Turbidity currents sculpt the seafloor into spectacular submarine channels, and are important conveyors of sediment from the continents into the deep ocean. Turbidity currents are so named because the current is a turbulent cloud of sediment flowing downslope under the influence of gravity (envisage a muddy avalanche). These flows are unpredictable and destructive, attaining speeds of up to 20 m/s (72 km/h) and flowing for hundreds of kilometres. Geologists study these events and their deposits in order to understand the processes that are responsible for the distribution of terrigenous sediment in the deep ocean as well as for natural hazard and resource prediction. A few spectacular examples of seafloor morphologies and historically observed turbidity currents are summarized below.
On 18 November 1929, an earthquake-generated turbidity current broke multiple submarine telecommunication cables south of Newfoundland, Canada (Heezen and Ewing 1952). Using the cable break times and seafloor mapping, the current thickness was estimated at 150 to 300 m thick and its speed was calculated to be 18 m/s (64 km/h). The turbidity current formed 2–5 m high gravel dunes and carried 100–200 km3 of sediment onto the abyssal plain (Piper et al. 1988).
Another turbidity current, this one human-induced, occurred on 16 October 1979 off the coast of Nice, France. Landfill operations for the Nice airport runway extension oversteepened the slope in the Var submarine canyon and caused a submarine landslide, which evolved into a turbidity current that travelled for more than 120 km, breaking cables on the way (Mulder et al. 1997). The current swept a bulldozer and pieces of the airport embankment more than 15 km from the airport, and unfortunately resulted in the deaths of numerous airport and construction workers.
Turbidity currents like these erode and deposit sediment on the seafloor, forming geomorphic features similar in geometry and scale to those found on earth’s subaerial surface. A beautiful example of this is the Bengal submarine channel–levee system, offshore east India (Kolla et al. 2012). This channel, fed by the Ganges–Brahmaputra river system, is more than 1000 km long and is highly sinuous. In cross-section, the Bengal channel is more than 1 km wide and highly aggradational. Using seismic reflection data to cut a horizontal plane through the channel system, lateral migration scroll bars and oxbow cut-offs form the majority of the stratigraphy. These features are very similar in geometry and scale to large, continental-scale river systems — just look around in Google Earth.
With ever-increasing quality and resolution of bathymetric, seismic, and flow-monitoring data, we will continue to advance the knowledge of submarine channel and fan systems and the turbidity currents that mould them.
Kolla, V, A Bandyopadhyay, P Gupta, B Mukherjee, and D Ramana (2012). Morphology and internal structure of a recent Upper Bengal Fan-Valley Complex, SEPM Special Publication 99, 347–369. The excerpted figure is copyright of SEPM, DOI 10.2110/pec.12.99.0347.
Mulder, T, B Savoye, and J Syvitski (1997). Numerical modeling of a mid-sized gravity flow: the 1979 Nice turbidity current (dynamics, processes, sediment budget, and seafloor impact). Sedimentology 44, 305–326, DOI 10.1111/j.1365-3091.1997.tb01526.x.
Piper, D, A Shor, and J Hughes Clarke (1988). The 1929 Grand Banks earthquake, slump, and turbidity current. GSA Special Paper 229, 77–92, DOI 10.1130/SPE229-p77.
Piper, D and B Savoye (1993). Processes of late Quaternary turbidity current flow and deposition on the Var deep-sea fan, north-west Mediterranean Sea. Sedimentology 40, 557–583, DOI 10.1111/j.1365-3091.1993.tb01350.x.