Except for abundant free oxygen released from plants, algae and cyanobacteria by photosynthesis, atmospheric gases initially come from the earth's interior. Gases within magma are dissolved because of high pressures beneath the earth's surface, but reduced pressure at the surface allows dissolved gases to expand and escape. Water vapor constitutes 70 to 95 percent of all eruption gases. The rest consists of carbon dioxide, sulfur dioxide and traces of nitrogen, hydrogen, carbon monoxide, sulfur, argon, chlorine and fluorine.

The different volcanic gases effect the earth, people and animals in different ways. Water vapor is beneficial, adding to the earth's water supply. Sulfur dioxide can form aerosols that reflect the sun's rays and cause cooling of the earth's surface, or cause harm by forming acid rains. Fluorine gases and their acid aerosols can be lethal to animals. Carbon dioxide can add to the effects of global warming. There is circumstantial evidence that volcanic eruptions can affect short-term weather patterns, and possibly trigger long-term climatic change. The U.S. Geological Survey is studying volcanic emissions and global change. The effects of human activities upon global climate becomes important to understand as the world population grows. One of the main concerns is global warming. This could cause ocean levels to rise if the glaciers in Antarctica melt, and seriously disrupt many large population centers on earth -- London, Tokyo-Yokohama, Los Angeles, New York, Buenos Aires -- as water levels rose. If there is some evidence that man's pollutants, beyond that of natural contaminates such as volcanic gases, can cause global warming, steps should be taken to reverse the trend.

Carbon Dioxide

Yearly averages of global temperatures have steadily increased since the industrial revolution, mid-1700's to mid-1800's in England, addition of carbon dioxide to the atmosphere from industrial processes and the internal combustion engine. Carbon dioxide is abundant in volcanic gases, but not enough to significantly contribute to the greenhouse effect. Volcanoes contribute about 110 million tons of carbon dioxide per year while man's activities contribute about 10 billion tons per year.

The problem is complex, for volcanoes can help cool the earth's surface by forming sulfuric acid aerosols that reflect the sun's rays, and also contribute to global warming by giving off carbon dioxide into the atmosphere, which contributes to the greenhouse effect. Greenhouses (or, hot houses) are heated by the sun's rays that enter through glass or plastic, and the heat is retained inside like a parked car on a hot day with the windows rolled up. Carbon dioxide behaves like a glass shield over the earth. The sun's rays penetrate the carbon dioxide gas but the carbon dioxide shield prevents heat from escaping into the atmosphere.

Sulfur Dioxide

The greatest volcanic impact upon the earth's short term weather patterns is caused by sulfur dioxide gas. In the cold lower atmosphere, it is converted to sulfuric acid by the sun's rays reacting with stratospheric water vapor to form sulfuric acid aerosol layers. The aerosol remains in suspension long after solid ash particles have fallen to earth and forms a layer of sulfuric acid droplets between 15 to 25 kilometers up. Fine ash particles from an eruption column fall out too quickly to significantly cool the atmosphere over an extended period of time, no matter how large the eruption.

Sulfur aerosols last many years, and several historic eruptions show a good correlation of sulfur dioxide layers in the atmosphere with a decrease in average temperature decrease of subsequent years. The close correlation was first established after the 1963 eruption of Agung volcano in Indonesia when it was found that sulfur dioxide reached the stratosphere and stayed as a sulfuric acid aerosol.

Without replenishment, the sulfuric acid aerosol layer around the earth is gradually depleted, but it is renewed by each eruption rich in sulfur dioxide. This was confirmed by data collected after the eruptions of El Chichon, Mexico (1982) and Pinatubo, Philippines (1991), both of which were high-sulfur compound carriers like Agung, Indonesia.


Chlorine gas can negatively effect the earth's environment. Chlorine is emitted from volcanoes in the form of hydrochloric acid (HCl), which breaks down into chlorine and chlorine monoxide (ClO) molecules. The sulfate aerosols furnish sites for chemical reactions that release the chlorine atoms. These eruption-derived chlorine atoms are added to man-produced chlorine already present in the stratosphere. The reactive chlorine atoms then proceed to destroy ozone, with each chlorine atom being recycled many times.

Ozone loss by volcanic eruptions combined with CFC use by people creates a greater threat to ozone depletion; we can only attempt to control man's contributions. The ozone layer begins at 12 kilometers (7.2 miles) above the earth s surface in temperate latitudes. It is a shield that protects living beings from the most harmful of the Sun's ultraviolet radiation known as UV-B. In high doses, UV-B damages cellular DNA in animals and plants. A well developed ozone layer absorbs this radiation, and protects us from these harmful effects.


Fluorine gas can condense in rain or on ash particles and coats grass and pollutes streams and lakes with excess fluorine. Animals that eat grass coated with fluorine-tainted ash are poisoned. Small amounts of fluorine can be beneficial, but excess fluorine causes fluorisis, an affliction that eventually kills animals by destroying their bones.


American Geophysical Union. "Volcanism and climate change." AGU Special Report. American Geophysical Union, Washington, DC., 1992, 27 pages.

Sigurdsson, H., 1990. "Assessment of the atmospheric impact of volcanic eruptions." In, Global catastrophes in earth history; and interdisciplinary conference on impacts, volcanism, and mass mortality, edited by Sharpton, V. L. and Ward, P. D. , 99-110. Geological Society of America, Special Paper 247.

Williams, H. and McBirney, A. R. Volcanology. San Francisco, California: Freeman, Cooper and Co., 1979, 397 pages.

Copyright (C) 1997, by Richard V. Fisher. All rights reserved.