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II. ÖRÆFAJÖKULL : GEOLOGY AND HISTORICAL FLOODS

Matthew J. Roberts * and Magnús T. Gudmundsson ** * Icelandic Meteorological Office **Nordic Volcanological Centre, Institute of Earth Sciences, University of

1. Introduction and scope magnitude. Eyewitness accounts of the 1727 jökulhlaup depicts a scene where floodwater Glacial outburst floods (jökulhlaups1) are a rushed from high on the side of Öræfajökull potent hazard in the proximal and distal to the adjacent floodplain (sandur) within tens regions of an erupting of minutes (Thorarinsson, 1958 and (Tilling, 1989; Roberts, 2005; Gudmundsson references therein). During both historical et al., 2008). Besides meltwater, volcano- floods, burst from two sets of combined genic jökulhlaups comprise fragmented : Falljökull and Virkisjökull (herein and primary and secondary volcaniclastic referred to as Falljökull) and Kotárjökull, and material (Major and Newhall, 1989; Rótarfjallsjökull (herein referred to as Tómasson, 1996). Such fluid-sediment mix- Kotárjökull), as shown in Figure II-1. There tures can produce a variety of flow properties, is also credible evidence of jökulhlaup ranging from turbulent, Newtonian discharge activity on the southern flanks of the ice-cap to cohesionless, hyper-concentrated torrents (Höskuldsson, personal communication, (Maizels, 1989). Moreover, volcanogenic October 2015), including a possible pre- jökulhlaups descending from steep, erodible historical route via Kvíárjökull (Thorarins- slopes often produce sediment-laden flows by son, 1958; Iturrizaga, 2008). Remarkably, entraining debris dynamically (e.g. Naranjo both historical floods deposited blocks of et al., 1986; Waythomas, 2015). glacial ice on the sandur that took decades to In 1362 CE, and again in 1727 CE, an melt. In several cases, these stranded masses at Öræfajökull — an ice- were renamed as glaciers as they melted capped located on the southern amongst jökulhlaup deposits (Sigurðsson and coast of Iceland — resulted in a massive, Williams, 2008). short-lived jökulhlaup that caused fatalities Despite the documented severity and lasting and extensive damage to farmland (Thora- geomorphic imprint of the 1362 and 1727 rinsson, 1958). The of 1362 jökulhlaups, there is scant information about is considered paroxysmal, equivalent to six the routing and extent of these floods. Using on the volcano explosivity index (VEI) published descriptions, field observations, (Gudmundsson et al., 2008), and the largest aerial photographs, and modern-day analo- explosive eruption in Europe since Mount gues, we reconstruct the 1362 and 1727 Vesuvius erupted in 79 CE. The following jökulhlaups. The goal is to constrain the eruption of Öræfajökull, 365 years later in duration, extent, composition, and maximum 1727, is thought to have been VEI ~4 in discharge of the two floods. The results

1 Note that the terms jökulhlaup and flood are used interchangeably in this chapter when describing -type flows from Öræfajökull.

Öræfajökull Volcano: Geology and historical floods 17

provide new insight into the routing and 2. Geological overview maximum discharge of volcanogenic floods from Öræfajökull, thereby contributing The Öræfajökull volcano is located about 50 toward hazard assessment in the region km southeast of the active zone in Iceland (Helgadóttir et al., 2015, Chapter IV) and forming, together with Esjufjöll and Snæfell, (Pagneux and Roberts, 2015, Chapter V). a 120 km long, discontinuous volcanic flank zone (Sæmundsson, 1979; Björnsson and A geological overview of Öræfajökull is Einarsson, 1990; Sigmundsson, 2006). presented next, summarising the stratigraphy, Öræfajökull is the highest volcano in Iceland, ice cover, and Holocene eruptive activity of rising from sea level to over 2,100 m to form the volcano. This is followed by descriptions Iceland’s highest peak, Hvannadalshnjúkur of the 1362 and 1727 jökulhlaups. The (~2110 m AMSL) (Figures II-1 and II-2). The chapter concludes by considering hazard- mountain massif of Öræfajökull is elongated related issues, including (i) floodwater slightly, with a north-south base diameter of routing, timing, and extent; (ii) flow 25 km, while the east-west basal diameter is properties; (iii) maximum discharge; and (iv) about 20 km. modern-day comparisons.

Figure II-1: Location of Öræfajökull, an ice-capped stratovolcano in south-east Iceland. The summit of the , Hvannadalshnjúkur, is ~2110 m AMSL and the highest point in Iceland. Radio-echo sounding measurements from the surface of the ice cap show that ice within the is up to 540 m thick (Magnússon et al., 2012b). The magnitude of the 1362 eruption may have caused deepening and widening of the volcano’s caldera. Both historical eruptions occurred either within the caldera or on its rim; however, in 1362 most flooding came from Falljökull, implying that the eruption site was within the caldera.

18 Öræfajökull Volcano: Geology and historical floods

A 14 km2 summit caldera exists in the extensively eroded northern part of southern part of the massif (Figures II-1 and Öræfajökull have progressively carved II-2). The ice-covered upper part of overdeepened valleys, resulting in up to 550- Öræfajökull is the southernmost region of m-thick glaciers such as Svínafells- Vatnajökull, connected to the main ice-cap at jökull (Figures II-1 and II-2; Magnússon et Hermannaskarð. Valley glaciers from the al., 2012b).

Figure II-2: Oblique aerial photographs of Öræfajökull. (A) View from the north- west; (B) southern flank; and (C) western flank. Photographer: O. Sigurðsson.

