THE GEOLOGY OF DOMINICA
Dominica lies at the center of the Lesser Antilles Island arc, where the islands of the Active Arc (see Lesser Antilles Arcs map on this website) are large and complex comprising many coalesced stratovolcanoes. The island has an area of 750 sq.km and a population of 74,000 including 3,000 Carib Indians. Whereas all the other volcanic islands of the Lesser Antilles have only one active volcano, Dominica has nine and yet there has been no major magmatic eruption since Columbus visited the island and as a result it has today the best and most extensively preserved tropical rainforests. The youngest dated volcanic deposits on the island are associated with the Morne Patates dome on the flanks of the large active Plat Pays Volcano that comprises the southwestern end of the island. This was a Pelean eruption (similar to the eruptions of Mt. Pelee on Martinique in 1902 and 1929) and radiocarbon ages from the block and ash deposits suggest it occurred about 500 year ago. In addition there have been two steam explosions (phreatic activity) in the Valley of Desolation in 1880 and 1997. Frequent seismic swarms and vigorous and widespread geothermal activity today characterize the island. In fact it is the most worrying of all the Caribbean volcanic areas and there is a general feeling that it (like Montserrat pre-1995) is long overdue for an eruption.
What is of particular concern is that the capital Roseau and most of the islands infrastructure lie on a pyroclastic flow fan derived from the Wotten Waven caldera situated on the eastern outskirts of the capital. The pyroclastic deposits of the Roseau area abound with ignimbrites (pumiceous pyroclastic flows), surge and airfall deposits with radiocaron ages ranging from 38,000 to 1000 years B.P. Volcanologist Haraldur Sigurdsson as long ago as 1972 described one of the units which he called the Roseau ash and together with other workers traced its submarine extension. As a result of this work they concluded that about 38,000 years ago the island erupted around 56 cubic kilometers of pumiceous materials in what was described as the largest eruption in past 200,000 years in the Caribbean . Pyroclastic flows deposited about 30 cubic km on the Caribbean floor and the remainder was deposited on the Atlantic floor from AntiTrade Wind dispersal. More recent work suggests that there are several ignimbrite sheets separated by ancient soils and the deposits may have resulted from several eruptions. However all conclusions indicate that the capital Roseau is situated in one of the most hazardous areas of the island.
The structure of the island is interesting as north of Dominica the Lesser Antilles arc divides into two, with the active arc lying to the west in Basse Terre of Guadeloupe and the extinct Limestone Caribbee arc lying to the east in Marie Galante and Grande Terre of Guadeloupe (see Lesser Antilles Arcs map on this website). On Dominica , Miocene rocks (7-5.3 million years) of the Extinct Limestone Caribbees occupy the east or windward coast where deeply dissected volcanoes form rugged mountains. The main bulk of the island was built up by coalesced volcanoes during the Pliocene (4.0-2.0 million years). During the Pleistocene (
Within the Central Graben two calderas have formed and occupy most of the floor of the graben. The Trois Piton caldera is to the north and the Wotten Waven caldera to the south. Following the eruption of pumiceous material from these calderas, the Trois Pitons and Micotrin centers formed. Both of these are Pelean dome complexes with surrounding aprons of block and ash flow deposits (from nuées ardente type pyroclastic flows named by Alfred LaCroix when describing the 1902 eruption of Mt. Pelée ).
Arcuate cliffs truncate the southwestern flanks of the Plat Pays stratovolcano and form the eastern margin of a depression now occupied by the Morne Patates and Crabier Pelean dome complexes and their pyroclastic aprons. The villages of Soufriere and Scotts Head also lie within this structure. Although there has been much debate as to whether the structure is a caldera or a sector collapse scar, the problem was resolved by Anne Le Fraint and others in 2002 when detailed bathymetric studies revealed that not only does the collapse scar continue down to depths greater than 2000m, but three collapses have resulted in the most extensive debris avalanche fan found associated with the Lesser Antilles arc with an area of 3,500 sq.km. On the proximal part of the fan numerous megablocks up to 2.8km long and 240m high exist and it seems that Scotts Head is such a block.
For a detailed description of the seismic and geothermal activity and a discussion of the types of volcanic hazard presented by the different active volcanoes on Dominica the reader is referred to the soon to be published VOLCANIC HAZARD ATLAS OF THE LESSER ANTILLES by the SEISMIC RESEARCH UNIT of Trinidad.