David W. McGarvie

Volcano-ice interactions at Prestahnukur, Iceland: rhyolite eruption during the last interglacial-glacial transition

Abstract Prestahnúkur is a 570m high rhyolite glaciovolcanic edifice in Iceland’s Western Rift Zone with a volume of 0.6 km3. Uniform whole rock, mineral and glass compositions suggest that Prestahnúkur was constructed during the eruption of one magma batch. Ar-Ar dating gives an age of 89± 24 ka, which implies eruption during the transition (Oxygen Isotope substages 5d to 5a) between the Eemian interglacial and the Weichselian glacial period. Prestahnu´kur is unique among published accounts of rhyolite tuyas because a base of magmatically-fragmented tephra appears to be absent. Instead, basal exposures consist of glassy lava lobes and coarse hyaloclastite, above which are single and multiple lava sheets with matrix-supported basal breccias and hyaloclastite upper carapaces. Steepening ramp structures at sheet termini are interpreted as ice-contact features. Interactions between erupting magma and water/ice have affected all lithologies. A preliminary model for the construction of Prestahnúkur involves an effusive subglacial eruption between 2–19 years duration which never became emergent, into an ice sheet over 700m thick. If 700m of ice had built up during this interglacial–glacial transition, this would corroborate models arguing for the swift accumulation of land-based ice in rapid response to global cooling.

New light on caldera evolution—Askja, Iceland

The large multiple-caldera volcanic system of Askja, central Iceland, is composed principally of subglacial basaltic hyaloclastite-pillow-lava formations and postglacial basaltic scoria and flows. Traditionally, such calderas are believed to be formed by downfaulting and ring-fracture collapse. Whereas this certainly applies to the smaller A.D. 1875 caldera, the older main caldera may have developed positive relief during subglacial construction of laterally confined hyaloclastite ridges above erupting fractures. This is supported by the evidence of a large negative gravity anomaly that reaches minima over the marginal low-density ridges but which is less negative within the caldera, where relatively dense postglacial lavas are believed to cover a more limited hyaloclastite succession beneath the caldera floor.

Physical volcanology of a subglacial-to-emergent rhyolitic tuya at Rauðufossafjöll, Torfajökull, Iceland

Abstract This paper presents the first modern volcanological study of a subglacial-to-emergent rhyolite tuya, at SE Rauðufossafjöll, Torfajökull, Iceland. A flat-topped edifice with a volume of c. 1 km3 was emplaced in Upper Pleistocene time beneath a glacier >350m thick. Although it shares morphological characteristics with basaltic tuyas, the lithofacies indicate a very different eruption mechanism. Field observations suggest that the eruption began with vigorous phreatomagmatic explosions within a well-drained ice vault, building a pile of unbedded ash up to 300m thick. This was followed by a subaerial effusive phase, in which compound lava flows were emplaced within ice cauldrons. Small-volume effusive eruptions on the volcano flanks created several lava bodies, with a variety of features (columnar-jointed sides, subaerial tops, peperitic bases) that are used to reconstruct spatially-heterogeneous patterns of volcano-ice interaction. Volcaniclastic sediments exposed in a stream section provide evidence for channelised meltwater drainage and fluctuating depositional processes during the eruption. Models are developed for the evolution of SE Rauðufossafjöll, and the differences between subglacial rhyolitic and basaltic eruption mechanisms, which are principally caused by contrasting hydrological patterns, are discussed.