Öræfajökull Volcano: Geology and historical floods 19

2.1. Stratigraphy the Holocene volcanic history of Öræfi. Björnsson (1988) published the first results of The oldest rocks are found in the northern radio-echo soundings from a north-south part of the Öræfajökull massif, with the traverse and measured the depth of the 14 km2 volcanic strata becoming progressively summit caldera. Magnússon et al. (2012b) younger on the volcano’s southern side. A performed an extensive radio-echo survey on boundary occurs roughly along a line Öræfajökull, deriving ice thickness for the between Svínafellsjökull, Tjaldskarð and caldera and the upper and lower areas of the Fjallsjökull. To the north of this line the rocks valley glaciers; the study’s results are are predominantly from the Matuyama summarised in § 2.2. magnetic chron (2.58–0.78 Ma) or older, as deduced from pronounced magnetic lows in Some of the nunataks on the caldera rim are aeromagnetic surveys (Jónsson et al., 1991) made of . formations are and confirmed by stratigraphic mapping and also found on the lower southwest slopes and radiometric dating (Helgason, 2007; on the eastern side where the Vatnafjöll Helgason and Duncan, 2001). South of the to the north of Kvíárjökull is made partly of a divide is the presently active Öræfajökull massive rhyolitic flow (Stevenson et al., stratovolcano, comprising normally magne- 2006). For the most part, the lower slopes tized rocks from the Brunhes chron (<0.78 consist of hyaloclastites and lava flows of Ma). The oldest dated rocks found near the basaltic to intermediate composition. In base of Svínafell have an Ar-Ar age of 0.76 summary, eruptions contributing to the Ma (Helgason and Duncan, 2001; Helgason, growth of the edifice are thought to have 2007). occurred mainly during glacial periods. This is also apparent in the form of the lower Thorarinsson (1958) published chemical slopes of Öræfajökull, which are steeper than analyses of the 1362 tephra from the upper slopes, suggesting partial Öræfajökull; he also described the overall confinement by glacial ice during extended morphology and geology of the volcano. periods over the volcano’s existence. Torfason (1985) compiled a geological map of southeast Iceland, including Öræfajökull. Later stratigraphy work was undertaken by 2.2. Ice cover Helgason and Duncan (2001, 2013) on the The upper parts of Öræfajökull, south of northern parts of the massif. The petrology of Hermannaskarð, have a mean slope angle of Öræfajökull was considered by Prestvik 15 degrees, with glacial ice covering most of (1982), whereas Stevenson et al. (2006) the volcano above about 1000 m AMSL. The analysed the physical volcanology of a large summit plateau between Hvannadalshnjúkur, Pleistocene rhyolitic lava flow on the Snæbreið and Hnappar has an elevation of southeast side of the volcano. Jakobsson et al. 1800–1850 m AMSL. The plateau is the (2008) classified the Öræfajökull central surface expression of the 14 km2 summit volcano as belonging to the transitional caldera, containing 3.9 km3 of ice at depths of alkalic series, together with other volcanoes up to 540 m in the caldera centre (Magnússon in the Öræfajökull-Snæfell flank zone. Other et al., 2012b). Ice flows out of the caldera in notable studies include that of Gudmundsson all directions, although mostly westwards to (1998) who used tephrochronology to study Falljökull and southeast to Kvíárjökull.

20 Öræfajökull Volcano: Geology and historical floods

Figure II-3: Geological map of Öræfajökull (modified from Torfason, 1985).

A small area near the southwest margin of the The steep-sloping ice falls of Falljökull and caldera drains to Kotárjökull. Radio-echo Kvíárjökull result in ice thicknesses of 50– soundings reveal that the lowest bedrock 100 m. Thicker ice exists near to the termini points are where Falljökull and Kvíárjökull of the valley glaciers, some of which have drain out of the caldera. These low points are eroded deep bedrock troughs (Magnússon et 270–290 m higher than the base of the al., 2012b). caldera.

Öræfajökull Volcano: Geology and historical floods 21

2.3. Volcanic production rate and jökull has been modest in Holocene times. It Holocene activity has been proposed that a lava- flow by the northern side of Kotárjökull, on The total volume of rocks above sea level the eastern side of Mount Slaga, is an ice- 3 south of Hermannaskarð is about 370 km . confined lava, emplaced during the 1727 The volume of ice in the same area is 25–30 eruption (Forbes et al., 2014). This location is 3 3 km . Of the 370 km massif, it appears that also the same area where floodwater burst roughly half the volume belongs to the from Kotárjökull in 1727 (§ 5.3). present volcano, younger than 0.79 Ma. A rough, lower bound for the production rate of the volcano may be obtained by assuming 3. Jökulhlaups due to that the present edifice has been built eruptions of Öræfajökull incrementally during this period. Conse- quently, the rate of volume growth is about Since Norsemen first settled Iceland in the quarter of a cubic kilometre every thousand late 9th Century CE, there are two written years. However, the long-term mean eruption accounts of volcanic activity at Öræfajökull. rate must have been considerably higher Before the 1362 eruption the ice-cap was given the erosive effects of repeated known as Knappafellsjökull, but in the glaciations and jökulhlaups. aftermath of the eruption the name was changed to Öræfajökull in recognition of the The two historic eruptions of 1362 and 1727 devastation wreaked by the eruption are discussed in more detail later, but the first (Thorarinsson, 1958). Before 1362, the one is considered to be the largest explosive lowlands flanking Öræfajökull hosted fertile eruption in Iceland in the last 1100 years. grazing land, which supported at least 40 Selbekk and Trønnes (2007) described farms in a regional settlement known rhyolitic tephra from 1362 as fine-grained formerly as Litlahérað (Ives, 1991 and vesicular glass, indicative of fast references therein). ascent to form a Plinian eruption plume. Rhyolitic tephra fell over large parts of Deposits from pyroclastic density currents Iceland during the 1362 eruption, although have been identified in the lowlands as the main area of deposition was oriented out belonging to the 1362 eruption. Tephra fall to sea, with a dispersal axis towards the east- was prevalent during both historical southeast (Thorarinsson, 1958). Thorarinsson eruptions, particularly in 1362. Excavations (1958) estimated the bulk volume of freshly of relic dwellings to the immediate south and fallen tephra at 10 km3. Deposits of west of the volcano show that, during the pyroclastic density currents have been found onset of 1362 eruption, several pyroclastic on the slopes and lowlands to the south and surges occurred (Höskuldsson and Thor- southwest of the volcano (Höskuldsson and darson, 2007), followed by extensive fall-out Thordarson, 2006, 2007). of rhyolitic ash (Thorarinsson, 1958). A wider examination of the region (Hö- Holocene volcanic activity before the 1362 skuldsson, 2012), reveals that pyroclastic eruption was modest, with two minor lava density currents from the 1362 eruption flows on the east side of the volcano. One is reached a distance of over 10 km from the on the lowlands west of Kvíárjökull while the centre of the caldera (Gudmundsson et al., other is higher up on the slopes in Vatnafjöll 2008). In this chapter, only deposits due to on the north side of Kvíárjökull. Tephro- jökulhlaups are considered; however it chronology of soils around Öræfajökull has should be borne in mind that tephra-related been studied, suggesting that a few, relatively hazards were probably responsible for the small rhyolitic eruptions occurred during the apparent total destruction of Litlahérað. period (Guðmundsson, 1998). Thus, apart from the 1362 eruption, activity in Öræfa-

22 Öræfajökull Volcano: Geology and historical floods

4. Methods identify and map flood deposits to the west and south of Öræfajökull. The DSM was Several methods were used to reconstruct the derived from an airborne LIDAR survey of timing, routing, and geomorphic impact of the region, performed during the summers of the 1362 and 1727 jökulhlaups. The sequence 2011 and 2012. The horizontal and vertical of events for both jökulhlaups was pieced accuracy of the initial scan was <0.5 m. These together mainly from published sources, as measurements were used to create a DSM explained in § 4.1. Similarly, palaeo- that depicted surface features exceeding 1 m2 estimates of subaerial floodwater routing and in area. The DSM was also used to measure floodwater extent at maximum discharge the depth of -holes and to extract cross- were derived from published sources, as well sectional profiles. In this context, the as an examination of aerial photographs (§ estimated vertical accuracy of the model is 4.2). The same mosaic of images was used to <0.5 m. For details of the LIDAR survey and map coarse-scale flood deposits and features data handling, see Jóhannesson et al. (2013). (§ 4.3). The following sub-sections outline Flood deposits and erosional features were the methodological details of each approach. studied during fieldwork that was carried out in 2003, 2005, and 2006 (Figure II-4). 4.1. Historical accounts Features including kettle-holes, boulder The pioneering monograph by Thorarinsson clusters, and terraces were mapped using a (1958) is the foremost resource about the Trimble Pathfinder backpack-mounted GPS. 1362 jökulhlaup; this source is used A differential correction was applied to the extensively here. Detailed first-hand accounts data using continuous measurements from a of the 1727 jökulhlaup exist (Thorarinsson, fixed GPS site in Reykjavík (baseline 1958 and references therein), and they are distance: ~247 km). The calculated accuracy used here to infer how the 1362 jökulhlaup of the results is ~0.7 m horizontally and ~1.3 developed. Likewise, qualitative compari- m vertically. Geomorphic features were sons are made with volcanogenic jökulhlaup identified from aerial photographs (§ 4.3) in Iceland from 1918 onwards (§ 10). using established criteria for the recognition of jökulhlaup deposits (Maizels, 1993, 1997; 4.2. Geomorphic mapping Russell and Marren, 1999; Marren, 2005; A digital surface model (DSM) and high- Russell et al., 2005) ( II-1). resolution aerial photographs were used to

