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.