Tsunamis are generally associated with earthquakes, not with largely seismically stable areas such as northern Europe. There is evidence, however, that somewhere around 6-8000 years ago, a major tsunami generated by a submarine landslide did occur in the North Atlantic, affecting the North Sea coasts and as far away as Iceland - and almost certainly wiping out any humans who were unfortunate enough to live within its reach.
What Are Submarine Slides?
Most continents are bounded by a submarine plateau known as a continental shelf, at the edge of which a steeper slope runs down to the depths of the continental basins. Although these slopes are not necessarily particularly steep (between 1° -15° according to the Oxford Dictionary of Earth Sciences) they mark significant changes in depth.
As with any slope, gravity causes the transfer of sediment from upper to lower slopes. Such submarine slides are common, and can occur on large scales, transferring material over hundreds of kilometres. As well as causing damage to oil installations and submarine cables, the significant displacement of large amounts of water can cause significant tsunamis, such as that off Newfoundland in 1929 (Farokh).
What was the Storegga Slide?
The Storegga slide was a major submarine landslide which took place off the Norwegian continental shelf. First identified by Bugge et al in 1893, the Storegga event was in fact three slides, the first and largest of which took place some time between 30-50,000 years ago, while the later two occurred close together in time during the Holocene period, probably between 4-6,000 BC.
Between them, the three slides moved a vast quantity of material: the first event is estimated to have had a volume of 3880 km³ and the combined volume of the second and third slides was probably around 1700 km³. Material was removed from an area of approximately 34000 km² and was carried a distance of 250 km from the source, possibly further (Bugge).
The mechanisms for triggering the slide are unclear. Bugge concluded that an earthquake was the most likely possibility: on the basis of studies of historic tremors, he and his colleagues suggested that a large earthquake capable of generating the slide (M6.0-M7.5) might well have occurred in the area within the last 10 000 years. The presence of gas hydrates in the area and other geophysical factors are also thought to have been influencing factors.
Impacts of the Storegga Slide: the Evidence
The displacement of vast amounts of material would have generated tsunamis. Britain was certainly settled at the time of the second and third slides, with many people living on the relatively hospitable and low-lying coastal plains. Although there is no written evidence of what took place, the archaeological record clearly shows that a huge wave will have swamped low-lying coastal areas.
Based on the evidence of a sand layer which will have been deposited by the tsunami and which has been found at a number of sites, Shi and Smith describe how the tsunami swamped coastal areas not just of Norway but also in Scotland, north-east England, Iceland and the Faeroe Islands.
The scale of the tsunami as it reached land is not clear, although Alastair Moffat suggests (without citing a specific source) that it may have been 8 metres high and Weiniger et al suggest an even greater height of 10-12 metres. At any rate it had the energy to take it considerable distances inland and it seems certain that many settlements would have been overwhelmed.
The fact that tsunami deposits are not recorded further south is, in itself, of interest. At the time of the event the North Sea had not been fully formed and there were significant areas of low-lying land, known as Doggerland. Although Doggerland may have been subsiding anyway, it may well be that the Storegga tsunami accelerated the process and contributed to the opening of the present North Sea Weiniger).
Sources and Further Information
T. Bugge, R. H. Belderson, N. H. Kenyon “The Storegga Slide” Philosophical Transactions of the Royal Society of London 1988 jstor.org accessed 14 March 2011
Moffat, Alastair Before Scotland Thames & Hudson 2009
Farokh Nadim Submarine Slides and their Consequences International Centre for Geohazards/NGI no date docstoc.com accessed 14 March 2011
S. Shi and I D. E. Smith Coastal Tsunami Geomorphological Impacts And Sedimentation Processes: Case Studies Of Modern And Prehistorical Events International Conference on Estuaries and Coasts 2003, irtces.org, accessed 14 March 2011
Bernhard Weiniger et al The Castastrophic Final Flooding of Doggerland by the Storegga Tsunami Documenta Praehistorica 2008 ucl.academia.edu accessed 14 March 2011
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