Heat and SO2 Emission Rates at Active Volcanoes – The Case Study of Masaya, Nicaragua

Abstract

The necessity of understanding volcanic phenomena, so as to assist hazard assessment and risk management, has led to development of a number of techniques for the tracking of volcanic events so as to support forecasting efforts. Since 1980s scientific community has progressively drifted research and surveillance at active volcanoes by integrated approach. Nowadays, volcano observatories over the world record and integrate real or near-real time data for monitoring and understanding volcano behaviour. Among the geophysical, geochemical, and volcanological parameters, the tracking of temperature changes at several volcanic features (e.g., open-vent systems, eruptive vents, fumaroles) and variations in sulphur dioxide flux and concentration at volcanic plumes are key factors for studying and monitoring active volcanoes.

Temperature is one of the first parameters that have been considered in understanding the nature of volcanoes and their eruptions. Thermal anomalies have proved to be precursors of a number of eruptive events, and once an eruption begins, temperature plays a major role in lava flow emplacement and lava field development. At active volcanoes, temperature has been measured by direct and indirect methodologies. Direct measurements represent the traditional thermal monitoring carried out at fumaroles, hot springs, molten lava bodies, and crater lakes, using thermocouples. Indirect measurements, also known as thermal remote sensing, can be performed by satellite, ground, and airborne surveys. Owing to the danger of most kinds of eruption, and the need of monitoring inaccessible areas on volcanoes, indirect measurements are especially attractive. Among them, thermal imagery is the most widespread and results from the capability to detect the infrared radiation emitted from the surface of hot bodies, and to provide the radiometric map of heat distribution of the body’s surface. This has been of primary importance for capturing the evolution of thermal anomalies, which shed light on magma movements at shallow depths. While magma is rising, hot gases separate from the melt and escape either directly from the main conduits, or indirectly by leaking through fumaroles, fractures, and faults, or by dissolving within crater lakes and hot spring waters, resulting in variations in their temperature and chemical composition. At the surface, these phenomena are also associated with radiative heat fluxes, which can be detected by infrared thermal detectors. The application of thermal imaging to volcanology was largely performed using satellite surveys, but in the last decade there has been increasing application of compact (hand-held or tripod-mounted) thermal imagers used from the air or ground.

Volcanic degassing plays a key role in magma transport and style, and timing of volcanic eruptions observed at the Earth’s surface. The assessment of volcanic gas composition and flux has become a standard procedure for volcanic monitoring and forecasting since degassing regimes are fundamentally linked to volcanic processes. Magma contains dissolved gases that are released into the atmosphere during both quiescent and eruptive stages. At high pressures, deep beneath the Earth’s surface, gases are dissolved in magma; however as soon as magma rises toward the surface, where pressures are lower, gases start to exsolve according to the solubility-pressure relationship of each species, as well as compositional and diffusional constraints. The abundance and final gas phase composition of the emitted plume depends on magma composition(s), volatile fugacities, crystallisation and on the dynamics of magma degassing, including kinetic effects. However, at the surface, the composition and flux of volcanic gases may change with time, reflecting variations in the magmatic feeding system of the volcano. Hence by studying and tracking this variability a number of parameters, such as magma residing depths and the amount of degassing magma bodies can be determined.

Among the volcanic gas species, sulphur dioxide (SO2) is one of the most-well investigated in remote sensing. As for temperature, SO2 concentration and emission rates can be measured using both direct sampling and non-contact, remote sensing measurements. The latter carried out during air- and ground-based surveys and satellite platforms, are based on optical spectroscopy. Since the 1970s, SO2 flux has been remotely measured using the COrrelation SPECtrometer (COSPEC) at several volcanoes worldwide. Over the last 10 years the advent of small, commercial and low cost spectrometers offered a valuable replacement to the outdated COSPEC. In particular, the combination of UV spectrometers with the Differential Optical Absorption Spectroscopy (DOAS) analytical method improved significantly data collection offering a number of advantages such as the possibility of obtaining measurements in the challenging environments typical of volcanic areas and detection of other plume species.

Our intent here is to discuss findings and implications arising from the integration of thermal imaging-derived temperature and SO2 emission rates. Calibrated temperatures from thermal imagery can provide qualitative as well as quantitative information, fundamental insights and parameters contributing to understanding and modelling of several eruptive features. Anomalies in SO2 emission rates have been often documented at several volcanoes prior to eruptive crisis. In syn-eruptive stages, anomalies in the SO2 flux pattern might indicate variations in the eruptive style and regime associated with changes in the volcano shallow feeder system. At open-vent systems, in non-eruptive phases, changes in SO2 flux emission have provided information on increases or decreases of magma supply in the shallow plumbing system suggesting likely volcanic unrests or magma migration towards peripheral areas of the volcano edifice, respectively. There is still much to explore about volcano behaviour and eruptive mechanisms, however, the combination of different types of monitoring techniques is crucial for constraining baselines for predicting phases of volcano unrests and for gaining useful insights for volcano hazard assessment.