Abstract
Several broadly circular structures up to 16 m in diameter, into which higher strata have sagged and locally collapsed, are present in a tephra outcrop on southwest Öræfajökull, southern Iceland. The tephra was sourced in a nearby basaltic tuff cone at Varða. The structures have not previously been described in tuff cones, and they probably formed by the melting out of large buried blocks of ice emplaced during a preceding jökulhlaup that may have been triggered by a subglacial eruption within the Öræfajökull ice cap. They are named ice-melt subsidence structures, and they are analogous to kettle holes that are commonly found in proglacial sandurs and some lahars sourced in ice-clad volcanoes. The internal structure is better exposed in the Varða examples because of an absence of fluvial infilling and reworking, and erosion of the outcrop to reveal the deeper geometry. The ice-melt subsidence structures at Varða are a proxy for buried ice. They are the only known evidence for a subglacial eruption and associated jökulhlaup that created the ice blocks. The recognition of such structures elsewhere will be useful in reconstructing more complete regional volcanic histories as well as for identifying ice-proximal settings during palaeoenvironmental investigations.Electronic supplementary materialThe online version of this article (doi:10.1007/s00445-016-1048-x) contains supplementary material, which is available to authorized users.
Highlights
Öræfajökull is the largest active stratovolcano in Iceland
This study suggests the following sequence of events: (A) Subaerial eruption of two agglutinate cones, probably on a fissure. (B) Subglacial eruption under an inferred Öræfajökull ice cap, possibly on a prolongation of the same fissure system inferred in (A). (C) Jökulhlaup associated with the subglacial eruption. (D) Numerous large ice blocks stranded preferentially on the local watershed by the drained jökulhlaup. (E) Subaerial hydromagmatic eruption of Varða from another part of the same fissure system, with magma interacting with groundwater or seawater in subsurface fractures. (F) Erosion by an ice advance and retreat, resulting in the present-day outcrops
Unusual circular structures found in lapilli tuffs of a tuff cone tephra blanket were probably formed by the melting of buried ice blocks, which caused the overlying tephra beds to collapse into and fill the voids created
Summary
Öræfajökull is the largest active stratovolcano in Iceland. It is situated on the southern margin of Vatnajökull and has a basal diameter of c. 23 km, rising to 2110 m a.s.l. at Hvannadalshnúkur a rhyolite lava dome, which is the highest peak in Iceland (Fig. 1). Öræfajökull is the largest active stratovolcano in Iceland It is situated on the southern margin of Vatnajökull and has a basal diameter of c. The eruption in 1362 was the most notable, with ejection of large volumes of rhyolite pumiceous tephra (Thorarinson 1958; Sharma et al 2008) It devastated a large settled farmed region that was deserted for more than 40 years and which became known as Öræfi, or wasteland. Both historical eruptions were associated with significant glacier outburst floods, or jökulhlaups (Thorarinson 1958; Roberts and Gudmundsson 2015). Varða (called Hofsfjall in previous publications) is a small hill situated above
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