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Gas and water chemistry directly relates to the amount and location of magma inside a volcano

Fumaroles steaming within Sherman Crater, Mount Baker, Washington....
Fumaroles steaming within Sherman Crater, Mount Baker, Washington.

Volcanic gases escape through fumaroles, porous ground surfaces, and active vents during different phases of a volcano's lifespan: as magma rises toward the surface, when it erupts, and even as it cools and crystallizes below ground. When rising gases encounter groundwater, the water acts as a filter and "scrubs" the gas of some chemicals, thereby changing the chemistry of the water.

Scientists can learn a lot about changes to the magma system within a volcano by 1) measuring changes in the emission rate of certain key gases, especially sulfur dioxide and carbon dioxide, and 2) collecting and analyzing water samples to look for chemicals, such as hydrogen chloride and hydrogen fluoride (both easily dissolve in water) that indicate volcanic gas has been filtered by the water.

Gas sampling with a CO2 Infrared Spectrometer at Mount St. Helens' ...
Gas sampling with a CO2 Infrared Spectrometer at Mount St. Helens' September Lobe lava dome, Washington.

Carbon dioxide may indicate new magma from deep below

Carbon dioxide (CO2) gas separates from magma deeper than other volcanic gases. If increased CO2 levels are detected at the surface, that may indicate new magma is entering the volcanic system. By regularly monitoring CO2 at volcanoes, scientists can easily detect those increases (as well as decreases) which leads to a greater understanding of what is happening inside the volcano. CO2 can also be hazardous – it can collect in soils and can cause trees and other vegetation to die, and if it does not dissipate quickly when it leaves the ground, it can collect in low-lying areas to fatal concentrations.

Camera captures SO2 released from Kīlauea's summit vent. HVO scient...
Camera captures SO2 released from Kīlauea's summit vent. HVO scientist tests the network link between the instrument and the Hawaiian Volcano Observatory.

CO2 is typically measured with an infrared absorption instrument. These instruments measure the concentration of CO2 in the air or in a volcanic plume. Alternately, the rate at which carbon dioxide is emitted through diffuse soil degassing can be determined by placing a so-called accumulation chamber (basically an inverted bucket) on the ground and measuring the increase in CO2 concentration within over time.

Sulfur dioxide indicates magma is near the surface

Gas flask sampling at West Astringent Creek, Yellowstone. Open tube...
Gas flask sampling at West Astringent Creek, Yellowstone. Open tube with attached gas chamber inserted into ground, gas travels through tube into vacuum flask being held by scientist.

Sulfur dioxide (SO2) is released from a volcano when magma is relatively near the surface. If SO2 is detected at a non-erupting volcano, it could be a sign that it will erupt soon. By monitoring the amount of SO2 being emitted from an active volcano, it is possible to calculate the amount of magma that is supplying the eruption. However, SO2 easily dissolves in water, so if the volcano has abundant surface or subsurface water (e.g. glaciers, crater lakes, a hydrothermal system) it becomes difficult and sometimes impossible to determine how much sulfur dioxide is actually being released. A variety of spectrometers (e.g., COSPEC and DOAS) are used to measure the volcanic emissionrate of sulfur dioxide gas.

When SO2 is injected into the atmosphere, it quickly forms potentially hazardous sulfate areosols. They can cause respiratory problems to people downwind of a volcanic plume (e.g VOG—volcanic smog). If injected into the upper atmosphere (stratosphere), they can cool the climate for years by reflecting incident sunlight back to space.

USGS employees deploys a water conductivity/temperature/depth probe...
USGS employees deploys a water conductivity/temperature/depth probe (AquaTroll) for a temporary study along the Madison River, Yellowstone.

Hydrogen sulfide indicates volcanic activity is relatively quiet

When sulfur gases are released from magma and encounter groundwater as they rise, the sulfur can react with water and form hydrogen sulfide (H2S). The presence of H2S typically indicates that volcanic activity is relatively quiet because the groundwater is able to filter out much of the sulfur gas that rises from the magma. Hydrogen sulfide can be measured by collecting a gas sample then analyzing the complete chemistry in a laboratory.

Water chemistry changes can be early signals of changing volcanic activity

Streams and rivers that flow on the flanks of volcanoes can transport chemical components that come from a deeper volcanic source. By collecting water samples and analyzing the chemical makeup, scientists can determine whether magma or magmatic gases have heated the water. This is especially valuable at quiet volcanoes where the very earliest signal of a change in activity can be a shift in water chemistry. For example, the chloride (Cl) content of streams near volcanoes is strongly tied to the amount of heated water contributed from hot springs and other types of thermal features. The USGS has been monitoring the Cl flux of Yellowstone streams since the 1970s in order to estimate the total heat flow from the Yellowstone system and has been monitoring the flux of Cl and other constituents throughout the Cascade Range since about 2009.