Acid hyaloclastites |
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| Authors: | Hans Pichler |
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| Institution: | 1. Mineralogisch-Petrographisches Institut der Universit?t Tübingen, Wilhelmstrasse 56, Tübingen, Germany
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| Abstract: | In exceptional cases also acid hyaloclastites can be originated by submarine extrusion of acid lavas. Acid hyaloclastites — which hitherto were not known — could be found for the first time on the western Ponza-Islands/Tyrrhenian Sea, Italy. The formation of acid hyaloclastites is closely connected with the process of auto-brecciation proceeding during the extrusion of acid lavas; both processes work together and cause the disintegration of parts of extruding lavabodies (domes etc.) into a mass of volcano-clastic materials. Hyaloclastization that is both the chilling of fluid lava in contact with water, ice or water bearing country rocks and the shattering of the congealed glassy skin by the pressure of the inflowing lava. Auto-brecciation that is destruction of parts of the extruding mass, not only by movements proceeding during the slow cruption but also by explosive phenomena depending on the release of the vapors. Hyaloclastization and auto-brecciation of acid lavas are an example of convergency: through both processes more or less similar deposits are formed. Thus brecciated acid hyaloclastites are not to be distinguished from auto-breccias. Two occurrences of brecciated lavas in California are interpreted as acid and intermediate hyaloclastites. Both are originated by the intrusion of rhyolitic respectively of andesitic lavas into water-bearing country rocks. In particularity for Iceland the possibility is discussed, that the formation of acid hyaloclastites also may occur in case of subglacial eruptions of acid lavas. |
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