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Through weathering processes, volcanic rocks contribute both to nutrient flux into the biosphere and atmospheric CO 2 drawdown. As rhyolitic rocks are of higher silica content and have lower concentrations of biologically-important elements than basalts they might be expected to be less easily weathered by a biota. Investigations on the microbial diversity and weathering of silica-rich rhyolitic glass (obsidian) from a lava flow in Iceland are reported. 16S rDNA analysis of rock whole genome DNA shows that the rock hosts remarkable eubacterial diversity. Irregular pitted weathering textures correspond to regions of eubacterial colonization as shown by FISH. Weathering processes proceed at alteration fronts, with a preference for potentially nutrient-rich regions containing plagioclase and pyroxene crystals, although these features are less well defined than those previously reported from basaltic glass, consistent with the lower rates of chemical weathering previously reported for rhyolites compared to ba...