The Steinsholtsjökull rockslide and GLOF in January 1967, South Iceland – a geophysical hazard likely to reoccur elsewhere in Iceland?

This paper presents new estimates of the dimensions and impact of the 1967 Steinsholtshlaup in Iceland in order to understand better the event, the hazards it generated, its long-term legacy and the implications for both landscape interpretation and hazard planning in areas of contemporary valley glaciation. On 15^th of January 1967 a major rockslide occurred on the northern face of the Innstihaus mountain in southern Iceland, which overlooked the valley glacier called Steinsholtsjökull. The slide occurred during intensive snowmelt, that followed heavy snow accumulation in December 1966. The landslide was a complex paraglacial response to decades of down wasting of Steinholtsjökull. Since the 19^th century high stands of the Little Ice age in Iceland, Icelandic glaciers have probably lost about 16% of their mass. Warm conditions in the 1920s and 1930s drove rapid glacier retreat in southern Iceland and resulted in the formation of many pro-glacial lakes, one of which formed in front of Steinsholtsjökull as the terminus of the glacier retreated up valley and the surface down wasted.  The Innstihaus rockslide displaced the southern margin of the glacier and broke up a large amount of the glacier surface. The resulting down valley avalanche of rock incorporated glacier ice, swept into a proglacial lake and the confined pro-glacial valley of Steinsholtsdalur, creating a GLOF that left a trail of ice, rock debris and landscape transformation that entirely overprinted the pre-existing pro-glacial landscape. The Steinsholtsá river was displaced from the centre line of the valley to its southern margin. About 5km from the site of the cliff collapse, boulders up to 80m³ in size were scattered immediately beyond the confluence of the proglacial valley with a wider valley sandur. A paper published by Kjartansson in 1967 recorded the immediate aftermath of the GLOF, but left many questions unanswered, and there have been no subsequent publications. A better understanding of this event is important because, circumstances similar to those found in the Steinsholtsdalur valley prior to 1967 have developed in numerous glacial environments around Iceland’s ice caps.  As in many other montane areas, increased temperatures over the last thirty years have driven renewed and rapid retreat of valley glaciers. Across Iceland, existing proglacial lakes have expanded and many new ones have formed. These glacier fluctuations have affected the stability of neighbouring mountain slopes, which are resulting in slope deformation and mass movements. The potential for a major geomorphological incident in areas that both attract tourists year-round and have seen a recent related infrastructure development raises serious concerns and stresses an urgent need to study and monitor these environments.