Volcano flank instability in the Lesser Antilles Arc: Diversity of scale, processes, and temporal recurrence

The 1997 Boxing Day collapse, a remarkable feature of the ongoing eruption of Soufrière Hills on Montserrat, has prompted new interest in the study of volcano stability in the Lesser Antilles. Building on a few cases documented in the literature, we have now identified at least 47 flank collapse events on volcanoes of the Caribbean arc where this type of behavior is characteristic and repetitive. About 15 events occurred on active volcanoes within the last 12,000 years. In the northern part of the arc, flank collapses are repetitive, do not exceed 1 km 3 in volume, occur in all directions, and are promoted by intense hydrothermal alteration and well‐developed fracturing of the summit part of the edifices. In contrast, infrequent but large sector collapses, with volumes up to tens of km 3 , are typical of the southern volcanoes. They are always directed to the west as a result of the high overall slopes of the islands toward the deep back‐arc Grenada Basin. Because Caribbean islands are small, a large part of the resulting debris avalanches have flowed into the sea thus contributing voluminous and sudden inputs of volcaniclastic sediments to the Grenada Basin. Deposits from such submarine flows have been identified during the recent AGUADOMAR and CARAVAL oceanographic cruises and traced to their source structures on land. Edifice collapses have a major influence on subsequent volcanic activity but also are of high concern because of their tsunamigenic potential.