VOLCANIC HAZARD ASSESSMENT ON PYROCLASTIC VOLCANOES WITH LONG REPOSE PERIODS

For volcanoes with short historic records and relatively infrequent eruptions, the only tool for understanding their volcanic histories and thus assessing their potential hazards is the interpretation of their stratigraphies. The volcanoes of the Lesser Antilles are excellent examples of such centers as historic records only began with European settlement in the mid-17th century, and only 4 of the approximately 20 potentially active volcanoes have had magmatic eruptions in the last 350 years. Also on three-quarters of the islands, many population centers are located within Integrated Hazard Zones designated Very High to High placing large percentages of the islands' populations at risk. Stratigraphic studies including the development of new geological maps, e.g. for Saba, St. Eustatius and Dominica, have shown that pyroclastic flows produced by various eruptive styles, ranging from small volume gravitational dome collapses to large volume Plinian eruptions producing thick, valley-fill ignimbrites, dominate both the historic and pre-historic record, with effusive volcanism being mainly restricted to the generation of felsic domes and occasional mafic/intermediate lava flows. An evaluation of the different styles of volcanic activity permits centers with similar volcanic histories, e.g. the Quill, St. Eustatius and Mt. Pelée, Martinique, to be grouped so that historic activity at one center can be used to predict possible future activity (precursor and eruptive) at analogous centers that have not erupted in historic times. Of particular importance for hazard assessment is the island of Dominica, which is unique in the Lesser Antilles in having 8 potentially active volcanoes in an area of 750 km2, most of which are within 10 km of the capital of Roseau. In addition, Dominica was the site of the largest eruptions (>58 km3 of felsic pyroclastic flow and fall) in the last 200,000 years in the entire Lesser Antilles. The island's capital of Roseau, with a population of approximately 50,000, is located on the ignimbrite and younger block and ash flow fan produced by these Plinian- and subsequent Pelean-style eruptions.