Sintered Pumice Aggregates: A Window into Primary Magmatic Fragmentation

Abstract:

Plinian eruptions are among the principal geological hazards along volcanic arcs. The vast majority of material ejected during such eruptions is pumice pyroclasts, which is produced by syn-eruptive explosive fragmentation of bubbly magma during rapid decompression in the conduit. During transport, disruptive collisions among ‘protopyroclasts’, the newly formed parcels of magma created upon fragmentation in the conduit, combined with their potential expansion and/or compaction, can modify their size distribution and textures, thereby erasing the record of primary fragmentation. Quantitative assessment of the conditions under which magma fragments and the characteristics of the products of primary fragmentation thus principally rely on laboratory experiments and numerical modeling. Here we show that some Plinian pumice lapilli from Medicine Lake Volcano (1060 CE Glass Mountain eruption), Newberry Volcano (700 CE Big Obsidian Flow eruption), and Crater Lake (both Cleetwood and climactic ~7.7 ka Mt Mazama eruptions) consist of an amalgamation of protopyroclasts that had partly sintered within seconds or less after primary fragmentation of magma. These pumices preserve the size distribution and textural characteristics of primary fragmentation, and therefore offer a unique window into the explosive fragmentation of magma in the conduit. Via image analysis from both Computed Tomography and Scanning Electron Microscopy, we measured the size distributions of 10^-6-to-10^-2-m protopyroclasts in two pumice lapilli from Medicine Lake. Both distributions follow a power law with a fractal dimension of ~2.4, consistent with experimental magma fragmentation by rapid decompression. The distributions are, however, significantly different from the final size distribution of the overall Glass Mountain Plinian fallout deposit, which is best fitted by a fractal dimension of 3.2–3.4. The difference between the fractal dimensions indicate that the pyroclast number density increased by 2 to 3 orders of magnitude between primary fragmentation and final deposition due to secondary fragmentation in the conduit and the plume. Some Plinian pyroclasts thus preserve a record of primary fragmentation that, with further study, can better place constraints on magma fragmentation and post-fragmentation mechanisms.