Figure II-4: Field assessment of jökulhlaup deposits. (A) Collection of bulk samples of sediment from the Kotá fan on 18 March 2003 (§ 5.3). (B) Boulder survey to the west of Falljökull on 27 August 2006. Note the person for scale.

Öræfajökull Volcano: Geology and historical floods 23

Table II-1: Criteria for the recognition of jökulhlaup deposits (after Marren 2005, p. 233).

Criteria indicative of high-magnitude flooding Sedimentary characteristics Hyperconcentrated flow: Poor sorting; massive; reverse grading; poor imbrication; floating clasts; traction carpets. Debris flow: Very poor sorting; massive; may show correlation between maximum particle size and bed thickness. Strongly uniform palaeo-flow: Indicative of a lack of reworking by falling-stage flows. Thick, inversely graded (upward coarsening) units: Inversely graded units in coarse sediment thicker than ~2 m. Formed under rising-stage conditions. Large-scale gravel foresets: Thick (>2 m) cross-bedded coarse gravel. Formed in expansion or pendant bars and in mega-dunes. Ice-block features: Steep-walled and inverse conical kettle-holes; circular ‘rimmed’ kettles; obstacle marks and tails; hummocky terrain. Rip-up clasts: Blocks of subglacial diamict, bedrock, or river- bank sediment uprooted and deposited out-of- place. Large-scale geomorphic features: Hummocky terrain; mega-scale bars and terraces; boulder fields; palaeo wash-limits.

ice-melt (e.g., Naranjo et al., 1986). In fact, 4.3. Analysis of aerial photographs anecdotal accounts of the 1362 eruption Using Thorarinsson’s (1958) delineation of describe every gully awash with floodwater flood routes, aerial photographs from (Thorarinsson, 1958). Thermal and mecha- Loftmyndir ehf. were used to classify surface nical erosion of the ice-cap by the passage of features indicative of flooding in 1362 and pyroclastic density currents could account for 1727. The imagery was made available in the deposition of some jökulhlaup deposits; geo-referenced format at a pixel resolution of however this is not addressed here. For <1 m. Combining the images with the DSM further details about tephra deposition, see enabled a detailed geomorphological view of Thorarinsson (1958) and Höskuldsson the region, allowing erosional and deposi- (2012). tional features to be classified using ArcGIS 10. Aerial photographs from the National 5.1. Prehistoric jökulhlaups Land Survey of Iceland were also used to aid According to Thorarinsson (1958) a pre- field investigations in 2005 and 2006. historic jökulhlaup burst from Kvíárjökull at a lateral breach known as Kambskarð in the 5. Course of events terminal (see also Iturrizaga, 2008) (Figure II-5). Sketchy accounts exist of the As well as considering the geomorphic legacy 1362 jökulhlaup draining partly from of prehistoric jökulhlaups, this section Kvíárjökull, but Thorarinsson disputed this. describes the development of the 1362 and He argued that tephra fall from the eruption 1727 jökulhlaups. As described in § 3, caused significant and widespread melting of pyroclastic density currents would have been the ice-cap, thereby causing a jökulhlaup that prevalent during eruptions of Öræfajökull. cascaded across the surface of Kvíárjökull. Partial collapse of the eruption plume could The Stórugrjót to the immediate have triggered pyroclastic density currents, west of Kvíárjökull extends into the sea. which would have scoured large zones of the Thorarinsson believed that Stórugrjót is ice-cap, causing significant and pervasive prehistoric as it underlies the terminal

24 Öræfajökull Volcano: Geology and historical floods

of Kvíárjökull, which is thought dates the aforementioned lava flow. widely to have formed ~500 BCE (Tho- According to Thorarinsson, the lava flow rarinsson, 1956). West of Kvíárjökull, originated to the east of Kvíárjökull; boulders from the Stórugrjót surface overtop therefore, an eruption occurred at a time when the fringe of a basaltic, postglacial lava flow Kvíárjökull was much farther up-valley than (Figure II-5). Thorarinsson (1956) claimed the position demarcated by the terminal that the of Kvíárjökull post- moraine.

Figure II-5: Oblique, aerial view of Kvíárjökull showing the lateral breach in the terminal moraine and the relic outwash-fan extending from it. Photographer: M. J. Roberts, July 2000.

this text is as follows: “At the same time [as 5.2. 1362 jökulhlaup the eruption] there was a burst from As Thorarinsson (1958) acknowledged, Knappafellsjökull [Öræfajökull] into the sea contemporary accounts of the 1362 eruption carrying such quantities of rocks, gravel and are vague, claiming that the entire settlement mud as to form a sandur plain where there was obliterated during the eruption. Likewise had previously been thirty fathoms [~55 m] other descriptions made decades after the of water.” eruption allude to complete destruction of Thorarinsson (1958) considered that the 1362 Litlahérað. The only direct reference to the eruption began in mid-June and it persisted 1362 jökulhlaup is found in the fragmented until the autumn. Flooding, though, was church annals of Skálholt, written at a confined mostly to the onset of the eruption monastery in Möðruvellir, Northern Iceland. and possibly the first 24 hours (c.f. Thorarinsson’s (1958, p. 26) translation of Magnússon et al., 2012b). The eruption

Öræfajökull Volcano: Geology and historical floods 25

created direct hazards of unprecedented farmland on the western side of Öræfajökull magnitude. Melting of ice through rapid heat at an initial elevation of ~80 m AMSL and a transfer from magma to ice, most likely distance of 10–30 km from the eruption site within the volcano’s ice-filled caldera, would (Figure II-6). Church annals written in the have generated masses of meltwater at a decades following the eruption depict a bedrock elevation of ~1600 m AMSL colossal flood that swept pieces of the ice-cap (Gudmundsson et al., 2015, Chapter III). The across Skeiðarársandur, cutting off all access ensuing jökulhlaup propagated through to the region (Thorarinsson, 1958). Falljökull and Kotárjökull before inundating

Figure II-6: Postulated routing of floodwater from Öræfajökull during the 1362 eruption (after Thorarinsson, 1958). Note the location of churches and farms along the flood path.

