High velocity debris avalanche at Lastarria volcano in the north Chilean Andes

A pre-historic collapse of the southeast flank of Lastarria volcano ( 5700 m) in the north Chilean Andes (25° 10 S), produced a fluidized volcanic debris avalanche whose morphology and surface structures are exceptionally well preserved. The avalanche travelled to the east-south-east, covering an area of 9.3 km², and came to rest after climbing and over-riding a 125 m high older scoria cone. The 0.091 km³ avalanche has an apparent coefficient of friction (H/L) of 0.15 and an excessive travel distance index (Le) of 5.1 km, indicating high emplacement velocity, perhaps of the order of 80 m s^–1. An important cause of the high mobility may have been the predominance of low-density, poorly cohesive scoriaceous and pumiceous layers in the source region. The flow may have had properties similar to those of a small ignimbrite.