)
6-14 oct. 2018: Montagnes de Feu - du Stromboli à l'Etna - Eoliennes + Etna
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Glossaire illustré de la volcanologie

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Volcanology: pierre ponce
Our tour guide Marta posing in Lipari's pumice...
Our tour guide Marta posing in Lipari's pumice...
Pumice from the Greek island Santorini
Pumice from the Greek island Santorini
Un très léger, habituellement brillant en couleur, écume roche volcanique formé par la lave solidifiée visqueux, contenant de nombreuses bulles de gaz.
Pumice is a textural term for a volcanic rock that is a solidified frothy lava composed of highly microvesicular glass pyroclastic with very thin, translucent bubble walls of extrusive igneous rock. It is commonly, but not exclusively of silicic or felsic to intermediate in composition (e.g. rhyolitic, dacitic, andesite, pantellerite, phonolite, trachyte), but occurrences of basaltic and other compositions are known. Pumice is commonly pale in color, ranging from white, cream or grey, but can be green brown or black. It forms when gases exsolving from viscous magma nucleate bubbles which cannot readily decouple from the viscous magma prior to chilling to glass. Pumice is a common product of explosive eruptions (plinian and ignimbrite-forming) and commonly forms zones in upper parts of silicic lavas. Pumice has an average porosity of 90%, and initially floats on water.

Scoria differs from pumice in being denser, with larger vesicles and thicker vesicle walls; it sinks rapidly. The difference is the result of the lower viscosity of the magma that formed scoria. When larger amounts of gas are present, the result is a finer-grained variety of pumice known as pumicite. Pumice is considered a glass because it has no crystal structure. Pumice varies in density according to the thickness of the solid material between the bubbles; many samples float in water. After the explosion of Krakatoa, rafts of pumice drifted through the Pacific Ocean for up to 20 years, with tree trunks floating among them. In fact, pumice rafts disperse and support several marine species. In 1979, 1984 and 2006, underwater volcanic eruptions near Tonga created large pumice rafts, some as large as 30 km that floated hundreds of miles to Fiji.

Famous pumice deposits in Europe include Lipari Island (Italy), Santorini and Yali (Greece), as well as the Laacher See (Germany).


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