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Skip Navigation LinksHome : Volcanoes : Monitoring : Volcanic / Hydrothermal Fluids
Monitoring - Volcanic / Hydrothermal Fluids

Most of the volcanic islands in the Lesser Antilles present some surface manifestation of 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 (i.e., dormant) and that magma is present at depth. The presence of magma underground has two main implications:

1. Magma is hot (hundreds of degrees Celsius) so when rain water penetrates the ground and circulates in depth, the latter will be 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.
2. Volcanic gases exit the magma relatively continuously. 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. Part of them will reach the surface without much interacting with the surrounding rocks but part of them (actually most of them 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.

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 off more than 80% of water vapour (H20) with some acidic gases such as H2S/SO2, CO2, HCl, HF and neutral gases (e.g., 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 30 and 200ºC. Hot springs have various pH (from acidic to basic), composition and temperature. 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 most of the time around 90ºC.

SRU monitors 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 at least twice annually and analysed at the University of the West Indies in Trinidad. In addition to the volcano monitoring aspect, SRU also conducts checks for environmental gas/water concentration in recreational areas to assess potential hazard to visitors (e.g. St Lucia).

Tech note:
Gases are sampled with Giggenbach bottles (i.e., quartz bottles partly filled with 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.