Volcanic Gas
Volcano Terms and Definition

Volcanic gas

Magma contains dissolved gases that are released into the atmosphere during eruptions. Gases are also released from magma that either remains below ground (for example, as an intrusion) or rises toward the surface. In such cases, gases may escape continuously into the atmosphere from the soil, volcanic vents, fumaroles, and hydrothermal systems. The most common gas released by magma is steam (H 2 O), followed by CO 2 (carbon dioxide), SO 2 (sulfur dioxide), (HCl) hydrogen chloride and other compounds.

Magma contains dissolved gases that are released into the atmosphere during eruptions. Gases are also released from magma that either remains below ground (for example, as an intrusion) or is rising toward the surface. In such cases, gases may escape continuously into the atmosphere from the soil, volcanic vents, fumaroles , and hydrothermal systems.

At high pressures deep beneath the earth's surface, volcanic gases are dissolved in molten rock. But as magma rises toward the surface where the pressure is lower, gases held in the melt begin to form tiny bubbles. The increasing volume taken up by gas bubbles makes the magma less dense than the surrounding rock, which may allow the magma to continue its upward journey. Closer to the surface, the bubbles increase in number and size so that the gas volume may exceed the melt volume in the magma, creating a magma foam. The rapidly expanding gas bubbles of the foam can lead to explosive eruptions in which the melt is fragmented into pieces of volcanic rock, known as tephra. If the molten rock is not fragmented by explosive activity, a lava flow will be generated.

Together with the tephra and entrained air, volcanic gases can rise tens of kilometers into Earth's atmosphere during large explosive eruptions. Once airborne, the prevailing winds may blow the eruption cloud hundreds to thousands of kilometers from a volcano. The gases spread from an erupting vent primarily as acid aerosols (tiny acid droplets), compounds attached to tephra particles, and microscopic salt particles.

An eruption cloud consisting predominantly of steam (water) and sulfur dioxide gas rises above a fountain of lava erupting from Pu`u `O`o spatter and cinder cone on Kilauea Volcano. The persistent release of sulfur dioxide gas from Kilauea's active vents into the atmosphere since 1983 has created volcanic smog ("vog") and acid-rain conditions on the Big Island of Hawai`i. Vog aggravates respiratory problems, and acid rain damages crops and corrodes metal

These dead and dying trees on the south side of Mammoth Mountain volcano (peak in distance) in eastern California were first noticed in 1990. Since then, about 170 acres of trees have died. When the area was examined in 1990, exceptionally high concentrations of carbon dioxide gas were found in the soil beneath the trees. What caused such high concentrations of carbon dioxide gas? The most likely sources of the carbon dioxide are (1) magma that intruded beneath Mammoth Mountain during an earthquake swarm in 1989; and (2) limestone-rich rocks surrounding the intrusion (CO 2 is released from limestone (CaCO 3 ) when heated).

Close view of the eruption column of Mount St. Helens on May 18, 1980; the volcano is visible in the bottom of the photograph. The explosive release of volcanic gases into the atmosphere fragmented the erupting magma into tephra, including frothy pumice and gritty volcanic ash. At the beginning of the eruption, the hot column of tephra, gases, and entrained air rose to about 22 km above sea level in less than 10 minutes.

The eruption continued for the next nine hours, reaching a climax in the late afternoon. The prevailing wind blew the ash and gases east across Washington at an average speed of 100 km/hr for the first 1,000 km. About 3 1/4 hours after the eruption began, volcanic ash began to fall on Spokane, Washington, about 400 km downwind from Mount St. Helens. At this distance, between 0.5 and 2 cm of volcanic ash fell to the ground. The eruption cloud encircled the Earth in about 2 weeks.

Volcanic gases undergo a tremendous increase in volume when magma rises to the Earth's surface and erupts. For example, consider what happens if one cubic meter of 900°C rhyolite magma containing five percent by weight of dissolved water were suddenly brought from depth to the surface. The one cubic meter of magma now would occupy a volume of 670 m 3 as a mixture of water vapor and magma at atmospheric pressure! The one meter cube at depth would increase to 8.75 m on each side at the surface. Such enormous expansion of volcanic gases, primarily water, is the main driving force of explosive eruptions

Types of volcanic gases

The most abundant gas typically released into the atmosphere from volcanic systems is water vapor (H 2 0), followed by carbon dioxide (C0 2 ) and sulfur dioxide (S0 2 ). Volcanoes also release smaller amounts of others gases, including hydrogen sulfide (H 2 S), hydrogen (H 2 ), carbon monoxide (CO), hydrogen chloride (HCL), hydrogen fluoride (HF), and helium (He).

