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Variations in hydration within perlitised rhyolitic lavas - evidence from Torfajökull, Iceland

Research output: Contribution to journalJournal article

Published

Journal publication date04/2012
JournalJournal of Volcanology and Geothermal Research
Journal number1
Volume223-224
Number of pages10
Pages64-73
Original languageEnglish

Abstract

Perlitic fractures form due to the hydration of glassy, rhyolitic lavas. Perlitised lavas are also an important industrial commodity yet there has been little study on the mechanisms of outcrop perlitisation. Here the fracture populations, perlitisation and volatile concentrations of subglacial rhyolitic glassy facies have been studied adding a quantitative dimension to previous qualitative studies. Samples include hyaloclastite, perlitised and non-perlitised obsidian and microcrystalline rhyolite, which are all present in lava lobes at Torfajökull, Iceland. Fractures formed through cooling increase in spacing with increased distance inwards from the margins of lobes. The size distribution of perlitic beads is shown to follow a log-normal distribution. The degree of perlitisation and the total volatile content (measured using thermogravimetric analysis, TGA) of samples are shown to decrease with distance inwards from lobe margins. In general, increased perlitisation is accompanied by increased hydration although complexity exists within single outcrops. The elevated total volatile content of perlite is shown to be due to hydration by environmental water with the structural location of the water changing as total volatile content increases.

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