26 Öræfajökull Volcano: Geology and historical floods

From historical descriptions and geomor- Secondly, clusters of angular-shaped rocks lie phological evidence, Thorarinsson (1958) ~4 km west from Falljökull (e.g. Figure II- concluded that the 1362 jökulhlaup burst 4B); projecting 4–5 m above the sandur, these primarily from Falljökull. Flood deposits, boulders are estimated to weigh more than recognisable by the presence of light- 500 tonnes and they are inter-bedded with coloured rhyolitic tephra, extend over a much jökulhlaup deposits (Thorarinsson, 1958). larger area than dark-coloured, basaltic Another notable boulder deposit is the deposits from the 1727 eruption (Figure II-7). smjörsteinn (butter stone) southeast of Moreover, rhyolitic tephra from 1362 Svínafell; it is believed that this boulder was comprises coarse silt-sized grains (e.g. transported to its present location by the 1362 Selbekk and Trønnes, 2007), whereas 1727 jökulhlaup (Thorarinsson, 1958) (Figure II- material is mostly coarse sands and pebbles. 7). In addition to the breccia of ice blocks, To the west and northwest of Falljökull, a boulders, and juvenile deposits known as boulder-strewn lag of vegetated, water-lain Langafellsjökull, three other named deposits deposits extends to the present-day course of have been associated with the 1362 Skaftafellsá (Figure II-7). Outcrops of the jökulhlaup; these are: Forarjökull, Gras- same surface continue west beyond jökull, and Miðjökull, which all contained Skaftafellsá to the former eastern edge of masses of ice and remained stranded at the Skeiðará. Large jökulhlaups from Skeiðar- foot of Öræfajökull for decades (Thora- árjökull (e.g. 1861, 1938, and 1996) would rinsson, 1958; Sigurðsson and Williams, have reworked or buried the Öræfajökull 2008) (Figure II-7). deposits, blurring the western extent of the In the foreground of Kotárjökull, evidence of sedimentary record on Skeiðarársandur the 1362 jökulhlaup is less obvious than at (Thorarinsson, 1959; Björnsson, 2003). Falljökull. From aerial assessments of Clearly, flows to the west and northwest of palaeo-flood extent and ground-based Falljökull carried large quantities of glacial surveys of sedimentary deposits, it is ap- ice and metre-scale boulders. This is parent that most of the 1362 deposits were supported by two lines of reasoning: Firstly, either buried or washed away by the 1727 the area was renamed at some point after the jökulhlaup. There are, however, occasional 1362 eruption as Langafellsjökull, signifying outcrops of lighter sediments within the distal that copious blocks of ice were left on the path of the 1727 jökulhlaup; Thorarinsson sandur (Thorarinsson, 1958; Guttormsson, (1958) described an area east of Kotá as an 1993; Sigurðsson and Williams, 2008). example (Figure II-7).

Öræfajökull Volcano: Geology and historical floods 27

Figure II-7: Extent of jökulhlaup deposits associated with the 1362 eruption of Öræfajökull 1727 jökulhlaup.

28 Öræfajökull Volcano: Geology and historical floods

5.3. 1727 jökulhlaup was sufficiently large and extensive to allow blocks of glacial ice to reach the sea, in The prelude to the 1727 eruption and the addition to depositing masses of sediment at consequent jökulhlaup was described by the the foot of the ice-cap. rector of Sandfell, Reverend Jón Þorláksson, who documented the course of events over 50 Decades elapsed before the stranded ice years after the eruption (Olavius, 1780). This around Sandfell disappeared. When explorers description was translated into English by Eggert Ólafsson and Bjarni Pálsson travelled Henderson (1818), with corrections made by through Öræfi in 1756, they described the Thorarinsson (1958). Whilst holding a terrain between Sandfell and Hof as a jumble sermon at Sandfell on 3 August 1727, the of debris-covered ice, ~3 km wide and ~13 congregation felt earthquakes that became km long (Ólafsson, 1974) (Figure II-8). Many progressively stronger. Damaging earth- pits and ravines were present in the melting quakes continued to occur on 4 August and it ice, making travel through the area difficult. was noted that booming noises, akin to Ólafsson (1974) likened the landscape to the thunder, radiated from the ice-cap (Hálda- appearance of Skeiðarárjökull, only much narson, 1918). Soon after 09:00 on 4 August, lower. The region to the immediate east of three particularly loud thunderclaps were Kotá, near to Goðafjall, was named heard, after which the jökulhlaup began. The Svartijökull (black glacier) in acknow- jökulhlaup affected Kotárjökull mainly ledgement of the lingering ice (Thorarinsson, (Háldanarson, 1918), but it is likely that some 1958; Guttormsson, 1993; Sigurðsson and floodwater drained via Falljökull. Traces of Williams, 2008); this name remains today. 1362 flood deposits between Sandfell and The uppermost surface of Svartijökull is Hof imply that the 1727 flood inundated characterised by closely spaced kettle-holes, roughly the same region, mostly likely resulting in hummocky topography (Figure covering pre-existing deposits. It can II-8). Angular blocks of palagonite also therefore be assumed that the 1727 jökul- project through the fan surface, implying hlaup from Kotárjökull was comparable in simultaneous incorporation and deposition of magnitude to the 1362 flood from the same glacial ice and bedrock from a high-energy, glacier (Thorarinsson, 1958). sediment-laden flow (e.g. Maizels, 1989; Russell and Knudsen, 2002). Figure II-9 The 1727 jökulhlaup caused three fatalities, shows seven surface profiles taken from the in addition to the loss of sheep, cows, and DSM of Svartijökull. These profiles depict a horses that were grazing in the path of the highly pitted surface, with some kettle-holes initial flood. From Thorarinsson’s (1958) forming inverse conical depressions, whereas translation of accounts, the jökulhlaup others are shallower and edged by a low- occurred as a series of floods, the last of amplitude mound of sediment. The former which was by far the greatest. Although the morphology is indicative of in-situ melt-out jökulhlaup is thought to have peaked within of buried ice, whereas the later signifies three to five hours, waning-stage discharge melting of a partially buried block with on 11 August from the remains of Kotárjökull resultant subaerial deposition of glacial was almost too warm for horseback riders to debris (Russell et al., 2005 and references cross. From experience gained at Eyjafjalla- therein). Viewed from above, the field of jökull in 2010 (Magnússon et al., 2012a), kettle-holes shows a distinct radial pattern, such high temperatures are a result of reflecting flow expansion from the valley meltwater interacting with advancing lava. between Mount Slaga and Goðafjall (Figure As the 1727 jökulhlaup subsided it was clear II-8). Additionally, kettle-hole diameters that Falljökull and Kotárjökull had “...slid diminish noticeably with increasing distance forwards over the plain ground, just like from the apex of the fan. melted metal poured out of a crucible...” (Thorarinsson, 1958, p. 31). The jökulhlaup

Öræfajökull Volcano: Geology and historical floods 29

Figure II-8: Extent of jökulhlaup deposits associated with the 1727 eruption of Öræfajökull.

