Cteklovatye rocks

Microscopic structure of volcanic products also varied. Some lava formed at solidification of glassy rocks, are in most cases trahitovye lava various localities. They usually represent highly lustrous masses with conchoidal fracture when broken, and give a translucent shards with sharp edges. Often they are black, rarely dark-brown, gray and green. This so-called obsidians: and they do not represent pure volcanic glass, and under the microscope can be seen in their bulk of many small crystals. Obsidian lava known in Europe for the Aeolian Islands north of Sicily, and in large numbers in Iceland, in addition, in Armenia, in the Canary Islands, some of the West India Islands, Mexico, the National Park Connect. States on the river Iellouston, and Java is a lot of rich silica glassy lavas. However, the vitreous structure of the lava is generally the exception, especially the rarely seen it from the poor in silica basaltic lavas. The edges of the basalt veins, which they are adjacent to a neighboring rock (so-called selvage), sometimes composed of basaltic glass - tahilita. The same structure was also observed in the lava flow Mouth of Hell (Gueule de l'Enfer) in the Department Ardem in central France, the lower parts of it, located directly on the ancient rocks (gneisses), have glassy structure. Modification of glassy basalt lavas in large numbers are on the island of Hawaii, is not alien to those of education and the influx of western North America. Reason, by which the lava acquire vitreous structure is not completely known to us. A common view on which the speed of cooling is crucial, can not be withheld because of the existence of powerful streams, consisting of obsidian and splitting into separate columnar. Maybe the malleability of acid lavas, which prevents the free movement of molecules is important for glassy structure. Contrast, represent the main lava, with a significant mobility of the particles, which allows them to fully crystallize. The reason for the high mobility of basic lavas Dana sees in ready fusibility their constituent parts, and the tendency of acid to form glassy lava rock Yudd ascribed Vaeth rich content in these sodic and potassium silicates, which, as the factory practice, generally favor the formation of glass.