by Taihisa Hill
The zone of westward subduction of the Atlantic plate beneath the Caribbean plate is marked by an 850 km arc of volcanic islands. This island arc stretches from Sombrero in the north to Grenada in the south and is separated in two to the north of Dominica. The older/outer arc, referred to as the ‘Limestone Caribbees’, dates back to early Eocene. This outer arc was once volcanic but moved away from the active area. The basement volcanic rocks are capped with non-marine and marine sedimentary rocks ranging in age from Oligocene to younger (Weiss, 1994). Inward from the Limestone Caribbee, is the ‘Volcanic Caribbee’, comprising of twenty-one live volcanoes, occupied by eleven volcanically active islands (Lindsay, et al., 2005).
Possessing one of the highest concentration of potentially active volcanoes in the world, Dominica is located in the middle of the Lesser Antilles. Its rugged 750 km² terrains is a result of the 9 active volcanoes, the highest being well over 1,000m (see Map 1)
Map 1 Showing Dominica and its volcanic peaks (Halamah, R. et al., 2006)
The magmas erupted north of Dominica to Saba and south to Saint Lucia have been dominantly dacite and andesite. The volcanoes in these areas are mainly stratovolcanoes, many with coalescing lava domes. South of Saint Lucia however, basalts and basaltic andesites are the dominant lava type. Based on current knowledge of the volcanoes and their present state of activity, six scenarios bases on future eruptions were drawn up by Lindsay et al. (2005).
Dominica houses three major lakes: Boiling Lake, Boeri Lake and Freshwater Lake, all of which are believed to be driven by a different mechanism. Based on previous studies (Werner et al., 1996) done on the water chemistry of these lakes, it was concluded that Freshwater Lake showed signs of being primarily rock dominance while the Boiling Lake was primarily controlled by evaporation and Boeri lake is precipitation dominance. Werner et al. (1996) made these conclusions based on “Gibbs’ (1970) scheme for classifying the mechanisms that control water chemistry of natural bodies of water”. In his scheme, Gibbs recognized three broad categories: 1) precipitation dominance, 2) rock dominance, and evaporation-fractional crystallization processes. Each of these categories had limitations based on the salinity of the lakes and major ions present. Precipitation dominated lakes were said to be of low salinity with Na and Cl as the dominant ions. Rock dominated lakes are moderately high salinity with Ca and HCO3 as the dominant ions. Evaporation dominated lakes were said to be of very high salinity and with sodium rich waters.
The only visible inlet into the Boiling Lake is in the form of a very small waterfall, this waterfall alone cannot aid in keeping the level of the lake constant with the rate of output. Therefore more water must be seeping into the lake. Delmelle et al. (2000) designed a cross-section of the hydrothermal system at Kawah Ijen. This model (Figure 1) supports the conclusions made from the Boiling Lake in terms of water source and mineral source. Labeling was edited to suit the purpose and location of this write up.
Figure 1. Showing theoretical cross-section of hydrothermal system (Delmelle et al 2000).
Figure 1 shows local meteoric water coming into contact with a magma source and is heated. The heated water interacts with rocks and dissolves minerals, and carries them in solution. The water continues to rise and seeps into the Lake providing it with water and a basin of precipitated elements. That process along with rainfall contributes to the water input of the lake. The diagram also shows possible fumaroles and lakes that may show up in the area, all of which are seen in the general area of the Boiling Lake.
The Freshwater Lake and Boeri Lake formed as a result of the filling of a depression that formed between the Morne Micotrin lava domes and the Wotten Waven Caldera crater rim, (Werner et al., 1996), with water. The water is not warm or boiling due to the absence of a direct magma source, however hot springs associated with Micotrin flows into the Fresh water Lake. The Boeri Lake, highest on the island, is a rather deep lake with a small surface area. Because of this characteristic Boeri Lake is of interest to many researchers as it fits the criteria of being a meromictic lake, of which only 200 have been discovered (Werner et al., 1996). However, Werner et al (1996) discovered that “Evidently, the morphometric factors that commonly determine meromixis are not at work in Boeri Lake”. Today, pipes leading from the Boeri Lake are transporting water into the Freshwater Lake as part of the island’s hydro development project. Other activities, such as kayaking and boating are done on the lake a few times a week year round and started in 2005.
These three lakes are major attractions in Dominica. Boiling Lake for being one of the largest of its kind in the world, Freshwater is the largest on the Island and Boeri Lake, the highest and deepest lake with the smallest surface area.