Potential effects of volcanic gases

The volcanic gases that pose the greatest potential hazard to people, animals, agriculture, and property are sulfur dioxide , carbon dioxide , and hydrogen fluoride . Locally, sulfur dioxide gas can lead to acid rain and air pollution downwind from a volcano. Globally, large explosive eruptions that inject a tremendous volume of sulfur aerosols into the stratosphere can lead to lower surface temperatures and promote depletion of the Earth's ozone layer. Because carbon dioxide gas is heavier than air, the gas may flow into in low-lying areas and collect in the soil. The concentration of carbon dioxide gas in these areas can be lethal to people, animals, and vegetation. A few historic eruptions have released sufficient fluorine-compounds to deform or kill animals that grazed on vegetation coated with volcanic ash; fluorine compounds tend to become concentrated on fine-grained ash particles, which can be ingested by animals.

Sulfur dioxide (SO 2 )

The effects of SO 2 on people and the environment vary widely depending on (1) the amount of gas a volcano emits into the atmosphere; (2) whether the gas is injected into the troposphere or stratosphere; and (3) the regional or global wind and weather pattern that disperses the gas. Sulfur dioxide (SO 2 ) is a colorless gas with a pungent odor that irritates skin and the tissues and mucous membranes of the eyes, nose, and throat. Sulfur dioxide chiefly affects upper respiratory tract and bronchi. The World Health Organization recommends a concentration of no greater than 0.5 ppm over 24 hours for maximum exposure. A concentration of 6-12 ppm can cause immediate irritation of the nose and throat; 20 ppm can cause eye irritation; 10,000 ppm will irritate moist skin within minutes.

Emission rates of SO 2 from an active volcano range from <20 tonnes/day to >10 million tonnes/day according to the style of volcanic activity and type and volume of magma involved. For example, the large explosive eruption of Mount Pinatubo on 15 June 1991 expelled 3-5 km 3 of dacite magma and injected about 17 million tonnes of SO 2 into the stratosphere. The sulfur aerosols resulted in a 0.5-0.6°C cooling of the Earth's surface in the Northern Hemisphere. The sulfate aerosols also accelerated chemical reactions that, together with the increased stratospheric chlorine levels from human-made chlorofluorocarbon (CFC) pollution, destroyed ozone and led to some of the lowest ozone levels ever observed in the atmosphere.

At Kilauea Volcano, the recent effusive eruption of about 0.0005 km 3 /day (500,000 m 3 ) of basalt magma releases about 2,000 tonnes of SO 2 into the lower troposphere. Downwind from the vent, acid rain and air pollution is a persistent health problem when the volcano is erupting.

Volcanic smog
Eruptions of Kilauea Volcano release large quantities of sulfur dioxide gas into the atmosphere that can lead to volcanic air pollution on the Island of Hawai`i. Sulfur dioxide gas reacts chemically with sunlight, oxygen, dust particles, and water to form volcanic smog known as vog.

Lava fountain is about 300 m tall


Lava lake is about 100 m in diameter (circular part of the lake).

In 1986 when the eruption of Kilauea Volcano changed from the episodic fountaining of lava and gas at Pu`u O`o cone every few weeks (top left) to the continuous outpouring of lava from a new vent (bottom left) only 3 km away, the volcano began releasing a large, steady supply of sulfur dioxide gas into the atmosphere. During the episodic activity, enough time elapsed between fountaining episodes for the prevailing trade winds (brisk winds from the northeast of Hawai`i) to blow volcanic gas away from the island. When the eruption style changed, however, the daily release of as much as 2,000 tons of sulfur dioxide gas led to a persistent air pollution problem downwind.