30 Öræfajökull Volcano: Geology and historical floods

Fluvial terraces incised into the head of obscures the surface topography. Beyond the Svartijökull show a ~25 m section of periphery of the Little (1750–1900 sediment in the form of a conformable CE) terminal moraines at Falljökull, pitted sequence of interbedded, laterally conti- and boulder-strewn surfaces remain intact nuous, water-lain deposits (e.g. Figure II- (Figure II-7). The moraines themselves and 4A). In places, up to eight nested terraces the intervening zone to the ice margin result remain intact. The deposits are dominated by presumably from glacially reworked flood angular, basaltic tephra typically ≤1 cm in deposits, particularly those of 1727. For diameter, which Thorarinsson (1958) attri- details about modern-day ice retreat at buted to the 1727 eruption. Thompson and Falljökull, see Bradwell et al. (2013) and Jones (1986) claimed that the fan contained Hannesdóttir et al. (2015). mostly air-fall pyroclastic deposits. This reasoning was based on the presence of dark, angular fragments of lacking matrix 6. Floodwater routing support. However, such massive, homo- Historical accounts and geomorphic evidence genous, granular sediment could equally have substantiate that the 1362 and 1727 eruptions been deposited under jökulhlaup conditions occurred in different locations of Öræ- (Maizels, 1991, 1997; Russell and Knudsen, fajökull. This is based mainly on the 1999, 2002). Thompson and Jones (1986) contrasting extent of dark-coloured, basaltic also argued that the distinctive terraces at the deposits in the river catchments of Falljökull head of Svartijökull developed after 1727 as and Kotárjökull (§ 5.3). In the vicinity of a result of gradual fluvial recovery from the Kotá, thick deposits of coarse-grained aggradational effects of the jökulhlaup. In basaltic tephra are present, whereas this contrast, Thorarinsson (1956, 1958) conclu- sediment type is less prominent near to ded that the terraces formed during the Virkisá. The 1362 eruption is thought to have waning-stage of the 1727 jökulhlaup. This is occurred within the caldera; this is supported entirely plausible as flooding occurred on two accounts. Firstly, the subglacial intermittently over four days (Thorarinsson, catchment of Falljökull extends toward the 1958). Furthermore the terrace tops show centreline of the caldera, where ice thickness hardly any signs of fluvial reworking, which exceeds 500 m (Magnússon et al., 2012b). would be expected if braided streams had Such a quantity of ice, coupled with an flowed over the area for sustained periods. eruption of very high mass-discharge rate Smaller jökulhlaup could have incised (Gudmundsson et al., 2015, Chapter III), unconsolidated sediments from the main could account for the volume of water outburst on 4 August 1727, as noted by required to deposit large boulders in high- Dunning et al. (2013) for the 2010 eruption energy, sediment-laden flows kilometres of Eyjafjallajökull. downstream from Falljökull. Secondly, large- In the foreground of Falljökull, the geo- scale mechanical break-up of Falljökull, as morphic impact of the 1727 jökulhlaup is less implied by former dead-ice masses such as prominent. Periods of glacier advance and Langafellsjökull, necessitates floodwater retreat have extensively reworked flood bursting from the ice surface to effectively deposits from 1362 and 1727; moreover the sever the lower part of the glacier from the area is vegetated by dwarf birch, which (e.g. Sturm et al., 1986).

Öræfajökull Volcano: Geology and historical floods 31

Figure II-9: Longitudinal and transverse profiles of Svartijökull – a mass of hummocky terrain arising from the 1727 jökulhlaup. (A) Map showing profile locations; (B) long-profile; (C) cross-sections depicted in (A). Note the location of Figure II-11 in cross-section 1. Survey data derived from a digital surface model (see § 4.2).

32 Öræfajökull Volcano: Geology and historical floods

As outlined in § 5.3, the 1727 jökulhlaup was region (Figure II-10). This, again, implies a preluded by thunder-like sounds. At floodwater source from within the caldera. It Eyjafjallajökull during the summit eruption should be noted, however, that Björnsson of 2010, booming sounds emanated from the (2005) disputed a caldera origin for the 1362 ice-cap on 15 April, followed immediately by eruption, believing instead that the eruption a volcanogenic jökulhlaup (§ 10.5). The occurred outside the caldera rim in an area of sound was attributed to floodwater cascading comparatively thinner ice, thus ruling out a down the lateral flanks of Gígjökull due to high-elevation origin for floodwater. Björns- outlets forming high on the glacier (Roberts son (2005) reasoned that an eruption within et al., 2011; Magnússon et al., 2012a). The the caldera would undoubtedly have affected similarity of the sounds and their timing gives Kvíárjökull. Mapping of bedrock topography confidence to the idea of subglacial flood- in the volcano’s caldera by Magnússon et al. water bursting from the upper slopes of (2012b) demonstrates that a water source Öræfajökull in 1727. With the benefit of within the subglacial catchment of Falljökull modern-day observations (Roberts, 2005; § would not necessarily cause flooding down 10), subglacial floodwater would have burst Kvíárjökull. This is an important point to preferentially from the thinnest section of consider in relation to Björnsson’s assertions. Falljökull, which would have been the icefall

Figure II-10: Northward cross-sectional profile of Falljökull, showing bedrock and ice-surface topography. The inset map shows the extent of the profile on the western flank of Öræfajökull, with shading denoting ice thickness in metres. Bedrock profile data derived from Magnússon et al. (2012b).

