What comes out when a volcano erupts

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Volcanic gases

Volcanoes not only produce lava, but also numerous gases. They are the driving forces behind the eruption mechanisms of a volcano. Usually the gases are dissolved in the magma. Under the high pressure at greater depths, undissolved gases take on a special form, which is known as the fluid phase. These phases combine properties of liquids and gases.

The most common gas in magma is the gas phase of water. This is often referred to as water vapor. But when the gas is already condensing and forming visible clouds of vapor, it is strictly speaking no longer a gas, but a condensate. The formation of steam clouds on the volcano is not only related to the amount of water vapor emitted, but also to the climatic conditions. Warm air can store much more vapor than cold air. In the mornings and evenings, or during winter, a volcano can steam significantly more, although the gas output has remained the same. The second most common volcanic gas is carbon dioxide, followed by sulfur dioxide and hydrogen sulfide. The latter is known for its rotten egg smell and is immediately associated with volcanic phenomena. Elemental sulfur can be deposited from the sulfur gases and acids are formed in connection with water. Halogens such as chlorine, bromine and fluorine come into contact with hydrogen and also form acids as condensate.

Most volcanic gases are also known as greenhouse gases and have a major impact on the climate. One theory of the formation of the atmosphere and the oceans is that volcanoes have exuded the gases required for this. Even today, volcanoes act as natural pressure relief valves that dissipate heat and gases.

Volcanic gases play an important role in predicting volcanic eruptions. A volcanic eruption is often heralded by increased gas emissions. Isotopes of helium and radon can provide the first clues. If elevated concentrations of these isotopes are detected in gases flowing from hydrothermal sources or fumaroles, magma is likely to penetrate the earth's crust. If sulfur gases are added, the volcano may be on the verge of an eruption.

The gases produced by a volcano can reach life-threatening concentrations. Carbon dioxide is heavier than oxygen and displaces it especially in valleys and hollows. Living beings suffocate without realizing it. Highly concentrated gases can poison a person after a few breaths.

During the eruption of the Icelandic volcano Bardarbunga in autumn 2014, so much sulfur dioxide was extracted that it endangered the health of the coastal inhabitants. The gas cloud was detected over Scandinavia and Europe. During the high phase of the eruption, up to 750 kg of gas per second were produced! Scientists were only able to stay in the vicinity of the crevice with gas masks and access to the eruption was temporarily blocked.
During the famous Laki eruption, which occurred in Iceland in 1783, the grass of the pastures withered by volcanic gases and the animals perished. A famine broke out. Acid rain also fell over Europe and created problems.

This image from Eumetsat shows the Ontake eruption cloud in visible light and in false color representation (SO2 RGB). The invisible sulfur dioxide cloud is neon yellow in the photo on the right.

The quantitative gas emissions of a volcano are usually measured with a spectrometer. For this purpose, volcanologists use a device called COSPEC (Correlation Spectrometer). Active volcanoes such as Etna and Kilauea emit quite a few thousand tons of sulfur gases per day. In the space age, volcanic gas clouds can be observed by satellite. In addition to spectrometers, radiometers (Advanced Very High Resolution Radiometers) are also used here. Qualitative measurements are carried out in the laboratory. This requires gas samples that are collected in glass flasks at the volcano.

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