Aspectos volcanológicos y estructurales. Evolución petrológica e implicaciones en riesgo volcánico de la erupción de 1730 en Lanzarote. Islas Canarias

Abstract

The eruption that took place in Lanzarote between 1 September 1730 and 16 April 1736 differs from the normal pattern of the historic (last 500 years) volcanism of the Canary Islands. The duration (2.053 days), extension (200 km²), volume of materials emitted (3-5 km³) and the evolution of magmas towards olivine tholeiite compositions are quite unique in the historic trend of volcanism in the Archipelago. However, no specific study has been undertaken until now to attempt its reconstruction. In this work the detailed geologic mapping (on a scale 1: 10.000) of the area was carried out, in addition to a thorough petrologic study and the correlation of data provided by eye witness accounts, one of which is a hitberto unpublished manuscript. The main phases of activity have been differentiated as a function of important changes in the composition of the magmas and of the relationship of tbe eruption with a large (at least 14 km) fracture. The petrologic and geochemical study of 51 stratigraphically and temporally well located samples shows significant changes in the magma generation processes and in the internal feeding system of the eruption. Magmas evolve from nepheline basanites to alkali basalts (7 to 12,5 % melting) during the initial phase of the eruption and from the second phase onwards from alkali basalts to olivine tholeiites (13 to 20 % melting), at shallower depths. Trace elements analysis indicates the predominant role of partial melting processes. The «anomalous» nature of this eruption in relation to the historic volcanism of the Canaries might be explained by the continuation of the eruption once the initial phase was completed (some 3-5 months), when the magma generation front was raised and placed at ao intermediate depth along a large fracture.