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Date of Award


Document Type

Campus Access Master's Thesis

Degree Name

Master of Science in Geology (MS)

Administrative Home Department

Department of Geological and Mining Engineering and Sciences

Advisor 1

William Rose

Advisor 2

Lucia Gurioli

Committee Member 1

Simon Carn


Textural characterization of pyroclasts coarser than ash have been extensively investigated, providing a corroborated protocol, however textural characterization of ash particles have been only recently investigated, therefore an established standard procedure has not been presented yet. A novel strategy is proposed here for 2D ash particles studies, involving sieving, selection of the main grain size mode, SEM image acquisition through thin section preparation, componentry analysis, image processing and application of stereological conversions. Textural and componentry comparisons between ash, lapilli and bombs in the course of the 1974 basaltic sub-plinian eruption of Fuego volcano provide relevant results in the understanding of how particle size influences vesicle size and vesicle number density (Nv). An important remark concerns the measured Nv value, which records a complex degassing history and therefore attention must be taken when the aim is to relate this parameter with the magma ascent rate or the intensity of the eruption. Vesicular ash preserve the signature of the fastest magma at the fragmentation level while the dense microlite-rich components witness the presence of small portion of stagnant or recycled degassed magma along the conduit walls. Large vesicles in vesicular ash fragments are liberated and therefore not counted, thus vesicular ash are denser than lapilli and bomb. The vesicles in the ash size pyroclasts present a narrow diameter range of ~20-120 μm, providing information about vesicles formed during fragmentation. Larger vesicles measured in larger pyroclasts are partly due to growth which happens after eruption and during longer cooling, before bubbles are fossilized.