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Petrographic studies of thin sections from the 1979 and 2017 Surtsey drill cores provide new insightsinto microstructural features in basaltic lapilli tuff sampled from the principal structural and hydrothermalzones of the volcano. These describe narrow rims of fine ash on altered glass pyroclasts in thin sections ofthe 2017 cores, characteristics of granular and microtubular structures in the original thin sections of the1979 core, and glass alteration in diverse environments. The narrow ash rims follow the outlines of glasspyroclasts in the subaerial tuff cone and in submarine and sub-seafloor deposits; they suggest complexeruptive and depositional processes. The tubular microstructures resemble endolithic microborings in olderoceanic basalt; they suggest possible microbial activity. Tubule lengths indicate rapid growth rates, up to30 µm in ~15 years. Comparisons of glass alteration in thin sections prepared immediately after drilling in1979 and 2017 indicate differential time-lapse alteration processes in the structural and hydrothermal zonesof the volcano. In contrast, thin sections of the 1979 core prepared after 38 years in the repository reveallabile glass alteration during archival storage. The oven-dry density of the sub-seafloor lapilli tuff decreasesin 2017 samples with high porosity and water absorption and increases in 2017 samples with a compactash matrix and lower water absorption. The petrographic descriptions and material measurements providea foundational reference for further investigations of explosive eruption and deposition of basaltic tephraat Surtsey and the subsequent alteration of these deposits in the volcanic environment and, potentially, thecuratorial environment.

Petrograpic and material observations of basaltic lapilli tuff, 1979 and 2017 Surtsey drill cores, Iceland