Pyroclastic Block Flow from the September, 1976, Eruption of La Soufri~re Volcano, Guadeloupe M.F. SHERIDAN Department o[ Geology, Arizona State University, Tempe, Arizona 85281, USA ABSTRACT ERUPTION A phreatic eruption occurred at 19:22 H from a new set of fissures on the southeast side of the summit dome. The steam jet from the nar- row fissure incorporated accessory fragments of the e~sting dome that ranged in size from 3 m blocks down to particles of a few microns in diameter. No juvenile material was erupted. The branching 550 m pyroclastic block flow is surrounded by a zone of total destruction ex- tending out to 900 m from the vent. An outer zone of directed blast projects to at least 1500 nm Profiles of the destructive cloud and the py- roclastic flow allow energy decay curves to be constructed for this eruption. From these curves the potential surface for flows associated with this type of phreatic eruption can be con- structed as an initial step in developing a vol- canic hazard map. INTRODUCTION In the fall and winter of 1976-77 La Soufri~re of Guadeloupe was in a state of activity that produced several phreatic eruptions from fissures on the summit dome. During the most active period more than 70,000 inhabitants were evacuated from the southwestern part of the island in anticipation of a violent Pel~ean erup- tiorL Although a devastating eruption did not occur, a number of violent phreatic blasts characterized the current eruptive patterns of La Soufri~re. This contribution deals with the largest pyroclastic block flow produced during the active phase, and its implications with regard to devel- oping a volcanic hazards map. Bull. Volcanol., Vol. 43-2, 1980 A phreatic eruption occurred at 19:22 H local time from a new set of fissures on the southeast flank of the summit dome (Fig. 1, 2). The eruption lasted 10 minutes with 9 minutes of harmonic tremor record- ed on the seismograph. The eruptive blast was heard at Fort St. Charles in Bas- se Terre, 8 km to the southeast, but cloud cover obscurred the eruption plume char- acteristics. An air-fall deposit spread 12 km eastward to the sea at Vieux Habitants LA SOUFRIERE 14 "I~ 76 t s Comou~ rate,vat 50m FIG. 1 - Map of the pyroclastic flow deposits: 1) pyroclastic block flow, 2) zone of total de- struction, 3) zone of directed blast, M) mud- flow. Dark areas on dome are active fissures.