Öræfajökull Volcano: Geology and historical floods 33

Additional insight into the subaerial routing upward-coarsening units such as the Kotá of the 1727 jökulhlaup can be gained from the fan; overall such sequences represent large- terrain surrounding Mount Slaga (Figure II- scale bedding deposited parallel to the slope 8). The southern part of the region is of the flooded surface. Such deposits would dominated by the hummocky, steep-sloping have originated from a pulsating, high-energy surface known as Svartijökull. A boulder- flow, limited mainly by sediment supply strewn surface to the northwest of rather than flood power (Maizels, 1997). The Svartijökull also radiates in a down-sandur architecture and vertical sedimentary direction from around the base of Mount structure of jökulhlaup deposits on the Slaga (Figure II-8). This surface, comparable western side of Öræfajökull represent to a debris-flow deposit (Pierson, 2005), continuous aggradation of sediment during a appears to represent the initial flood-wave rapid, linear rise to maximum discharge, akin from Öræfajökull, before floodwater focus- to a dam burst (c.f. Russell et al., 2010). sed on the present-day route of Kotá. It is Scant geomorphic features preserve the possible that the boulder-strewn surface also downstream extent of the 1362 and 1727 underlies deposits at Svartijökull. The routing jökulhlaups. As flows expanded from the of the debris-flow deposit to the north-west of western flank of Öræfajökull, floodwater Svartijökull is uncertain. Some of the flow would have drained across the eastern side of could have been routed between Mount Slaga Skeiðarársandur. In distal regions, mostly and Goðafjall, although the adjacent valley sand to cobble-sized sediment would have between Mount Slaga and Sandfell could been deposited from turbulent flows. Despite have conveyed some of the flow. For this to being laterally extensive, such deposits occur, the Kotá valley must have filled with would either be eroded by Skeiðará or buried floodwater, allowing discharge from the by subsequent jökulhlaups on Skeiðarár- western branch of the glacier to descend into sandur. During the fourteenth century, the neighbouring valley. This hypothesis is climate-induced thickening and advance of especially plausible if floodwater descended Skeiðarárjökull forced the drainage of over the surface of Kotárjökull (c.f. Roberts meltwater to the western and eastern edges of et al., 2011). the glacier (Björnsson, 2003). Over subsequent centuries Skeiðará would have 7. Flood timing and extent flowed over distal flood deposits from Öræfajökull. This process would have been The exact timing of both historical particularly effective during large, eruption- jökulhlaups is difficult to ascertain. Of the related jökulhlaups from Skeiðarárjökull, two eruptions, only accounts of 1727 contain especially in 1861, 1938, and 1996 (Þóra- any detail (§ 5.3). As noted by Thorarinsson rinsson, 1974; Snorrason et al., 1997). (1958), the 1727 eruption began soon after 09:00 on 4 August, and it is thought to have peaked within three to five hours. 8. Flow properties Nevertheless, the actual duration of the main Both the 1362 and 1727 jökulhlaups would rise to maximum discharge could have been have transported masses of freshly erupted two to four hours. An hour could have material, especially while the eruptions were elapsed between the beginning of the confined beneath ice (Gudmundsson et al., and the onset of flooding 2015, Chapter III). As ice blocks became from the ice-cap (Gudmundsson et al., 2015, entrained in the developing floods, this would Chapter III). Jökulhlaup deposits from 1727 have increased the volume of the jökulhlaups shed light on the form of the palaeo- significantly. In this section we review both hydrograph. Sediments ranging from coarse the rheology and ice-content of the two sands to large, angular boulders were historic floods. deposited simultaneously within individual,

34 Öræfajökull Volcano: Geology and historical floods

8.1. Rheology 8.2. Role of ice From existing sedimentological studies at The extent of glacial ice on Öræfajökull Öræfajökull (Thorarinsson, 1958; Maizels, would have been significantly greater in 1727 1991) and inferences from other volcano- than during the 21st Century. In the 1750s, genic floods in Iceland (Tómasson, 1996; Kvíárjökull is thought to have reached the Russell et al., 2005; Duller et al., 2008), it is crest of the terminal moraines (Hannesdóttir possible to speculate on floodwater et al., 2015), so it is probable that Kotárjökull composition during the 1362 and 1727 jökul- was advancing also (Guðmundsson et al., hlaups. Explosive fragmentation during both 2012). When the 1727 eruption occurred, subglacial eruptions would have created a Kotárjökull was at least 30% more extensive copious supply of fine-grained volcani- than it was in 2011 (Guðmundsson et al., clastic material (Gudmundsson et al., 2015, 2012); this explains why ice-release was so Chapter III). Combined with fast-flowing ubiquitous during the 1727 jökulhlaup. water due to steep terrain, sediment would The 1362 and 1727 eruptions were noted for also have been eroded from the entire flood widespread deposition of glacial ice by tract, including subglacial pathways. At the floodwater (see § 5.2 and 5.3). Densely- onset of flooding, when the amount of clustered kettle holes in the foreground of floodwater was minor compared to the Falljökull and Kotárjökull are indicative of volume at maximum discharge, sediment downstream flow expansion and a cor- concentrations could easily have ranged from responding reduction in flood power, leading hyperconcentrated (40–80% solids by mass) to ice-block grounding (Baker, 1987; Fay, to debris flow conditions (>80% solids by 2002; Russell and Knudsen, 2002) (Figure II- mass). The initial front of both floods would 11). Ice blocks that were buried by rising- have reached the lowland as a fast-moving, stage sediment aggradation led to the debris-laden wall of muddy material (c.f. formation of circular kettle holes (e.g. Háalda Russell et al., 2010; Waythomas et al., 2013). in between Sandfell and Hof), whereas Maizels (1991) ascribed debris-flow condi- partially buried fragments gave rise to scour- tions to matrix-supported clastic deposits at like formations (e.g. lower parts of Svarti- the base of the Kotá fan; the implication being jökull) (Figure II-8). From eyewitness that clasts were supported by a fabric of fine- descriptions of the 1727 jökulhlaup (§ 5.3), grained pyroclasts as the 1727 flow emanated large sections of Falljökull and Kotárjökull from Kotárjökull. were broken from Öræfajökull; smaller As both the 1362 and 1727 floods continued pieces even reached the coastline, over 18 km to rise, water-flood conditions would have away. Grounding of ice blocks during prevailed (Maizels, 1991). However, owing waning-stage flows could have caused to high discharge, steep water-surface slopes, floodwater to pond behind an in and topographic constrictions, flows would regions of flow expansion. Ice blockades, have remained deep and fast enough to either close to the eruption site, or in the produce high shear stresses and strong proximal region of Kotá, could account for turbulence (Pierson, 2005). Such conditions the series of 1727 floods noted by would allow for prodigious quantities of Thorarinsson (1958) (see § 5.3). sediment transport, ranging from granular- to boulder-sized clasts (c.f. Duller et al., 2008).

Öræfajökull Volcano: Geology and historical floods 35

The densely pitted sandur around Kotá flood-wave was a slurry mixture, then the affirms to a colossal release of ice from the density of the flow itself may have been upper flanks of Öræfajökull. For the 1727 sufficient to raft tabular sections of Kotár- jökulhlaup, mechanical break-up of Kotár- jökull downstream within minutes of the jökull by floodwater travelling beneath, jökulhlaup beginning; this image is consistent along, and on top of the glacier would have with accounts from 1727 (see § 5.3). readily produced fragmented ice. If the initial

Figure II-11: Kettle-hole on the surface of Svartijökull – note the person for scale (photographer: P. Alho, September 2005). The depression formed due to melting of stranded blocks of ice, which were deposited in the region during the 1727 jökulhlaup (Ólafsson, 1974; Thorarinsson, 1958). For the location and dimensions of the kettle-hole, see Figure II-9.

