The morphology and texture of Plinian pyroclasts reflect their lateral sourcing in the conduit

•>3500 Plinian clasts analyzed from proximal and medial pits of the 1060 CE Glass Mountain eruption, Medicine Lake Volcano, CA.•Changes in clast density are correlated with change in vesicle shape, clast shape, and clast size.•Morphology and texture of Plinian pyroclasts reflect their formational location in the conduit.•Study of μm- to cm-size clasts is necessary to capture breadth and complexity of eruptive processes. We investigated the relationships between the size, shape, density and texture of thousands of juvenile pyroclasts from the Plinian phase of the 1060 CE Glass Mountain eruption of Medicine Lake Volcano (California, USA). Analyzed pyroclasts, which originate from representative beds of both proximal and medial deposits, range from 0.125 mm to several centimeters in diameter. We show that the size, shape, density and texture of juvenile Plinian pyroclasts are intimately interrelated: as the size of pyroclast decreases from 64 mm to 0.125 mm, their shape shifts from equant to tabular to elongate. Moreover, while rounded vesicles dominate the largest clasts, elongate and distorted vesicles become progressively more present with decreasing clast size. Concurrently, clast density continually increases in a sigmoidal way from ∼0.4 gcm−3 (>80% porous) in the largest clasts to ∼1.7–1.9 gcm−3 (20–30% porous) by 0.125 mm. These observations are identical for both the proximal and medial locations. We show that expansion, compaction and secondary fragmentation did not significantly change the texture of juvenile pyroclasts after fragmentation. The size dependency of density and texture of Plinian pyroclasts is instead interpreted to originate from the lateral position of the magma in the conduit prior to fragmentation: smaller clasts result from higher shearing closer to conduit walls, where greater shear stresses increasingly elongate vesicles and fragment particles in smaller pieces. Moreover, ∼5% of pyroclasts 1–8 mm in size are likely agglomerates of mono-texture ash particles that sintered within the conduit during or shortly after primary fragmentation. Our work shows that future studies should analyze the full spectrum of clast sizes, from the micron- to centimeter-scale, in order to fully capture the complexity and breadth of eruptive processes across the volcanic conduit.