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The 10th century Eldgjá fissure eruption is the largest in Iceland in historical time. It erupted 21.0 km3 of magma, with 1.3 km3 as tephra in at least 16 explosive episodes from subaerial and subglacial vents, producing magmatic and phreatomagmatic deposits respectively. Grain-size distributions for these end-members show distinct differences at comparable distances from source: the former are coarser and unimodal; the latter are finer and bimodal. These distributions appear to record different primary fragmentation histories. In contrast, the vesicle-size distributions of pyroclasts from each type of deposit show the pyroclasts underwent similar vesicle nucleation and growth prior to fragmentation. This indicates that the role of glacial water was comparatively late-stage, re-fragmenting an already disrupting magma by quench granulation. The presence of microlite-rich domains within clasts reveals a history of complex conduit evolution, during the transition from a continuous dyke to focussed, discrete vents.
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