Chronology and volcanology of the 1949 multi-vent rift-zone eruption on LaPalma (Canary Islands)

The compositionally zoned San Juan eruption on La Palma emanated from three eruptive centers located along a north-south-trending rift zone in the sou th of the island. Seismic precursors began weakly in 1936 and became strong in March 1949, with their foci progressing from the north of the rift zone towards its south. This suggests that magma ascended beneath the old Tabur iente shield volcano and moved southward along the rift. The eruption began on June 24, 1949, with phreatomagmatic activity at Duraznero crater on the ridgetop (ca, 1880 m above sea level), where five vents erupted tephritic lava along a 400-m-long fissure, On June 8, the Duraznero vents shut down a bruptly, and the activity shifted to an off-rift fissure at Llano del Banco , located at ca, 550 m lower elevation and 3 km to the northwest. This erup tive center issued initially tephritic aa and later basanitic pahoehoe lava at high rates, producing a lava flow that entered the sea. Two days after basanite began to erupt at Llano del Banco, Hoyo Negro crater (ca. 1880 m a si), located 700 m north of Duraznero along the rift, opened on July 12 and produced ash and bombs of basanitic to phonotephritic composition in viole nt phreatomagmatic explosions (White and Schmincke, 1999). Llano del Banco and Hoyo Negro were simultaneously active for 11 days and showed a co-varia nce of their eruption rates indicating a shallow hydraulic connection. On J uly 30, after 3 days of quiescence at all vents, Duraznero and Hoyo Negro b ecame active again during a final eruptive phase. Duraznero issued basaniti c lava at high rates for 12 h and produced a lava flow that descended towar ds the east coast. The lava contains ca, 1 vol.% crustal and mantle xenolit hs consisting of 40% tholeiitic gabbros from the oceanic crust, 35% alkalin e gabbros, and 20% ultramafic cumulates, The occurrence of xenoliths almost exclusively in the final lava is consistent with their origin by wall-rock collapse at depth near the end of the eruption. The volcanic evolution of the 1949 eruption is typical of La Palma eruptions generally. Considerable shallow magma migration prior to and during eruption is manifested by stron g seismicity, intense faulting, and the almost unpredictable opening of spe cific vents which can be spaced three or more km apart. (C) 1999 Elsevier S cience B.V. All rights reserved.