Abstract
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.
Highlights
Icelandic eruptions can have significant impacts on the atmosphere and on aviation and the global economy, as demonstrated by the 2010 eruption of Eyjafjallajökull [Gudmundsson et al 2012; Langmann et al 2012]
This paper explores and quantifies shallow conduit and vent processes which influenced the nature of the explosive episodes of Eldgjá
The explosive episodes of Eldgjá provide the perfect opportunity to examine the role of water during a subglacial eruption because: 1) activity took place at both subglacial and subaerial vents and; 2) the magma composition remained relatively constant throughout the eruption
Summary
Icelandic eruptions can have significant impacts on the atmosphere and on aviation and the global economy, as demonstrated by the 2010 eruption of Eyjafjallajökull [Gudmundsson et al 2012; Langmann et al 2012]. This is primarily due to the wind-advected ash plumes produced by these events. Grímsvötn 2011 [Stevenson et al 2013], it is thought that explosive magma–water interaction (phreatomagmatism) will increase the abundance of fine ash particles present in the tephra [Kokelaar 1986; Wohletz et al 2013; Wohletz 1986]. Given the importance of fine-ash to impacts on aviation and infrastructure [Wilson et al 2012], it is critical to assess the role and timing of external water in ‘wet’ tephra-producing eruptions
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