Submarine Mass Movements

The biggest landslides occur on the seafloor however, until recently it was thought that these types of events were rare and limited to areas of high slope, deltas or areas of high seismic activity.  The increasing knowledge of seafloor topography correlated with exponential use of acoustic remote systems of observation (e.g. multibeam bathymetry, sidescan sonar, seismic profiles), resulting not just from the technical advances but also to the growing development of offshore activities, have uncovered that landslides have been far more frequent than previously believed. This represents a substantial risk to the coastline, as the collapse of large amounts of sediment could induce the formation of tsunamic waves with catastrophic consequences, especially to offshore structures such as pipelines, as the cable breaks on the Grand Banks slide are an example Piper et al., 1988).

Slope instability can occur in many environments: fjords, active river deltas, submarine canyons, oceanic volcanic islands or ridges and open continental margin slopes (Lee et al., 1991), and result from an increase in the driving stresses, a decrease in strength, or a combination of the two. In a situation of slope instability, mass movement can be induced by several trigger mechanisms, including:

·   earthquakes (e.g. Grand Banks Slide, Piper et al., 1988);

·   sedimentary accumulation (e.g. Mississippi Fan, Coleman et al., 1993),

·   oversteepening (e.g. Canary Islands, Gee et al., 2001);

·   loading by human activity (e.g. Nice, Assier-Rzadkieaicz et al., 2000);

·   diapirism (e.g. Gulf of Mexico, Prior & Hooper, 1999);

·   storms (e. g. Mississippi Hurricane Camille, Bea et al., 1983);

·   sea-level changes (e.g. Madeira Abyssal Plain, Weaver & Kuijpers, 1983)

Regardless of the mass transport process acting, all continental margins shaped by these processes make slope instability an important factor in continental margin evolution. However it can be consider that there is a mixture of “local” and “universal” mass movement processes, since glacigenic debris flows and volcanic island collapse only occur in specific environments, whereas large-scale sediment slab failure is a universal processes.

The classification of submarine mass movements has been the object of continuing debate, in part due to the fact that during and following slope failure, sediments can be transported by a broad variety of processes, from rigid block motion to turbulent flow generation.