Global cooling and ozone depletion
Measurements from recent eruptions such as Mount St. Helens, Washington (1980), El Chichon, Mexico (1982), and Mount Pinatubo, Philippines (1991), clearly show the importance of sulfur aerosols in modifying climate, warming the stratosphere, and cooling the troposphere. Research has also shown that the liquid drops of sulfuric acid promote the destruction of the Earth's ozone layer

Volcanic ash vs sulfur aerosols
The primary role of volcanic sulfur aerosols in causing short-term changes in the world's climate following some eruptions, instead of volcanic ash, was hypothesized by scientists in the early 1980's. They based their hypothesis on the effects of several explosive eruptions in Indonesia and the world's largest historical effusive eruption in Iceland. Scientists studied three historical explosive eruptions of different sizes in Indonesia--Tambora (1815), Krakatau (1883), and Agung (1963). They noted that decreases in surface temperatures after the eruptions were of similar magnitude (0.18-1.3 °C). The amount of material injected into the stratosphere, however, differed greatly. By comparing the estimated amount of ash vs. sulfur injected into the stratosphere by each eruption, it was suggested that the longer residence time of sulfate aerosols, not the ash particles which fall out within a few months of an eruption, was the paramount controlling factor.	In contrast to these explosive eruptions, one of the most severe volcano-related climate effects in historical times was associated with a largely nonexplosive eruption that produced very little ash--the 1783 eruption of Laki crater-row in Iceland. The eruption lasted 8-9 months and extruded about 12.3 km 3 of basaltic lava over an area of 565 km 2 . A bluish haze of sulfur aerosols all over Iceland destroyed most summer crops in the country; the crop failure led to the loss of 75% of all livestock and the deaths of 24% of the population. The bluish haze drifted east across Europe during the 1783-1784 winter, which was unusually severe. Clearly, these examples suggested that the explosivity of an eruption and the amount of ash injected into the stratosphere are not the main factors in causing a change in Earth's climate. Instead, scientists concluded that it must be the amount of sulfur in the erupting magma.
The eruption of El Chichon, Mexico, in 1982 conclusively demonstrated this idea was correct. The explosive eruption injected at least 8 Mt of sulfur aerosols into the atmosphere, and it was followed by a measureable cooling of parts of the Earth's surface and a warming of the upper atmosphere. A similar-sized eruption at Mount St. Helens in 1980, however, injected only about 1 Mt of sulfur aerosols into the stratosphere. The eruption of Mount St. Helens injected much less sulfur into the atmosphere--it did not result in a noticeable cooling of the Earth's surface. The newly launched TOMS satellite (in 1978) made it possible to measure these differences in the eruption clouds. Such direct measurements of the eruption clouds combined with surface temperatures make it possible to study the corrleation between volcanic sulfur aerosols (instead of ash) and temporary changes in the world's climate after some volcanic eruptions.

The most significant impacts from large explosive eruptions come from the conversion of sulfur dioxide (SO 2 ) to sulfuric acid (H 2 SO 4 ), which condenses rapidly in the stratosphere to form fine sulfate aerosols. The aerosols increase the reflection of radiation from the Sun back into space and thus cool the Earth's lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere.

Ozone depletion promoted by volcanic sulfur aerosols. The sulfate aerosols also promote complex chemical reactions on their surfaces that alter chlorine and nitrogen chemical species in the stratosphere. This effect, together with increased stratospheric chlorine levels from chlorofluorocarbon (CFC) pollution, generates chlorine monoxide (ClO), which destroys ozone (O 3 ).

Hydrogen sulfide (H 2 S)

Hydrogen sulfide (H 2 S) is a colorless, flammable gas with a strong offensive odor. It is sometimes referred to as sewer gas. At low concentrations it can irritate the eyes and acts as a depressant; at high concentrations it can cause irritation of the upper respiratory tract and, during long exposure, pulmonary edema. A 30-minute exposure to 500 ppm results in headache, dizziness, excitement, staggering gait, and diarrhea, followed sometimes by bronchitis or bronchopneumonia.

Carbon dioxide (CO 2 )

Volcanoes release more than 130 million tonnes of CO 2 into the atmosphere every year. This colorless, odorless gas usually does not pose a direct hazard to life because it typically becomes diluted to low concentrations very quickly whether it is released continuously from the ground or during episodic eruptions. But in certain circumstances, CO 2 may become concentrated at levels lethal to people and animals. Carbon dioxide gas is heavier than air and the gas can flow into in low-lying areas; breathing air with more than 30% CO 2 can quickly induce unconsciousness and cause death. In volcanic or other areas where CO 2 emissions occur, it is important to avoid small depressions and low areas that might be CO 2 traps. The boundary between air and lethal gas can be extremely sharp; even a single step upslope may be adequate to escape death.