Island-Jökulhlaup August 2006
After a few months in Iceland, being privy to conversations about jökulhlaups – glacial outburst floods - becomes common place. Recent research by Kate Smith - University of Icelandhighlighted the presence of jökulhlaup deposits in the Western outwash plain of Mýrdals- and Eyjafjallajökull glaciers bringing the fact that jökulhlaups were natural hazards that could affect populated areas of Iceland to the attention of the authorities (whereas before they were believed to affect only uninhabited part of the country).
The spectacular pictures of the floods caused by the eruption of the Gjálp fisssure under Vatnajökull glacier in 1996, washing away roads and bridges, give an indication of the potential strength of the phenomenon at play. Looking at older events, Icelanders still tell stories about the large jökulhlaup from the Katla eruption under Mýrdalsjökull in 1918 and on how its sediment load extended the land area up to 4km in some coastal areas.
Geothermally-triggered and ice-dammed lake jökulhlaups are becoming increasingly well-understood. I was, however, surprised to hear recently about the existence of another phenomenon: the rockfall-triggered jökulhlaup.
Only one such event has been documented to date to my knowledge: the jökulhlaup from Steinsholtsjökull (outlet glacier of Eyjafjallajökull - Southern Iceland) in January 1967.
Due to the remoteness of the site, there was no direct witness of the rockfall and the unfolding of events had to be rebuilt a posteriori. The first anybody knew of the event was when floodwaters reached inhabited coastal areas to the West of the glacier. Local populations then raised the alarm as no volcanic eruption had been reported in the area at the time. Tracing back the path of the water, scientists discovered that a rockfall had occurred from the valley walls of Steinsholtsjökull and very large boulders had fallen on the terminus of the glacier. The total volume of rock that fell on the glacier was so large that it caused high frictions within the ice triggering significant melting and leading to the formation of a jökulhlaup. Approximately 15million m3 of basalt and hyaloclastite fell off the cliff, less than half of which directly on the glacier, covering an area of 280,000m2 and rising up to 50m above the ice surface. The impact was so large that microseismic waves were recorded on the country’s seismograph network.
The reason why such rockslides occur is not certain. They are believed to be the result of rapid thaw followed by very heavy rainfalls, leading to a large block of rock breaking away from the rockface.
Nowadays the valley in front of the terminus of the glacier is still dotted with boulders (some exceeding 5m high), either left in situ where they broke through the surface of the ice or where the jökulhlaup ceased to carry them. The valley is incredibly quiet, with just a peaceful glacial river running through it. But when you hear the story of what happened there nearly 40 years ago, you can’t help but feeling very small. Such a volume of rock breaking off a cliff is next to impossible to imagine, even when you stand among all these boulders.
Other rockslides on the surface of glaciers have been recorded during the latter part of the 20th century (e.g. Jökulsárgilsjökull) but none have been large enough to trigger jökulhlaups like the events at Steinsholtsjökull in 1967. In the absence of jökulhlaups and microseismicity, these events are generally only noticed through visual observation when flying over the glacier or analysing aerial/satellite images.