What major gases do volcanoes emit?

Volcanic gases contained within the magma (molten rock) are released as they reach the Earth's surface, escaping at the major volcanic opening or from fissures and vents along the side of the volcano. The most prevalent gases are carbon dioxide (CO2) and hydrogen sulfide (H2S). Carbon dioxide is a dangerous gas; it is invisible and odorless, and can kill within minutes.

One example in which volcanic gases proved dangerous involved the Dieng Volcano Complex (or Dieng Plateau) in Java, Indonesia. It consists of two volcanoes and over 20 craters and cones, and is noted for its poisonous gas emissions at some craters. In 1979, at least 149 people were killed by poisonous gases as they fled eruptions at two of the craters: the Sinila and Sigludung.

Did volcanoes play a role in creating Earth's atmosphere?

Scientists now believe that much of our planet's atmosphere was generated by carbon dioxide, water vapor, nitrogen, argon, and methane spewing out of volcanoes. When life began to form as primitive plant cells, the carbon dioxide issued from volcanoes was absorbed by these plants and then released as oxygen. At first, the oxygen reacted with iron and other metals in the Earth's crust, creating iron oxides that form the commonly seen reddish earth in the ground. Eventually, though, there was enough oxygen that it became part of the atmosphere, and breathable air was created.

Can volcanic eruptions affect the global climate?

Most volcanic eruptions do not affect the global climate, although larger ones can cause disruptions—albeit for a relatively short period of time. Large eruptions tend

An ash plume erupts from the Cleveland Volcano in Alaska

An ash plume erupts from the Cleveland Volcano in Alaska's Aleutian Islands on May 23, 2006. International Space Station flight engineer Jeff Williams took this photo after warning the Alaska Volcano Observatory of the activity. (NASA)

to eject gases and dust high into the stratosphere. From there, prevailing winds carry the particles around the world—sometimes with interesting results.

For example, in 1815, Mt. Tambora on the island of Sumbawa (near Java, Indonesia) erupted, putting out a record amount of ash that briefly changed the world's climate. Huge amounts of volcanic dust rose high into the atmosphere, reaching around the globe. That year (and for some of the following year), volcanic particles screened out some sunlight, causing the global temperatures to fall. In Europe and other parts of the Northern Hemisphere, winter never seemed to end, with frosts occurring throughout the summer. Hence, 1816 is known as "the year with no summer."

Scientists used to believe that cooling of the atmosphere could result from volcanic eruptions because of the amount of ash that was thrown up into the air. Now they know, however, that most of these fine particulates return to the Earth within about six months. What actually has a greater effect is the sulfur dioxide (SO2) that volcanoes produce. Sulfur dioxide reacts with water vapor, and the result is a long-lasting haze that blocks out a considerable amount of the Sun's radiation.

Who first theorized that there was a link between volcanoes and climate?

As well as being one of the Founding Fathers of the United States, an inventor, and a diplomat, Benjamin Franklin (1706-1790) is often credited as the first person to notice that volcanic activity might be affecting weather. Observing that, after the eruption of Iceland's Laki volcano in 1783, there seemed to be a period of cooler weather lasting into 1784, Franklin believed that the more common incidence of fogs in Europe was a consequence of the eruption.

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