Jökull


Jökull - 01.01.2016, Side 44

Jökull - 01.01.2016, Side 44
Jonathan L. Carrivick et al. Glacier surface morphology Surge-type outlets of Drangajökull dip on aver- age 6.5–7.5◦and small cirque surge-type glaciers in Tröllaskagi peninsula dip 12–14◦(Björnsson et al., 2003; Björnsson and Pálsson, 2008). Since the slope of the vast majority of the surface of Kverkjökull is >4◦, a surge there is not typical in an Icelandic con- text. Furthermore, none of the glaciers mentioned in Table 1 are generally steeper than 4◦ except perhaps in small localised parts. The exception is the surge of Horcones Inferior glacier because that glacier is steep and fed by ice falls (Pitte et al., 2015). The surging glaciers in the Karakoram are also generally low an- gle in their ablation areas but with steep ice falls to connect with accumulation areas. The orientation of crevasses is normally depen- dent on the direction of maximum extending strain- rates, with a fracture typically forming perpendicu- lar to the direction of maximum extending strain-rate (Cuffey and Paterson, 2010). In our data, crevassing in 2007 was generally restricted to the margins of the glacier and to a few centrally-situated zones associ- ated with proximity to bedrock outcrops (Figure 2A). In contrast, in 2011 crevassing extended throughout the central portion of the glacier and was intense along the eastern margin and in the south-western part of the terminus region (Figure 2B). Between 2008 and 2011, the evolving orientation of the crevasses close to the centreline of Kverkjökull demonstrates differ- ential flow between the middle of the glacier and its margins. Longitudinally compressive stresses are in- terpreted in the highly-crevassed central lobe, whereas longitudinal extension is prevalent in the reservoir zone in the upper glacier. Our surface velocities derived from manual fea- ture tracking in the terminus area (Figure 8) and in the mid-elevation parts of the glacier near the Kverk (Fig- ure 9) both suggest a deceleration of the glacier be- tween 2012 and 2013 of ∼20 to 25%. This slowdown is from velocities of up to 80 m yr−1 (Figure 8) but the fastest sections of the glacier are probably those most crevassed and they were moving so quickly that our imagery did not have sufficient temporal resolution to confidently track features (Figure 8). The fastest ve- locities that we could measure relate to a 21 m dis- placement over 30 days in 2012 (Figure 9), equiva- lent to 0.7 m day−1, which is not uncommon for ‘nor- mal’ (non-surge) ice flow. The bias in our velocity measurements towards slower velocities probably ex- plains why the surface speeds that we have measured are low in comparison to the surface velocity of land- terminating surging glaciers elsewhere (Table 1), but might also be an indication that we are measuring the final stages of the surge. Our surface observations did not reveal any change in the number or position of glacier river out- lets, which have been noted to commonly occur dur- ing other Icelandic glacier surges (Björnsson, 1998; Björnsson et al., 2003). Surge timing and duration We are limited by data availability for determining exactly when the Kverkjökull surge started and how long it lasted. However, we can say that it started af- ter 2008 (Figure 3) and before 2011. The surge front was well progressed by 2009 (Figure 7) and had not reached the glacier terminus in 2010 (Figure 6) but had by 2011 (Figure 2C). The terminus continued to advance through 2013 (Figure 8). Ice surface veloci- ties apparently slowed between 2012 and 2013 (Fig- ures 8 and 9). The discrepancy between the daily dis- charge and seasonal volume of meltwater runoff be- tween the years 2008, 2009 and 2014, and the years 2010, 2011, 2012 and 2013, in the Jökulsá á Fjöl- lum and the Kreppa suggest that the Jökulsá á Fjöllum river received an unusual amount of meltwater that was not seen in neighbouring catchments and so un- likely to be due to climate. Considering these obser- vations together, the timing and duration of the surge of Kverkjökull might therefore be tentatively put at 2009 to 2013, and 4 years, respectively. The duration of the Kverkjökull surge is longer than those at many temperate surge-type glaciers in regions such as Alaska, which typically surge for 1 to 2 years (Jiskoot et al., 2000). Other Icelandic glaciers that surge typically do so over ‘several years’ (Björns- son et al., 2003). The Kverkjökull surge is, how- ever, apparently rather short in duration in comparison with some other high-latitude and polythermal glacier surges, such as those in Alaska, Svalbard and Green- land, which often last 10 to 15 years (Jiskoot et al., 44 JÖKULL No. 66, 2016
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