Chemical and Textural Surface Features of Pyroclasts from Hydrovolcanic Eruption Sequences

摘要

Hydrovolcanic pyroclasts are produced by the interaction of erupting magma with surface or near-surface water. Eruption energy determining pyroclast transport and depositional modes is dependent upon the amount of water interacting with magma. Grain morphologies, size distributions, surface textures, and chemical effects studied record the history of eruption cycles for volcanoes under consideration. Samples of crater-rim stratigraphic sequences from Crater Elegante and Cerro Colorado, Mexico, and from Panum Crater and Obsidian Dome, California, illustrate a basaltic tuff ring and tuff cone and two rhyolitic tuff rings respectively. Grain morphologies observed by scanning electron microscopy (SEM) reveal information on the process of melt fragmentation. Surface alteration and diagenesis have greater effect on pyroclasts from wet eruptions compared to dry ones. These textures are patchy overgrowths of microcrystalline and hydrated materials. Surface chemical characteristics observed with energy dispersive spectral analysis (EDS) show relative gains or losses of elements and an apparent enrichment of silica on altered surfaces. (ERA citation 08:024491)