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Chapter III. Öræfajökull Volcano: Eruption Melting Scenarios

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Chapter III. Öræfajökull Volcano: Eruption Melting Scenarios III. ÖRÆFAJÖKULL VOLCANO: ERUPTION MELTING SCENARIOS Magnús T. Gudmundsson *, Þórdís Högnadóttir *, Eyjólfur Magnússon * * Nordic Volcanological Centre, Institute of Earth Sciences, University of Iceland 1. Introduction associated with volcanic activity in Öræfajökull. The approach is therefore to Observations of recent volcanic unrest consider what can be defined as realistic demonstrate that melting of ice in eruptions worst case scenarios. This needs to be kept in within glaciers can be extremely fast. The mind when considering the results. The best documented cases have occurred in the scenarios with the highest probability are less last quarter of a century in Grímsvötn, Gjálp, extreme. Three types of eruptions/events are Eyjafjallajökull and Redoubt in Alaska (e.g. considered. (1) Eruptions within the caldera Gudmundsson et al., 1997, 2004; Gud- of Öræfajökull (thick ice), (2) eruptions on mundsson, 2005; Magnússon et al., 2012a; the flanks (thin ice), and (3) pyroclastic Waythomas et al., 2013) and some earlier density currents (PDCs). The values of events such as the Katla 1918 eruption various parameters used in calculations and (Tómasson, 1996; Björnsson, 2003) and the definitions of terms are listed in Table III-1. eruptions of Mount St. Helens in 1980–1983 In this chapter a short overview of the area (Pierson, 1999), Nevado del Ruiz in 1985 being considered is given in Section 2 while (Pierson et al., 1990) and Redoubt in 1989– the magnitudes of eruptions that occur in 90 (Dorava and Meyer, 1994; Trabant et al., Iceland are reviewed briefly in Section 3. In 1994). For eruptions observed in Iceland, the Section 4, calorimetric considerations on the highest rates of heat transfer and melting various types of volcanic events are presented occur in the early, fully subglacial phases of and empirical data used to constrain explosive eruptions where the magma is efficiencies of processes. The jökulhlaups, fragmented into glass particles, typically in their entrainment of volcanic material and the the size range 0.01–1 mm (e.g. Gud- onset times are considered in Section 5. It is mundsson, 2003). More gradual melting is assumed that a flood breaks through the ice expected to occur when heat transfer takes and starts to cascade downslope mostly on the place largely by free convection of water surface and along the margins of outlet above rapidly cooled lava under ice (e.g. glaciers where ice on the slopes is shallow as Höskuldsson and Sparks, 1997; Gud- on Öræfajökull. This behaviour was for mundsson, 2003; Woodcock et al., 2012, example observed in Eyjafjallajökull in 2010 2014). Thus, because of their greater potential (Magnússon et al., 2012a). The propagation to melt large amounts of ice in a short period of the flood once it has reached the upper of time, eruptions where fragmentation is parts of the outlets is not considered further dominant are more dangerous. The analysis here since it is dealt with in Chapter IV presented here is therefore mostly con- (Helgadóttir et al., 2015). The results for the centrated on eruptions dominated by fra- various catchments and outlet glaciers for the gmentation and their consequences. three types of events considered are presented The purpose of the present work is to estimate in Section 6. the potential hazard due to jökulhlaups Öræfajökull Volcano: Eruption melting scenarios 45 Table III-1: List of symbols, abbreviations and numerical values of parameters. Symbol definition Unit PDC Pyroclastic density current - MER Mass eruption rate kg/s EOT Eruption onset time min STT Subglacial transport time min FTT Flank transit time min dE Rate of heat transfer / energy flux W dt Ep Energy available to melt snow and ice in a PDC J f Efficiency of heat transfer (0–100%, in reality fmax~80–90%) Dimensionless χ Fraction of tephra entrained in phoenix cloud during PDC formation Dimensionless 휉 Fraction of PDC flowing over a particular catchment Dimensionless 3 Qm Volumetric flow rate of magma m /s 푀̇ 푚 Mass eruption rate kg/s 푚̇ Mass eruption rate per unit length of volcanic fissure (kg/s)/m Mass generation rate of pyroclastic material at eruption site (usually equal to 푀̇ kg/s 푝 mass eruption rate) 푀̇ 푤 Mass generation rate of meltwater at eruption site kg/s Q , Q , 1 2 Rate of liquid water generation by ice melting m3/s Qw 3 QT Discharge of jökulhlaup (liquid water + entrained ice and pyroclasts) m /s 3 m Magma density kg/m 3 g Tephra kg/m 3 i Density of ice kg/m 3 w Density of liquid water kg/m Ti, Tf, T Temperature, i: initial, f: final, T: temperature difference °C Te Emplacement temperature of pyroclastic density current °C T0 Ambient air/snow temperature (~0°C) °C 5 Li Latent heat of solidification of ice, Li = 3.34x10 J/kg J/kg Cg Specific