Explosive subglacial rhyolitic eruptions in Iceland are fuelled by high magmatic H2O and closed-system degassing

Rhyolitic eruptions beneath Icelandic glaciers can be highly explosive, as demonstrated by Quaternary tephra layers dispersed throughout northern Europe. However, they can also be small and effusive. A subglacial rhyolitic eruption has never been observed, so behavioral controls remain poorly understood and the influence of pre-eruptive volatile contents is unknown. We have therefore used secondary ion mass spectrometry to characterize pre-eruptive volatile contents and degassing paths for five subglacial rhyolitic edifices within the Torfajokull central volcano, formed in contrasting styles of eruption under ice ∼400 m thick. This includes the products of the largest known eruption of Icelandic subglacial rhyolite of ∼16 km 3 at ca. 70 ka. We find pre-eruptive water contents in melt inclusions (H 2 O MI ) of up to 4.8 wt%, which indicates that Icelandic rhyolite can be significantly more volatile-rich than previously thought. Our results indicate that explosive subglacial rhyolite eruptions correspond with high H 2 O MI , closed-system degassing, and rapid magma ascent, whereas their effusive equivalents have lower H 2 O MI and show open-system degassing and more sluggish ascent rates. Volatile controls on eruption style thus appear similar to those for subaerial eruptions, suggesting that ice plays a subsidiary role in influencing the behavior of subglacial rhyolitic eruptions.

Will subglacial rhyolite eruptions be explosive or intrusive? Some insights from analytical models

Abstract Simple analytical models of subglacial eruptions are presented, which simulate evolving subglacial cavities and volcanic edifices during rhyolitic eruptions beneath temperate glaciers. They show that the relative sizes of cavity and edifice may strongly influence the eruption mechanisms. Intrusive eruptions will occur if the edifice fills the cavity, with rising magma quenched within the edifice and slow melting of ice. Explosive magma–water interaction may occur if a water- or steam-filled gap develops above the edifice. Meltwater is assumed to drain away continuously, but any gap above the edifice will be filled by meltwater or steam. Ductile roof closure will occur if the glacier weight exceeds the cavity pressure and is modelled here using Nye’s law. The results show that the effusion rate is an important control on the eruption style, with explosive eruptions favoured by large effusion rates. The models are used to explain contrasting eruption mechanisms during various Quaternary subglacial rhyolite eruptions at Torfajökull, Iceland. Although the models are simplistic, they are first attempts to unravel the complex feedbacks between subglacial eruption mechanisms and glacier response that can lead to a variety of eruptive scenarios and associated hazards.

Remote sensing and geologic mapping of glaciovolcanic deposits in the region surrounding Askja Dyngjufjöll volcano, Iceland

The surface geology of the Northern Volcanic Zone in Iceland is dominated by volcanic ridges, central volcanoes, shield volcanoes, and tuyas. The largest features are typically ice-confined (glaciovolcanic) in origin, and are overlain by voluminous Holocene (subaerial) lavas and glacial outwash deposits. The literature has focused heavily on prominent or very young features, neglecting small and older volcanic features. The purpose of this study is to demonstrate the application of remote-sensing mapping techniques to the glaciovolcanic environment in order to identify dominant lithologies and determine locations for textural, stratigraphic, and age studies. The deposits targeted in this study occur on and around Askja volcano, in central Iceland, including Pleistocene glaciovolcanic tuffs and subaerial pumice from the 1875 rhyolitic eruption of Askja. Data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) were used in conjunction with previously published geologic and remote-sensing data sets and recent field work on glaciovolcanic deposits of Askja for validation. Remotely acquired data sets include aerial photographs and one ASTER scene obtained in August 2010. Visible and near-infrared (VNIR) and thermal infrared (TIR) classifications and linear deconvolution of the TIR emissivity data were performed using end-members derived from regions of interest and laboratory spectra. End-members were selected from samples of representative lithologic units within the field area, including glaciovolcanic deposits (pillow lavas, tuffs, etc.), historical deposits (1875 pumice, 1920s basaltic lavas), and Holocene basaltic lavas from Askja. The results demonstrate the potential for remote sensing-based ground cover mapping of areas of glaciovolcanic deposits relevant to palaeo-ice reconstructions in areas such as Iceland, Antarctica, and British Columbia. Remote sensing-based mapping will benefit glaciovolcanic studies, by determining the lithologic variability of these relatively inaccessible massifs and serving as an important springboard for the identification of future field sites in remote areas.