Large debris avalanches on volcanic island flanks - what can the deposits 
tell us about emplacement processes?

Douglas G. Masson
Southampton Oceanography Centre, European Way, Southampton SO14 3ZH

Large scale landslides have been a key process in shaping the Canary Islands. 
Evidence for geologically recent landslides includes landslide scars and 
debris deposits on the island flanks and volcaniclastic turbidites on the 
adjacent ocean basin floor. Debris avalanches, long runout catastrophic 
failures which affect the superficial (1-2 km thick) part of the island 
volcanic sequence, are the commonest type of landslide. Individual debris 
avalanches are 50-500 km3 in volume, cover several thousand km2 of seafloor, 
and have runouts of up to 130 km. Debris avalanche deposits have a blocky 
morphology, with individual blocks up to a kilometre in diameter, although 
considerable variation exists between different avalanches.  At one extreme, 
El Golfo avalanche on El Hierro has few large blocks scattered randomly 
across the avalanche surface.  At the other, Icod on Tenerife has much more 
numerous but smaller blocks over most of its surface, with a few very large 
blocks at the deposit margins.  Icod also exhibits flow structures 
(longitudinal shears, pressure ridges) which are absent in El Golfo.  The 
primary controls on block structure and distribution are inferred to relate 
to the nature of the landslide material and to flow processes.