Volcanos are among the most awesome and destructive forces in nature. A single volcano can inject massive amounts of fine ash and gas high into the atmosphere producing global climactic effects. High speed, sometimes explosive flows of hot, poisonous gas with entrained ash and rocky debris, called pyroclastic flows, blast down volcanic slopes destroying everything in their path. Powerful, rapidly flowing muddy mixtures of rock and debris, called lahars, are mobilized by water high on volcanic flanks and sent careening down to valleys below with often disastrous effects. Slower, steadier flows of hot lava and cement-like mud made up of volcanic ash mixed with rainwater can inundate vast areas of land surrounding a volcano.

Volcanic eruptions pose numerous hazards and can have substantial environmental effects. In the immediate vicinity of an eruption the primary hazards are debris falls, pyroclastic flows and lahars. Ash plumes can cover a wide region down-wind and can be hazardous to aircraft. Mud flows can cover fertile regions far from the volcano, destroying crops and villages in their path. Large eruptions inject massive amounts of water vapor, carbon dioxide, sulfur dioxide and other gasses into the stratosphere. Most of the sulfur dioxide is converted into sulfuric acid aerosol particles which can remain aloft for a very long time.

The crew of STS 67 took this picture of the Barren Island volcanic plume in March, 1995. Barren Island is a small volcanic island in the Andaman Islands west of Malayasia. This view shows a well developed volcanic plume rising several thousand feet into the atmosphere. The main plume is drifting westward over Andaman Island toward India, a smaller plume close to the ground is dispersing to the south. This photograph shows the regional extent of the hazard from a volcanic ash plume. The Indian authorities issued a Notice to Airmen (NOTAM) concerning the Barren Island volcano in March, 1995 advising all aircraft to avoid over flying the area. This NOTAM was valid for three months, and then re-issued for an additional three months. Neither the location of the plume nor the time of its dispersal were accurately known. This tied up a large volume of airspace causing airlines to divert a considerable distance around the volcano for months on end, at great expense in additional fuel. With accurate and timely information provided by new remote sensing systems under development, the hazard of accidentally encountering a volcanic ash plume and the expense of avoiding them will be greatly reduced.

Pinatubo in the Philippines, which had no documented activity for over 600 years, erupted on June 15, 1991, spewing approximately 20 million tons of sulfur dioxide gas into the stratosphere. The Total Ozone Mapping Spectrometer (TOMS) instrument on the Nimbus-7 satellite was able to measure the amount of sulfur dioxide from this eruption and map its distribution as the cloud drifted westward around the world. The images above show the distribution of sulfur dioxide in the atmosphere two and three days after the eruption. The highest concentration of sulfur dioxide is shown in white and the lowest is blue. The red dot marks the location of the eruption. The upper map is from June 17, the lower map is from June 18. The cloud is seen to be separated from the volcano, drifting westward and slightly southward, and dispersing in longitude. By July 7, 22 days after the main eruption, the sulfur dioxide cloud had traveled completely around the planet.

Most of the sulfur dioxide gas injected into the stratosphere is slowly converted to sulfuric acid, which condenses into a mist of fine particles called aerosols. These aerosol particles gradually settle out but they can remain suspended in the stratosphere for several years. During that time they can have global climatic effects by reflecting radiation from the sun back into space, thus shading and cooling the earth. Stratospheric aerosols from the Mt. Pinatubo eruption are believed to have caused a .5 degrees Celsius drop in the average global temperature by the following summer.

The Stratospheric Aerosol and Gas Experiment II (SAGE II) on the Earth Radiation Budget Satellite was able to measure the distribution of stratospheric aerosols from the eruption of Mt. Pinatubo for a period of two years or more. The images above show the global evolution of aerosols from a period just after the eruption to some 20 months later. In these maps the highest aerosol concentration is shown as magenta to red with the lowest being violet to blue. The upper map is for a period covering 40 days after the eruption. It shows the aerosol concentration being confined to low equatorial latitudes but completely encircling the globe. The lower map is for a 40 day period starting about 20 months after the eruption. By this time the aerosol distribution completely covers the globe.

The crew of STS 67 took this picture of Mt. Pinatubo in March, 1995. The winter dry season in the Philippines reveals the full extent of the mudflows. The light tan parts of the flows are dried mud deposits, which have now claimed hundreds of square kilometers of formerly rich farm land. Darker channels in the flows are stream channels within the mud-filled drainages.