THE GEOLOGY OF DOMINICA, LESSER ANTILLES

The island of Dominica located in the central Lesser Antilles has one of the highest concentrations of potentially active volcanoes in the world. As part of an collaborative effort to assess the volcanic hazards of the island the first two authors have produced a geological map of the island (the other authors are undergraduate students who participated in field work on Dominica as part of CSUSB’s OEDG grant). Mapping combined with 95 radiometric dates has allowed the geology of the island to be divided into 4 units: Miocene; Pliocene; ‘Older Pleistocene’; ‘Younger Pleistocene’-Recent. The Miocene rocks, which are deeply dissected and weathered are only exposed along the east coast. Overlying a major unconformity are a number of eroded stratovolcanoes of Pliocene age often composed of pillow lavas and submarine volcanic breccias, overlain by thick subaerial lava flows interbedded with pyroclastic deposits. In the northern part of the island a thick saprolite-laterite horizon has been developed on top of these Pliocene rocks. Centers belong the ‘Older Pleistocene’ are all confined to the N of the island and are characterized by the presence of Pelean domes and associated aprons of block and ash flow deposits. The two most important centers are Morne aux Diables and Morne Diablotins. Around 1 Ma activity switched from the N to the S, where six major volcanoes, (Morne Trois Pitons, Wotten Waven/Micotrin, Watt Mountain, Grand Soufriere Hills, Morne Anglais, and Morne Plat Pays) developed. Activity also continued in a reduced manner at Morne aux Diable and Morne Diablotins. Within the last 100,000 years, 3 major periods of Plinian activity produced extensive subaerial and submarine pumiceous deposits. These eruptions, each of which is estimated to have produced tens of km3 of pyroclastic material are associated with Morne Diablotins, and the calderas of Morne Trois Pitons and Wotten Waven/Micotrin. Morne Plat Pays in the SW corner of the island has also been subjected to at least 3 sector collapses during the past 300,000 years. Historical activity has included 2 phreatic eruptions (1880, 1997) from the Valley of Desolation, a major geothermal area, and 17 volcano-seismic crises.