11 
All of the above events require the volcano to be located in or close to the ocean. According 
to a map of historic volcano eruptions (Latter 1981), tsunami have occurred in the North 
Atlantic Ocean after volcanic eruptions on Martinique (Monte Pelée) and Iceland. The 
Russian tsunami catalogue (10.2: HTA03) lists seven tsunami which were triggered by 
Icelandic volcanoes. Imsland (1988), however, considers the tsunami risk from Icelandic 
volcanoes to be very low or even non-existent. 
Tsunami generation by atmospheric pressure waves following a volcanic eruption at a large 
distance from the volcano has been observed. So far, there has been no evidence of such 
phenomena in the North Atlantic Ocean. 
3.3 Slope failures 
Earthquakes may trigger slope failures on steep, vulnerable continental slopes and coasts. 
Volcanic eruptions may be involved in slope failures in several ways. Prehistoric slope 
failures have been attributed to meteoritic impacts. Also a potential de-stabilisation of slopes 
by mining of mineral resources has been discussed. Gas hydrates have an influence on local 
slope stability in the area of the hydrate reservoir. They are very vulnerable to changes in 
ambient conditions, especially pressure conditions. These may be influenced by 
climatological changes or by the exploitation of gas hydrates or other natural resources. Also 
minor slope failures triggered by various factors may change pressure conditions within the 
slope, thus modifying the ambient parameters for the gas hydrates and causing secondary 
slope failures (Parlaktuna 2003). 
The mechanism of tsunami generation by slope failure is movement of the slope material 
itself or the impact of rapidly moving material on the water. The oscillation period increases 
with the size of the slide and steepness of slope (Bryant 2001). 
Fig. 3.3.1: Schematic representation of a submarine slide (Bryant 2001, Fig. 6.3) 
Several large slope failures occurred on the Norwegian continental slope in the past. One of 
them is the Storegga slide around 8000 bp which caused a well-known tsunami. From the 
thickness of the sediment layer flowing down the slope, a wave height of 2.30 m was 
computed for the tsunami in the deep ocean. Its oscillation period was 2 to 3 hours, which is 
clearly longer than that of tsunami caused by earthquakes (Blasio et al. 2003). 
Reconstruction based on geological data, with different assumptions regarding slide velocity, 
yielded computed water levels between 5.3 and 18 m near Scotland (Harbitz 1992). With sea 
level rising 0.30-0.35 m per century (Streif 2003), the North Sea level at 8000 bp was about 
20 m below the present mean sea level. Off Scotland, it was only 6 m below current mean 
sea level (Long et al. 1989). This has been taken into account in the above reconstruction. 
Model computations indicate water levels from 3 m to 5.5 m on the coast of East Scotland 
(Harbitz 1992 and Bondevik et al. 2005). According to computations by Henry et al. (1992)