Dominica is built on a broad submarine ridge of (probably) Early Tertiary age (that is about 60 million years ago) or perhaps even older deposits. It is one of the Volcanic Caribees, the more western line of a double island-arc, the other line of which is mainly built of limestone. Dominica is almost entirely composed of volcanic rocks.
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 a 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.
The Historic Past and the Present
Activity during the historic period has been mainly of the form of phreatic and phreato-magmatic explosions, (phreato meaning that water draining from the surface was involved), the last having been located in the Valley of Desolation in 1880, often leaving maare (lakes like that at Woodford Hill) or craters (Du Mas Estate, top end of Morne Prosper e.g)
Present activity:
- soufrieres (Soufriere, Roseau Valley, Morne aux Diables, Valley of Desolation)
- hot springs (Check Hall, Layou,Titou, etc)
- geysirs (Wotten Waven)
- Boiling Lake
- seismic activity (mainly in the south, especially La Plaine area
Rocks
Main rock types:
- basaltic lavas (Antrim, eg)
- andesitic tuffs, (some welded) and ashes, (some dacitic) and agglomerates (widespread, but dacitic materials which include little quartz crystals of diamond shape are confined to the NE, eg Calibishie)
- dacitic andesitic lavas (Micotrin, Trois Pitons) [also microdiorite, (Imray's)]
- reef limestones and conglomerates (west coast)
Economic geology:
- geothermal power (potential high, twice that of St Lucia) untapped
- aggregate, including lightweight (pumice) some exported
- minor accumulations of minerals (low grade sources)
- exploration has found copper
- minor diatomaceous earth, impure clays, low grade thin layer alumina and limestone
"sponge" which holds and guides water to our very productive rivers