9. Maximum discharge volume of the jökulhlaup. With this in mind, Thorarinsson (1958) postulated that the 1362 Historic descriptions of the 1727 jökulhlaup, jökulhlaup peaked at > 1×105 m3/s. together with the geomorphic consequences The 1727 jökulhlaup burst primarily from of the 1362 and 1727 eruptions, are clear Kotárjökull, and the extent of flooding was evidence for a rapid, ephemeral rise to similar to that of 1362 (§ 5.3), however the maximum discharge. For instance, Reverend 1362 jökulhlaup drained foremost from Jón Þorláksson (§ 5.3) recalled that the 1727 Falljökull (§ 5.2), signifying that the 1727 jökulhlaup on 4 August peaked within 3–5 jökulhlaup was lower in magnitude. From hours. Thorarinsson (1958) favoured flood- slope-area calculations based on the width of ing analogous to volcanogenic jökulhlaups the Kotá valley between Mount Slaga and from (Tómasson, 1996), thereby Goðafjall (Figure II-12) and a corresponding implying a rapid rise to a maximum discharge surface velocity of 12.1 m/s, the maximum that would be very high compared to the

36 Öræfajökull Volcano: Geology and historical floods

discharge of the 1727 jökulhlaup is estimated (§ 6) and exceptional amounts of ice-release at ~4×104 m3/s (Figure II-12). Palaeo- (§ 8.2). discharge estimates are, of course, hindered Further credence for a rapid rise to maximum by the masses of sediment, rock, and glacial discharge comes from a sedimentological ice that are known to have been transported interpretation of palaeo-hydrograph form. onto the sandur. The maximum discharge Large-scale, upward-coarsening units of from the eruption site (Gudmundsson et al., sand- to cobble-sized deposits (§ 7) 2015, Chapter III) is naturally lower than the demonstrate high-energy flow conditions downstream equivalent, as bulking factors equivalent to the passage of a lahar (Way- such as sediment and ice need to be thomas et al., 2013). Such sequences could considered. For volcanogenic floods from from only under sustained high discharge, Öræfajökull, a bulking factor as high as ~25% resulting in a rising-stage hydro-graph akin to seems reasonable, especially when consi- a dam-burst. dering initially hyperconcentrated conditions

Figure II-12: Reconstructed maximum discharge during the 1727 jökulhlaup from Kotárjökull. (A) Cross-section of the Kotá valley from Mount Slaga to the uppermost surface of Svartijökull (see Figure II-9); (B) calculated slope of the palaeo water-surface; (C) channel cross-section and hydraulic data. Survey data derived from a digital surface model (see § 4.2).

Öræfajökull Volcano: Geology and historical floods 37

10. Modern-day of 1510 m AMSL. Floodwater exited Grímsvötn through a rapidly expanding comparisons subglacial conduit. The initial flood-wave This section highlights occasions when took ~10.5 hours to travel the 50 km distance supraglacial flooding has occurred in from Grímsvötn to the edge of Skeiða- connection with volcanic activity. The rárjökull; at peak flow the transit time purpose is to use modern-day analogues to decreased to about 3 hours (Björnsson, 1998). The jökulhlaup ceased after 40 hours, having better understand how the 1362 and 1727 3 jökulhlaups developed. Examples are taken released ~3.6 km of floodwater onto from Iceland and , U.S.A. The Skeiðarársandur (Gudmundsson et al., 1997). Icelandic examples from Eyjafjallajökull and During the initial rising stage of the Sólheimajökull are especially relevant, as the jökulhlaup, floodwater blasted through the affected glaciers are similar in surface profile surface of Skeiðarárjökull, producing multi- and ice thickness to the Öræfajökull flood ple supraglacial outbursts across the terminus paths. (Roberts et al., 2000). In some locations, floodwater burst through ~350 m of ice before reaching the glacier surface. Where 10.1. Redoubt: 1989–1990 and 2009 floodwater burst through the ice surface close The surface of Drift glacier has been to the margin, large volumes of ice were disrupted on several occasions by subglacial released (Roberts et al., 2002). at in 1989–1990 and 2009 (Trabant et al., 1994; Waythomas et 10.3. Sólheimajökull: 1999 al., 2013). Instead of draining entirely beneath Drift glacier, debris-laden out- Sólheimajökull drains from the Mýrdals- pourings of floodwater have broken repeated- jökull ice cap, which is underlain by the Katla ly through the glacier’s surface at high volcano. Sólheimajökull is a 9 km long, non- surging valley glacier, with a surface area of elevation (Trabant et al., 1994). In some 2 locations, glacial ice has been stripped away ~78 km and a terminus ~1 km wide. On 10 to bedrock by repeated floods. Distinctive July 1999, the river issuing from Sólheima- ‘ice diamict’ deposits have been mapped on jökull (Jökulsá á Sólheimasandi) was the glacier surface and also several kilometres abnormally high. People travelling across downstream, revealing the extent of supra- Sólheimasandur between 14 and 17 July glacial flooding (Waitt et al., 1994). During informed local authorities that the river was the 2009 eruption of Redoubt, floods and unusually dark, high, and extremely odorous pyroclastic flows removed 0.1–0.2 km3 of ice (Sigurðsson et al., 2000). At 17:00 UTC on from Drift Glacier (10–20% of total ice 17 July, prolonged seismic tremors were volume) (Waythomas et al., 2013). detected from beneath Mýrdalsjökull; this seismicity intensified through the evening, culminating at ~02:00 hours on 18 July. This 10.2. Skeiðarárjökull: 1996 peak in seismic activity was concomitant with Skeiðarárjökull is a -type piedmont the release of a jökulhlaup from Sólheima- glacier draining from the Vatnajökull ice cap. jökull (Roberts et al., 2000). The northern edge of the glacier’s water- During the jökulhlaup, numerous high- divide neighbours the Grímsvötn subglacial capacity outlets developed across the termi- . From 30 September 1996 to early nus, western lateral flank and surface of October 1996, a subglacial eruption took Sólheimajökull (Roberts et al., 2000). Peak place north of Grímsvötn (Gudmundsson et discharge at the terminus and 6 km down- al., 1997). Late on 04 November 1996, 35 stream was estimated at ~5,000 m3/s and days after the start of the eruption, floodwater 1,940 m3/s, respectively (Sigurðsson et al., began to drain from Grímsvötn at a lake-level 2000; Russell et al., 2010); these values