heat capacity of fresh volcanic glass J/(kg °C) Cp Specific heat capacity of pyroclastic material in collapse J/(kg °C) l Length – used here for volcanic fissure at base of glacier m x Length m 2 q Rate of meltwater production per unit length of fissure m /s 3 Vi Volume of ice m Mp Massi of pyroclastic material kg Mg Mass of pyroclasts interacting with glacier/snow in pyroclastic density current kg 46 Öræfajökull Volcano: Eruption melting scenarios Symbol definition Unit Duration of plume collapse forming pyroclastic density current. s trun Period it takes a ground hugging PDC to flow over snow/ice and release heat s 휑 Static fluid potential of water flow under ice Pa g Acceleration due to gravity g = 9.82 m/s2 m/s2 zb Height of glacier bed (usually above sea level) m zs Height of glacier surface (usually above sea level) m 2. Öræfajökull and its iv) Rótarfjallshnjúkur-Hnappur and glaciers to the south of these nunataks: The upper potential to generate boundary of this segment is the southern jökulhlaups caldera rim. Can be affected by flank eruptions and pyroclastic density currents. The height and overall morphology of v) Kvíárjökull: This includes a large part of Öræfajökull with an ice-filled caldera and ice the caldera, the slopes of Kvíárjökull and its covered upper slopes makes jökulhlaups an lower part. Can be affected by caldera almost inevitable consequence of eruptions eruptions, flank eruptions and pyroclastic on the upper parts of the volcano. The two density currents. historical examples of 1362 and 1727 demonstrate this, as shown by Thorarinsson vi) Eastern flank of Öræfajökull north of (1958) and Roberts and Gudmundsson (2015; Kvíárjökull: The upper slopes are similar to this volume, chapter II). The part of the those on the west side and can be affected by mountain massif considered here is the flank eruptions and possibly pyroclastic presently active volcano south of density currents. However, since the Svínafellsjökull and Hermannaskarð (Figures inundation area of jökulhlaups is not III-1 and III-2). The ice-covered part of the inhabited, this segment is not considered in volcano has recently been mapped with radio- the same way as those on the west and south echo soundings (Magnússon et al., 2012b). side. For the jökulhlaup hazard, the following water catchment basins were considered: 3. Magma discharge in i) The southern catchment of Svínafellsjökull: Only considered here as a potential source of eruptions jökulhlaups caused by pyroclastic density Models exist that relate magma flow rate in currents. an explosive eruption with eruption plume ii) Virkisjökull-Falljökull: This includes a height (Sparks et al., 1997; Mastin et al., section of the caldera and the flanks north of 2009; Woodhouse et al., 2013; Degruyter and Sandfell. Can be affected by caldera Bonadonna, 2012). These equations, how- eruptions, flank eruptions and pyroclastic ever, are very sensitive to the plume height, density currents. This also includes the plume height is related to both magma Grænafjallsgljúfur, to the south of Falljökull. flow rate and wind speed and discrepancies between predicted and observed flow rate iii) Kotárjökull: This catchment reaches the may be as much as a factor of 3–4 (Oddsson caldera rim but is mainly confined to the et al., 2012). These equations are not used slopes. Flank eruptions can occur here and the here. flanks of the catchment can be affected by pyroclastic density currents. In Table III-2 the estimated magma flow rate of several Icelandic eruptions are given Öræfajökull Volcano: Eruption melting scenarios 47 together with known fissure lengths. In most However, the peak values may well have cases the numbers in the table are mean been 2–3 times higher in some cases and for values over some interval during the most the largest ones 푀̇ may have reached or powerful phase of the eruption. exceeded 108 kg/s. Figure III-1: Öræfajökull and surroundings, Surface topography and ice catchment basins. The main pathways of the jökulhlaups of 1362 and 1727 were down Falljökull and Kotárjökull. 48 Öræfajökull Volcano: Eruption melting scenarios Figure III-2: Bedrock topography of Öræfajökull (after Magnússon et al. 2012b). The bottom of the caldera is an enclosed depression that would collect water if it were not ice-filled. 4. Models of ice melting in Magma fragmentation within a fissure through ice, with rapid initial widening of the eruptions fissure through melting. This model applies The conceptual models of magma melting where rapid opening to the ice surface takes considered here concur with the highest place and ice deformation is small in relation melting rates observed at certain ice to vertical ice melting rates. This applies to thicknesses and eruption rates (Figure III-3): relatively thin ice, but the thickness at which this occurs is expected to depend on the Magma fragmentation under thick ice intensity of the eruption.
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