Over 500 active volcanoes have been noted in history, with approximately 20 currently being active on the earth’s surface today. North America is the third most active region behind Indonesia and Japan in the number of currently active volcanoes, with more than 50 eruptions recorded in the last four centuries. While not all these volcanoes are visibly violent, the more dramatic eruptions, the ones we most often see photos of, are those spewing ash many miles into the atmosphere. Is there reason to be concerned about volcanic activity in North America? Might they affect our weather and crop production? How about volcanoes in the southern hemisphere or those along the western Pacific rim? What effects do volcanoes have on our weather patterns? To answer these questions we must understand how volcanic ash is created and how its constituent elements affect the environment.
Volcanic ash is formed as a result of the explosive release of magma through vents in the earth’s surface. As the magma is propelled upward, expanding gasses trapped in the magma cause a fracturing of the material into small particles, which are then driven up into the atmosphere with the rising column of heat. While larger aggregated particles may ‘rain’ out within a few miles of the eruption, the smallest of the particles can remain in the atmosphere for days, weeks, or months, coming to rest on the earth’s surface only when conditions permit water or condensation to increase their mass sufficiently to result in their settling to the surface.
The amount of ash distributed into the atmosphere can be limited to an initial ejection or result from a continuous or multiple ejections, and maybe somewhat ameliorated by expelled steam that forms rain from the atmospheric uplift and the resultant cooling in the immediate vicinity of the eruption. Besides the quantity of ash ejected, the effect of ash on weather is determined in part by prevailing winds, the height to which the ash is ejected, the direction of the ash plume and possibly the time of year the eruption occurred (a factor not yet fully investigated). There have been a number of eruptions which scientists have associated with a direct impact on weather. All of these impacts have resulted in a cooling of the earth’s temperature for periods of several months. The most recent of these events was the 1991, Mount Pinatubo eruption on the Luzon island in the Philippines which resulted in a one degree Fahrenheit drop in the earth’s ambient temperature for a year. While events of this size that are significant to the entire planet are not frequent, they may be expected to occur once or twice every century.
Current volcanic activity in North American extends through a region along the west coast from Alaska to Panama. The greatest probability of the continental U.S. receiving ash from an ejecting volcano would be from the prevailing westerly winds blowing ash from events occurring in the state of Washington or Alaska. Significant ash deposition from volcanoes occurring in Central America is somewhat less likely. For ash to reach the U.S. from events occurring along the western edge of the Pacific Rim, the amount of ejected ash would have to be very significant.
While smaller volcanic events with ash ejection are quite common, the effects tend to be limited to a region close to the volcano. Except for some temporary blockage of sunlight, these events are less likely to affect the weather and more likely to result in a troublesome deposition of the pumice and ash. In areas receiving ash deposition, the mechanical and chemical (toxic) effects on plant and animal life can be significant. The extremely fine nature of ash particles can irritate the lungs and digestive system of livestock and can smother plants. Mechanical support equipment will also be affected by plugged air and oil filters and cooling systems, abrasion of moving parts, and rust and corrosion of metal surfaces. The abrasion typically results from the high degree of silica in most ash, although calcium, magnesium, iron and aluminum are often found in quantities sufficient that amend the abrasion characteristics of the ash and increase its abrasive qualities. The amount of ash deposition can range from a few millimeters to several feet and will depend on the factors mentioned above; distance from eruption, amount of ejection, and displacement of the plume. For an interesting graphic refer to the USGS map below. It shows a forecast of a possible Yellowstone Caldera month-long eruption based on winds for the month of July, 2001
Other components of ash particles can be toxic. Flourine, chlorine and sulfates are often found at toxic levels and can result in livestock poisoning through ingestion. Plants can also suffer from suffocation and burning of leaf tissue from leachates, most often sulfates. While leachates seldom reach groundwater, they can have an effect on the plant ecosystem once they reach the soil profile, favoring an occasional species that is naturally tolerant to the toxin. Deleterious effects could last several years depending on the level of toxicity and the prevailing precipitation that might dissipate the toxin via leaching. Other immediate effects include an increase in surface water turbidity, reduced traction on roads, ash drawn into machinery by wheel traffic and winds, and arcing of power lines when ash deposited on insulators becomes wet.