Jökulhlaups, or glacial outburst floods (GLOFS) are a key part of the deglaciation of alpine mountains and ice sheet margins. They are a hazard to people and infrastructure, and can cause widespread landform modification through erosion and deposition. These floods occur very suddenly discharge large volumes of water, ice and debris from a glacial source. They can occur anywhere where water accumulates in a subglacial lake beneath a glacier. The flood is initiated following the failure of an ice or moraine dam.
Geological importance of jökulhlaups
Jökulhlaups with a large landscape impact have been called ‘megafloods’, which have an extremely large channel geometry, flood volume and peak discharge. They can erode gorges and spillways, dry waterfalls (‘cataracts’), streamlined obstacles, scour marks and longitudinal grooves. These individual landforms can form by several different mechanisms, and so in order to diagnose a jökulhlaup their association and sediment-landform assemblages must be closely examined.
Depositional evidence of megafloods includes giant gravel bars and megaripples, which may be formed of boulders. Slackwater deposits may include some finer material, such as sand, which may be dateable using Optically Stimulated Luminescence.
Deglacial association of jökulhlaups
Jökulhlaups are a key part of deglaciation and result in landscape change on land and on near-shore continental shelves. Soon after the LGM, major jökulhlaups from the ice sheet margins disrupted the ocean thermohaline circulation, excavated terrestrial and submarine canyons and redistributed sediment across the land and into the oceans.
Periods of rapid deglaciation may promote volcanically-generated jökulhlaups, making them more frequent and of a higher magnitude. This is because rapid deglaciation can result in increased volcanic activity following unloading and stress release within the Earth’s mantle and lithosphere.
Present deglaciation is resulting in more frequent jökulhlaups from moraine and ice-dammed lakes, with recent examples from mountain glaciers in Scandinavia, Alaska, Canada, Patagonia, the Himalaya, New Zealand and Antarctica. They may be increasing because of the apparent world-wide accelerated recession and downwasting of alpine mountain glaciers. However, their identification and measurement is difficult due to their unpredictable nature.
Case study: volcanically-triggered jökulhlaups at Eyafjallajökull, Iceland
Eyafjallajökull is a glaciated volcano in southern Iceland. Its eruption on 13th April 2010 disrupted flights across Europe. On the 14th of April, a large subglacial explosive eruption started beneath the 2.5 km-wide summit caldera, melting through the 200 m thick ice cap within hours, and generating a major 8.5 km high plume. By 0700 GMT, rapid melting of the ice cap produced volcanogenic jökulhlaups that cascaded from Gígjökull and down Núpakotsdalur on the northern and southern flanks of the volcano. The flood reached peak discharge several hours later, damaging Iceland’s ring road. Later, further eruptions generated more jökulhlaups that inundated the town of Markarfljót, resulting in evacuation of the population. The sediment-laden jökulhlaup had a smooth hyper-concentrated fontal wave followed by a more turbulent fluid flow body.