Ice-confined construction of a large basaltic volcano—Austurfjöll massif, Askja, Iceland

Abstract

Austurfjöll is the largest basaltic glaciovolcanic massif at Askja volcano (Central Iceland), and through detailed studies of its volcanological and geochemical characteristics, we provide a detailed account of the sequence and structure of the ice-confined construction of a large Icelandic basaltic volcano. In particular, Austurfjöll represents a geometry of vents, and resulting glaciovolcanic morphology, not previously documented in ice-confined basaltic volcanoes. Austurfjöll was constructed during two major phases of basaltic volcanism; via seven eruptive episodes through disperse fissure-dominated eruptions. The earliest episode involved a rare and poorly-exposed example of subaerial activity, and this was succeeded by six episodes involving the eruption of ice-confined pillow lavas and numerous overlapping fissure eruptions of phreatomagmatic tephra. Evidence of local subaerial lavas and tephras indicates the local growth of eruptive centers above englacial lake levels, and subsequent flooding, but no prolonged subaerial activity. Localized ice-contact facies, paleowater levels and diamictons indicate the position and thickness of the ice was variable during the construction of Austurfjöll, and eruptive activity likely occurred in multiple and variable level melt-water lakes during the last glacial period. Lithofacies evidence including gradational transitions from effusive to explosive deposits, superposition of fragmental facies above coherent facies, and drainage channels suggest that changes in eruptive style were driven largely by external factors such as drainage and the increasing elevation of the massif. This study emphasizes the unique character of Austurfjöll, being composed of large pillow lava sheets, numerous (>40) overlapping glaciovolcanic tindars and only localized emergent deposits, as a product of its prolonged iceconfined eruptive history, contrasts with previous descriptions of tuyas and tindars.

The glaciovolcanic evolution of the Copahue volcano, Andean Southern Volcanic Zone, Argentina-Chile

“…There are several cases of pillow lavas formed in early stages of subglacial eruptions in thick ice (e.g. Skilling, 1994;Edwards et al, 2009;Pollock et al, 2014;Russell et al, 2014;Graettinger et al, 2019). The peperitic contact with the underlying glacial deposits also supports the inference that pillow lavas are coeval with ice (Smellie and Edwards, 2016).…”

Hydrothermally altered deposits of 2014 Askja landslide, Iceland, identified by remote sensing imaging

“…Moreover, the area contains the outcrops of two (2) rather small basaltic lava flows that erupted in 1922-23 in the SE part of the caldera from vents north and south of the landslide (Thorinasson and Sigvaldason, 1962;Sigvaldason, 1964;Thorinasson, 1968), and two (2) rhyolite lava bodies of unknown age situated in the SE corner of the caldera (McGarvie, 2009;Graettinger et al, 2019). The southern of the two (2) domes is located in the alleged source region of the landslide, where it is attached to and conformably overlain by the subglacial deposits of the Austurfjöll massif (Graettinger, 2012) forming the part of the caldera wall that later collapsed including its pumice cover.…”

On the Origin of Seismic Anisotropy in the Shallow Crust of the Northern Volcanic Zone, Iceland

“…The last eruption of Askja was in 1961, when a 2 km‐long fissure opened up, with lava breaching the eastern side of the main caldera wall. Surface mapping around Askja has revealed a complex pattern of both caldera‐concentric and rift‐parallel features, including faults and surface fractures, which deviate in orientation from those observed elsewhere in the associated rift zone (Graettinger et al., 2019; Hjartardóttir et al., 2016).…”