THE IMPORTANCE OF MICROBIOLOGICAL ACTIVITY IN THE ALTERATION OF NATURAL BASALTIC GLASS

The textural development of palagonite may differ profoundly depending on whether alteration occurred in the outermost 6-7 mm thick light-exposed surface zone of deposits, or elsewhere. In the former case, a pit-textured development of the parent basaltic glass develops as a consequence of local establishment of a highly alkaline micro-environment (pH > 9) for which the light-dependent cryptoendolithic cyanobacteria are considered most likely to be responsible. A highly porous, sponge-textured variety of palagonite, frequently defining zoned layers, contains abundant examples of bacteria. The shape and size of the pores combined with the geochemical development strongly suggest that bacteria have played an important role in the development of this texture.