Volcanic & Hydrothermal Fluids
Most of the volcanic islands in the Lesser Antilles manifest some form of surface volcanic activity such as hot springs and fumaroles (i.e.hot gases). Those manifestations indicate that, although some of the volcanoes are not erupting, they are still active and that magma is present at depth. The presence of magma underground has two main implications:
- Magma is hot (hundreds of degrees Celsius, °C), so when rain water penetrates the ground and circulates at depth, it is warmed up. This is called a hydrothermal system (“hydro-“ for water and “-thermal” for heat). As it gets warmer, its density decreases and it tends to rise up through fractures and faults – which are common on volcanoes – towards the surface. At the surface, they can produce hot springs.
- Volcanic gases are relatively continuously released from the magma. Those gases are hot (originally at magma temperature) and very acidic and, as they are extremely light, they tend to rise up to the surface through fractures and faults. A fraction of them will reach the surface with little interaction with the surrounding rocks, but a larger fraction (in many islands) will interact with local hydrothermal systems and/or fresh ground water bodies. Manifestations at the surface of those gases are fumaroles and hot springs.
Scientist collecting gas samples at the Valley of Desolation, Dominica.
What happens at volcanoes in tropical regions – hence marked rainy seasons (e.g., Lesser Antilles) – is usually a mixture of both phenomena. Volcanic gases are typically made up of more than 80% of water vapour (H20) with some acidic gases such as Hydrogen Sulphide/Sulphur Dioxide, Carbon Dioxide, Hydrogen Chloride, Hydrogen Flouride (H2S/SO2, CO2, HCl, HF) and neutral gases (e.g., Methane, Carbon Monoxide – CH4, CO – Radon). Their temperature ranges from atmospheric temperature to magmatic temperature (i.e, ~25-1000ºC). In the Lesser Antilles, with the exception of Montserrat, gas temperature at the surface usually ranges between 30ºC and 200ºC. Hot springs have various pH (from acidic to basic), composition and temperature.
Boiling Lake, Dominica
Most extreme temperatures for hot-springs can be found in the Valley of Desolation in Dominica, especially at the Boiling Lake, where water temperature is generally around 90ºC. The SRC initiated routine geothermal monitoring (measurement of temperature, pH, and chemical composition) of hot springs and fumaroles associated with Lesser Antilles volcanoes in 2001. Routine sampling in islands including Dominica, Saint Lucia, Grenada, St. Vincent, St. Kitts, and Nevis have taken place on an almost annual basis. We monitor hot springs and gas chemical composition through time as changes in physico-chemical properties through time can indicate a change in the volcanic activity. Samples are collected and analysed at The University of the West Indies St. Augustine Campus (Trinidad). In addition to the volcano monitoring aspect, we also conduct checks for environmental gas/water concentration in recreational areas to assess potential hazard to visitors (e.g. Saint Lucia).
Tech note
Gases are sampled with Giggenbach bottles (i.e., quartz bottles partly filled with Sodium Hydroxide – NaOH, the remaining volume being vacuum). During the sampling, the gas bubbles through the NaOH solution. Acidic gases (e.g., HCl, S(g), CO2 dissolve in the basic solution whereas neutral gases go in the vacuum part. Two types of analysis are therefore needed: analysis for the liquid phase (Wet Chemistry, Ion Chromatography, Atomic Absorption) as well as for the gas phase (Gas Chromatography). Hot springs are sampled using Nalgene® bottles. Onsite measurements include temperature, pH and conductivity. Samples are analysed using mainly Ion Chromatography and Atomic Absorption.