38 Öræfajökull Volcano: Geology and historical floods

indicate marked downstream flow magnitude flooding (c.f. Duller et al., 2014). attenuation, analogous to flash-floods in In 1362 floodwater was routed primarily via ephemeral regions. Eyewitness accounts Falljökull, whereas in 1727 floodwater from the bridge over Jökulsá á Sólheimasandi affected Kotárjökull more so, implying suggest that the jökulhlaup persisted for ~6 different eruption sites within the caldera for hours, having peaked within an hour the two eruptions. Both historical jökulhlaup (Sigurðsson et al., 2000). were fleeting in nature, rising to maximum discharge in a matter of hours. Although 10.4. Eyjafjallajökull: 2010 difficult to constrain, the maximum discharge of the 1362 jökulhlaup was on the order of Sourced from within the volcano's ice-filled 1×105 m3/s; the peak of the 1727 jökulhlaup, caldera, the April 2010 eruption of although smaller, was in the region of 4×104 Eyjafjallajökull stratovolcano caused repea- m3/s — a flood discharge equivalent to the ted jökulhlaups in response to initial height of the November 1996 jökulhlaup subglacial volcanism, followed by phreato- from Grímsvötn. magmatic activity and lava-flow confined by ice (Roberts et al., 2011; Magnússon et al., A first-hand account of the 1727 jökulhlaup 2012a). The ice-surface in the summit caldera described floodwater rushing from Falljökull lies at 1500–1600 m AMSL, with the ice and Kotárjökull, followed by the complete being up to 200 m thick. This ice mass forms break-up and removal of Kotárjökull. Gígjökull – a northward flowing valley Flooding peaked during the 1727 eruption in glacier. The summit eruption began at 01:15– a matter of hours; this timeframe necessitates 01.30 UTC on 14 April. By 06:45, stage rapid run-off from the eruption site, measurements 1 km from Gígjökull con- combined with swift drainage of floodwater firmed the onset of flooding. Gauged 18 km to the lowlands. Although onlookers’ downstream from Gígjökull, the initial descriptions of the 1727 jökulhlaup do not jökulhlaup reached a discharge of 2,700 m3/s refer explicitly to supraglacial outbursts, it is within 88 minutes of arrival. A smaller, asserted here that such flooding dominated concurrent jökulhlaup also burst from the the onset of both the 1362 and 1727 southern flank of Eyjafjallajökull, carving a jökulhlaups. From modern-day measure- 3-km-long trench into the ice surface. On ments of subglacial bedrock topography and both 14 and 15 April 2010, floodwater ice-surface elevation at Öræfajökull, it is descended across the surface and flanks of evident that floodwater draining from the Gígjökull as it broke through the glacier at an caldera region would have broken through the elevation as high as 1400 m AMSL. Such ice surface at ~1,500 m AMSL. The breakout pits formed in several places on the implication of this is twofold: Firstly, glaciers upper reaches of Gígjökull and allowed ice- such as Falljökull and Kotárjökull would laden slurries to debouch across the ice- have been severed by fractures conveying surface (Roberts et al., 2011; Magnússon et floodwater to the ice surface; and secondly, al., 2012a). such a process would lead to rapid fragmentation and eventual ice removal, as attested by written accounts. By bypassing 11. Summary subglacial drainage routes, supraglacial The stark geomorphic imprints of the 1362 outbursts of floodwater would have caused a and 1727 jökulhlaups are a testament to the rapid rise to maximum discharge — a impact of historical volcanism at Öræfa- situation akin to a dam-burst. Rapid mecha- jökull. Despite only two confirmed volcanic nical disruption of the lower reaches of eruptions during the past thousand years, the Falljökull and Kotárjökull would have led to landscape in the vicinity of Virkisá and Kotá ice-blocks being incorporated constantly into is almost entirely a consequence of high- rising-stage flows.

Öræfajökull Volcano: Geology and historical floods 39

The findings of this chapter provide Bradwell, T., Sigurðsson, O., & Everest, J. (2013). constraints for estimating the melting Recent, very rapid retreat of a temperate glacier potential of Öræfajökull eruptions, as studied in SE Iceland. Boreas, 42, 959–973. in Chapter III by Gudmundsson et al. (2015); Duller, R. A., Mountney, N. P., Russell, A. J., & they are also pertinent to the simulation of Cassidy, N. C. (2008). Architectural analysis of a volcaniclastic jökulhlaup deposit, southern volcanogenic floods from Öræfajökull, as Iceland: sedimentary evidence for supercritical explored in Chapter IV by Helgadóttir et al. flow. Sedimentology, 55, 939–964. (2015). Furthermore, insights into flood Duller, R. A., Warner, N. H., McGonigle, C., De extent, floodwater composition, and the Angelis, S., Russell, A. J., & Mountney, N. P. prevalence of ice blocks provides an (2014). Landscape reaction, response, and empirical basis for the rating of flood hazards recovery following the catastrophic 1918 Katla in the Öræfi region (Pagneux and Roberts, jökulhlaup, southern Iceland. Geophysical 2015, Chapter V). Research Letters, 41, 4214–4221. Dunning, S., Large, A. R., Russell, A. J., Roberts, M. J., Duller, R., Woodward, J., Mériaux, A. S., 12. Acknowledgements Tweed, F. S., and Lim, M. (2013). The role of multiple glacier outburst floods in proglacial This study was funded by the Icelandic landscape evolution: the 2010 Eyjafjallajökull Avalanche Mitigation Fund, the National eruption Iceland. Geology, 41(10), 1123–1126. Power Company, and the Icelandic Road and Fay, H. (2002). The formation of ice block obstacle Coastal Administration. Fieldwork in 2003 marks during the November 1996 glacier was undertaken with funding from the outburst flood (jökulhlaup), Skeiðarársandur, Iceland. In I. P. Martini, V. R. Baker, & G. Earthwatch Institute. Fieldwork in 2006 was Garzon (Eds.), Flood and Megaflood Deposits: supported by a grant to Matthew J. Roberts Recent and Ancient Examples (pp. 85–97). from Kvískerjasjóður. Oddur Sigurðsson International Association of Sedimentologists. provided the photographs for Figure II-2. Forbes, A. E., Blake, S., Tuffen, H., & Wilson, A. Emmanuel Pagneux is thanked for assistance (2014). Fractures in a trachyandesitic lava at with figure preparation. We are grateful to Öræfajökull, Iceland, used to infer subglacial Andrew J. Russell, Trausti Jónsson, Tómas emplacement in 1727–8 eruption. Journal of Jóhannesson, Oddur Sigurðsson, and Philip Volcanology and Geothermal Research, 288, 8– 18. M. Marren for valuable feedback on this chapter. Gudmundsson, M. T., Högnadóttir, Þ., & Magnússon, E. (2015). Öræfajökull: Eruption melting scena- rios. In E. Pagneux, M. T. Gudmundsson, S. Karlsdóttir, & M. J. Roberts (Eds.), Volcanogenic 13. References floods in Iceland: An assessment of hazards and Baker, V. R., & Costa, V. R. (1987). Flood power. In risks at Öræfajökull and on the Markarfljót L. Mayer, & D. Nash (Eds.), Catastrophic (pp. 45–72). Reykjavík: IMO, Flooding (pp. 1–21). Boston: Allen and Unwin. IES-UI, NCIP-DCPEM. Björnsson, H. (1988). Hydrology of ice caps in Gudmundsson, M. T., Larsen, G., Höskuldsson, Á., & volcanic regions. Soc. Sci. Isl., 45, 139, 21 maps. Gylfason, Á. G. (2008). Volcanic hazards in Iceland. Jökull, 58, 251–258. Björnsson, H. (1998). Hydrological characteristics of the drainage system beneath a surging glacier. Gudmundsson, M. T., Sigmundsson, F., & Björnsson, Nature, 395, 771–774. H. (1997). Ice-volcano interaction of the 1996 Gjálp subglacial eruption, Vatnajökull, Iceland. Björnsson, H., & Einarsson, P. (1990). Volcanoes Nature, 389, 954–957. beneath Vatnajökull, Iceland: evidence from radio echo-sounding, earthquakes and jökul- Guðmundsson, H. J. (1998). Holocene glacier hlaups. Jökull, 40, 147–168. fluctuations and tephrochronology of the Öræfi district, Iceland. Edinburgh: University of Björnsson, S. (2003). Skeiðarársandur og Skeiðará. Edinburgh. Náttúrufræðingurinn, 71(3–4), 120–128. Björnsson, S. (2005). Gos í Öræfajökli 1362. Náttúrufræðingurinn, 73(3-4), 120–128.

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