Phreatomagmatic eruptive and depositional processes during the 1949 eruption on La Palma (Canary Islands)

In 1949, a 5-week-long magmatic and phreatomagmatic eruption took place alo ng the active volcanic ridge of La Palma (Canary Islands). Two vents, Duraz nero and Hoyo Negro, produced significant pyroclastic deposits. The eruptio n began from Duraznero vent, which produced a series of deposits with an up ward decrease in accidental fragments and increase in fluidal ash and spatt er, together inferred to indicate decreasing phreatomagmatic interaction. H oyo Negro erupted over a 2-week period, producing a variety of pyroclastic density currents and ballistic blocks and bombs. Hoyo Negro erupted within and modified an older crater having high walls on the northern to southeast ern edges. Southwestern to western margins of the crater lay 50 to 100 m lo wer. Strongly contrasting deposits in the different sectors (N-SE vs. SW-W) were formed as a result of interaction between topography, weak eruptive c olumns and stratified pyroclastic density currents. Tephra ring deposits ar e thicker and coarser-grained than upper rim deposits formed along the high er edges of the crater, and beyond the crater margin, valley-confined depos its are thicker than more thinly bedded mantling deposits on higher topogra phy. These differences indicate that the impact zone for the bulk of the co llapsing, tephra-laden column lay within the crater and that the high crate r walls inhibited escape of pyroclastic density currents to the north and e ast. The impact zone lay outside the low SW-W rims, however, thus allowing stratified pyroclastic density currents to move freely away from the crater in those directions, depositing thin sections (< 30 cm) of well-bedded ash (mantling deposits) on ridges and thicker sections (1-3 m) of structureles s ash beds in valleys and small basins. Such segregation of dense pyroclast ic currents from more dilute ones at the crater wall is likely to be common for small eruptions from pre-existing craters and is an important factor t o be taken into account in volcanic hazards assessments. (C) 1999 Elsevier Science B.V, All rights reserved.