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An Unusual Jökulhlaup Resulting from Subglacial Volcanism, Sólheimajökull, Iceland

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An Unusual Jökulhlaup Resulting from Subglacial Volcanism, Sólheimajökull, Iceland Newcastle University ePrints Russell AJ, Tweed FS, Roberts MJ, Harris TD, Gudmundsson MT, Knudsen Ó, Marren PM. An unusual jökulhlaup resulting from subglacial volcanism, Sólheimajökull, Iceland. Quaternary Science Reviews 2010, 29(11-12), 1363-1381. Copyright: NOTICE: this is the author’s version of a work that was accepted for publication in Quaternary Science Reviews. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Quaternary Science Reviews , Volume 29, Issues 11-12, (June 2010) http://dx.doi.org/10.1016/j.quascirev.2010.02.023 Further information on publisher website: http://www.elsevier.com/ Date deposited: 09-07-2014 Version of file: Author Final Version This work is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License ePrints – Newcastle University ePrints http://eprint.ncl.ac.uk An unusual jökulhlaup resulting from subglacial volcanism, Sólheimajökull, Iceland Andrew J. Russell* Geography, Newcastle University, Claremont Road, Newcastle-upon-Tyne, NE1 7RU, UK Fiona S. Tweed Geography, Staffordshire University, College Road, Stoke-on-Trent, Staffordshire, ST4 2DE, UK Matthew J. Roberts Monitoring and Forecast Division, Icelandic Meteorological Office, Bústaðavegur 9, Reykjavík IS-150, Iceland Tim D. Harris Geography, Staffordshire University, College Road, Stoke-on-Trent, Staffordshire, ST4 2DE, UK Magnús T. Gudmundsson Institute of Earth Sciences, University of Iceland, Herbergi 329, Náturufræðihús, Sturlugata 7, 101 Reykjavík, Iceland Óskar Knudsen Verzlunarskóli Íslands, Ofanleiti 1, 103 Reykjavík, Iceland Philip M. Marren Department of Resource Management and Geography, University of Melbourne, Victoria 3010, Australia *E-mail: [email protected] ABSTRACT Jökulhlaups (glacial outburst floods) are frequent in glaciated terrain. Jökulhlaups exhibiting a sudden rise to peak discharge have not been subject to detailed investigation. A volcanically-generated flood burst from Sólheimajökull, Iceland in July 1999. This paper accounts for the causes, characteristics and impacts of this flood. Pre- and post-flood fieldwork was carried out at Sólheimajökull allowing the reconstruction of mean flow velocities and peak discharge. Flood onset was rapid, rising to a peak discharge of 4.4 x 103 m3s-1 (± 1.2 x 103 m3s-1) within one hour. High basal water pressures resulted in floodwater Russell et al., “An unusual jökulhlaup.……Sólheimajökull, Iceland.” bursting through the glacier surface. Ice rip-up clasts containing glacial diamict provided evidence of floodwater contact with the glacier bed. Within the glacier, jökulhlaup sedimentation occurred within hydrofractures and conduits generating complex hydrofracture fills and esker ridges. In the proglacial zone, regions of flow expansion associated with rapid reductions in sediment transport capacity controlled the locations of major jökulhlaup sedimentation. A large fan composed of material of up to boulder size was deposited at the glacier snout. Two ice-marginal basins filled and drained during the jökulhlaup. One of the basins, Jökulsárgil, emptied rapidly during the flood, accentuating peak jökulhlaup discharge. High rates of downstream peak discharge attenuation during the July 199 jökulhlaup provides an analogy with flash flood hydrographs in semi-arid regions and the catastrophic failure of man-made dams. The July 1999 jökulhlaup was initially triggered by the subglacial volcanic eruption, but the characteristics of the flood were accentuated by within-event meltwater storage and release. This unusual jökulhlaup provides an important addition to our understanding of the spectrum of distinctive jökulhlaup characteristics. Keywords: jökulhlaup, discharge attenuation, catastrophic drainage, esker, fracture-fill, Iceland INTRODUCTION Jökulhlaups or glacier outburst floods are a frequent hazard in glaciated regions, and have a substantial impact on human activity (Thorarinsson, 1939; Young, 1980; Ives, 1986; Sturm and Benson, 1985) and the physical environment (Shakesby, 1985; Russell, 1989, 1992; Desloges and Church, 1992). Sudden releases of meltwater from glaciers and ice sheets can be initiated by a variety of mechanisms, including surging (Kamb et al., 1985), spring events (Anderson et al., 1999; Skidmore and Sharp, 1999), intense rainfall (Barrett and 2 Russell et al., “An unusual jökulhlaup.