Except for abundant free oxygen released from plants, algae(海藻) and cyanobacteria(藍綠藻) 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(一氧化碳), sulfur, argon(氬), 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(氣候) 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(污染) such as volcanic gases, can cause global warming, steps should be taken to reverse the trend.
The problem is complex(複雜), 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(溫室效應). 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.
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(硫磺) acid aerosol layers. The aerosol(液化氣體) 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.
Chlorine(氯) gas can negatively effect the earth's environment. Chlorine is emitted from volcanoes in the form of hydrochloric acid, 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.
Fluorine(氟) gas can condense in rain or on ash particles and coats grass and pollutes(污染) streams and lakes with excess fluorine. Animals that eat grass coated with fluorine-tainted ash are poisoned. Small amounts of fluorine can be beneficial(有幫助), but excess fluorine causes fluorisis, an affliction that eventually kills animals by destroying their bones.