A fumarole is a vent in the Earth’s surface that emits steam and volcanic gases without accompanying liquid or solid material. These vents provide a direct pathway from the volcano’s plumbing system to the atmosphere. Fumaroles connect directly to the volcano’s internal heat and magma system, making them a primary tool geologists use for monitoring volcanic activity. Analyzing the gases and heat escaping from these vents provides a real-time window into the subterranean processes driving a volcano.
The Chemical Composition of Fumarole Emissions
The mixture of gases released from a fumarole reflects conditions deep within the volcanic structure. The majority of the emission, typically 70 to 90 percent by weight, is water vapor (\(\text{H}_2\text{O}\)), which often originates from meteoric groundwater heated by the magma body. The remaining fraction consists of gases released directly from the magma, primarily carbon dioxide (\(\text{CO}_2\)), sulfur dioxide (\(\text{SO}_2\)), and hydrogen sulfide (\(\text{H}_2\text{S}\)). Trace amounts of hydrogen chloride (\(\text{HCl}\)) and hydrogen fluoride (\(\text{HF}\)) may also be present.
Gases like \(\text{CO}_2\) and \(\text{SO}_2\) are considered true magmatic volatiles, meaning they are dissolved in the molten rock at depth. Hydrogen sulfide (\(\text{H}_2\text{S}\)), which has a characteristic rotten-egg smell, is often associated with the interaction of magmatic gases with groundwater in a shallow, cool hydrothermal system. The relative concentrations of these gases are continuously tracked because they reflect changes in the magma’s depth and temperature.
Monitoring Techniques and Data Collection
Geologists use a combination of direct and remote sensing techniques to gather data from fumaroles. Direct sampling involves physically collecting the gases at the vent, a procedure that requires specialized, heat-resistant equipment. Scientists insert an inert tube, often made of titanium, directly into the high-temperature vent to draw off gas. The gas is then collected in pre-evacuated glass bottles, which often contain an alkaline solution like sodium hydroxide to capture and concentrate acidic gases such as \(\text{SO}_2\) and \(\text{HCl}\). This method provides the most detailed chemical analysis of the gas composition, but the analysis must be completed later in a laboratory.
For real-time and remote measurements, scientists use specialized instruments to measure gas flux and temperature. Spectrometers, such as the Differential Optical Absorption Spectrometer (DOAS) or the Correlation Spectrometer (COSPEC), are used to measure the total emission rate of \(\text{SO}_2\) in the plume from a distance. Ground-based MultiGAS instruments are deployed to continuously measure the relative concentrations of multiple gases, including \(\text{CO}_2\), \(\text{SO}_2\), and \(\text{H}_2\text{S}\), right at the vent. Temperature measurements are collected using thermocouples, which are simple probes inserted into the vent to record the temperature of the escaping steam and gas.
Interpreting Changes as Volcanic Warning Signs
The data collected from fumaroles are interpreted by volcanologists to detect subterranean changes that may precede an eruption.
Temperature Increase
A sustained, anomalous increase in fumarole temperature is a clear warning sign. This indicates a significant rise in heat flow, caused by a body of magma moving closer to the surface or by an increased supply of hot magmatic fluids to the system.
Gas Ratio Changes
Changes in the relative abundance of magmatic gases are an important indicator of unrest. A precursory signal involves a relative increase in \(\text{CO}_2\) compared to \(\text{SO}_2\), monitored through the \(\text{CO}_2/\text{SO}_2\) ratio. Since carbon dioxide has a lower solubility in magma, it escapes first at greater depths. An increase in this ratio signals the arrival of a new batch of deep, \(\text{CO}_2\)-rich magma into the system.
Shift to Magmatic Gases
A shift from gases associated with a cool hydrothermal system to purely magmatic gases is a definitive alert. This is observed as a rapid increase in the total flux of \(\text{SO}_2}\) and a simultaneous drop in the \(\text{H}_2\text{S}/\text{SO}_2\) ratio. High rates of \(\text{SO}_2\) emission suggest that rising magma has created dry, open pathways, allowing sulfur-rich magmatic gas to escape directly without reacting with groundwater. This change indicates that magma is now at a shallow level, increasing the likelihood of an eruption.