……Sólheimajökull, Iceland.” Collins, 1997), snow melt (Lawson, 1993; Röthlisberger and Lang, 1987) and floods caused by the breaching of moraine dams (Costa and Schuster, 1988; Clague and Evans, 2000) volcanic activity (Tómasson, 1996) or the sudden drainage of stored meltwater (Thorarinsson, 1939; Björnsson, 1992; Tweed and Russell, 1999). Within glaciated environments, flood generation mechanisms control flood magnitude and hydrograph shape (Thorarinsson, 1955; Haeberli, 1983; Costa and Schuster, 1988; Björnsson, 1992; Walder and Costa, 1996; Tweed and Russell, 1999; Roberts, 2005), which in turn have important implications for proglacial inundation rates and areas. Both of these factors are crucial for the management of flood hazards and understanding of the geomorphological and sedimentary impact of jökulhlaups. In July 1999 a volcanically-generated jökulhlaup burst from Sólheimajökull, southern Iceland. This flood was unexpected and characterised by an unusually rapid rise to peak discharge (Sigurðsson et al., 2000; Roberts et al., 2003). The July 1999 jökulhlaup marked the onset of a period of volcanic unrest from Katla subglacial volcano (Einarsson, 2000; Einarsson et al., 2005; Soosalu et al., 2006; Gudmundsson et al., 2007; Sturkell et al., 2009). The term ‘Katla’ is traditionally used for the eastern part of the large caldera under Mýrdalsjökull. Katla erupts roughly twice a century, a prime hazard constituting the flooding generated as the heat of the volcanic eruption melts the glacier base (Rist, 1983; Larsen, 2000, 2009). Jökulhlaups created by Katla eruptions have been principally responsible for the form of the sandar (outwash plains) between Mýrdalsjökull and the coast (Fig. 1). Floodwater paths have varied significantly during successive Katla eruptions due to changes in the location of the volcanic edifice and the geometry of Mýrdalsjökull (Björnsson et al., 2000; Larsen, 2000). This paper determines the controls on (i) the characteristics of the July 1999 jökulhlaup and (ii) the geomorphological and sedimentary impact of the jökulhlaup. In order 3 Russell et al., “An unusual jökulhlaup.……Sólheimajökull, Iceland.” to fulfil these aims we: (a) present evidence of the jökulhlaup generation mechanisms; (b) reconstruct the temporal evolution of jökulhlaup drainage routeways; and (c) present geomorphological and sedimentary evidence of jökulhlaup impact. FIELD AREA Sólheimajökull is an 8 km-long temperate, non-surging outlet glacier draining the Mýrdalsjökull ice cap, southern Iceland (Fig. 1). Mýrdalsjökull and the Katla volcanic system belong to the southern volcanic zone, which is part of the active region of tectonic rifting (Sigbjarnarson, 1973). Sólheimajökull has a surface area of c. 78 km² (Lawler et al., 1996), a snout 1 km wide, and is slightly overdeepened, reaching a maximum thickness of 433 m (Mackintosh et al., 2000). Jökulsá á Sólheimasandi, the main glacial river draining Sólheimajökull, has three main sources. The first exits Jökulsárgilsjökull, an outlet glacier approximately 3 km to the north of Sólheimajökull (Fig. 1). In 1999 this water flowed through Sólheimajökull via a 1 km long subglacial tunnel before joining the second glacial meltwater source from Sólheimajökull itself (Tweed, 2000b). Fjallgilsá, the third river, joins the Jökulsá approximately 200 km downstream of the glacier snout, and the river receives input from several small streams before passing under the road bridge approximately 5 km from the glacier terminus. Sólheimajökull is thought to have been subject to at least eight major jökulhlaups from the Katla volcanic system between 4500 BP and 1357 AD (Thorarinsson, 1975). Although most historic Katlahlaups exited Kötlujökull (Fig. 1), a series of floods inundated Sólheimasandur and Skógasandur in the early 10th century (Dugmore, 1987; Dugmore et al., 2000; Larsen, 1978, 2009) and a small component of floodwater associated with the 1860 Katla eruption was routed through Sólheimajökull (Hákonarson, 1860; Björnsson et al., 2000; Larsen, 2000). The eruption centre is then believed to have migrated to the northeast, routing 4 Russell et al., “An unusual jökulhlaup.……Sólheimajökull, Iceland.” floodwater through Kötlujökull, a glacier to the east of Katla, accounting for more recent jökulhlaups on Mýrdalssandur (Björnsson et al., 2000; Larsen, 2000). Analysis of the timing of the most recent Katla eruptions in 1721, 1755, 1823, 1860 and 1918, suggests that a major eruption is overdue (Rist, 1983). The dramatic 1918 Katla eruption released a jökulhlaup on Mýrdalssandur, which may have had a peak discharge of 250,000 - 300,000 m3s-1 (Tómasson, 1996; Elíasson et al., 2007), five to six times higher than the November 1996 jökulhlaup at Skeiðarárjökull (Björnsson, 2002). Currently, the ice and water divide of Sólheimajökull continues into the southwestern rim of the Katla subglacial caldera (Björnsson et al., 2000); therefore, geothermal ice
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