The types of eruptions most likely to be influenced by weather are small ones that expel only rock fragments entrained in steam or magmatic gas, and no new lava. In at least one series of such eruptions, at Mount St. Helens, Wash. between 1989 and 1991, an association with rainfall has been statistically documented.
Some blasts were powerful enough to hurl blocks of rock a meter (3 feet) in diameter as far as a kilometer (0.6 mile) from the vent and to destroy three nearby seismic stations. Plumes of ash that wafted up to 5 kilometers (3 miles) above the crater floor were carried by wind nearly as far as Yakima, Wash., 140 kilometers (90 miles) away, and in one case prompted flight cancellations at the Portland, Oreg. airport. Within the crater, debris was ejected or avalanched northward onto the crater floor (fig. 1). Some of the hot ejecta mixed with snow and produced small floods or slurries of rock and water (termed debris flows) that flowed out of the crater.
Compared with the eruptions of the early 1980's these events were small, but had there been visitors in the crater the eruptions could have been deadly. In January and March 1993, eight volcanologists and three tourists lost their lives in two similar explosions in South America: at Galeras Volcano in Colombia and Guagua Pichincha Volcano in Ecuador. Another explosion of this type killed nine tourists on Mount Etna, Italy in 1979.
Small gas eruptions are potentially deadly because they are hard to predict. Between 1981 and 1986, magmatic eruptions at Mount St. Helens were preceded by uplift of the crater floor and by earthquakes. In nearly all of those cases, scientists accurately predicted eruptions a few days to a few weeks in advance. No such precursors preceded the gas eruptions in 1989-91.
How could rainfall have caused the release of this gas? A little information on the dome might provide some clues. From magnetic measurements, we know that the interior of the dome is still hotter than 350 C (650 F)--hot enough to ooze like warm tar--even though the outer several tens of meters of the dome has solidified (fig. 3). Over time, the dome cools from the outside inward. As it cools, the rock shrinks, fractures, and opens pathways to greater depth. Cooling and fracturing take place primarily during storms when water flows into the dome.
At the surface the pervasive growth of cooling fractures has caused entire outcrops to disintegrate to piles of rubble. Cooling fractures also reduce the strength of rock masses that cling precariously to the dome's slopes, promoting slides and avalanches of debris. Between 1989 and 1991, large avalanches removed massive amounts of rock from the vent area (fig. 3). Much of the avalanching appears to have occurred during eruptions, although it is not known whether the eruptions caused the avalanches or vice versa.
One could speculate at least two ways that rainfall could have caused these eruptions. First, during storms, some fractures may have opened pathways for the release of pressurized gas from within or below the dome. Second, storms may have triggered rock slides or avalanches that exposed zones of pressurized gas. In either case, the infiltration of water into the dome probably acted to open some escape route for gas that was already present.
Mastin, Larry G., 1994, Explosive tephra emissions at Mount St. Helens, 1989-1991: The violent escape of magmatic gas following storms?: Geological Society of America Bulletin, V. 106, p. 175-185.
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