||Sizes of Eruptions
Volcanic eruptions come in all sizes: small, medium, large, extra large, giant economy size, and super - huge -knock - your - socks - off - more - than - you - could - possibly - imagine size. Giant eruptions can literally affect the whole world. On the other hand, small eruptions may affect only a single hillside or valley. Let's look at the characteristics and effects of volcanic eruptions of different sizes, starting with small and working our way up.
How close? A small volcanic eruption may consist of a single small burst of steam and volcanic ash, such as the initial eruption of Mount St. Helens, or a single lava flow like those that make the local evening news in Hawaii. By exercising some caution, you can view many small eruptions from a reasonable distance. What is reasonable? It all depends on the type of eruption. Explosive eruptions (high-water, high-silica), even small ones, are best seen from a distance of miles. On the other hand, effusive eruptions (low-water, low-silica) can be viewed from quite close if you position yourself properly. Large flows of very fluid lavas can attain speeds of 50 to 60 mph, so you would not want to try standing in front of such a flow! With care and proper clothing, however, these flows can be observed and even sampled from the side at a distance of a few feet.
C.R. Chapman tells of two volcanologists walking beside a lava flow during a long hike to the source vent. They had been carrying a frozen pizza with them for a snack. When they grew hungry, they decided to cook the pizza on the solid, but hot crust forming at the edge of the lava flow. Unfortunately for their stomachs, while the pizza was cooking, the "oven" of unstable rock crust broke free and drifted into the flow, carrying their pizza out of reach. Because wading through liquid rock is not recommended, they went on, hunger unsatisfied. Photo: Courtesy of NGDC/NOAA.
Some flows are very slow, and even the fastest flows eventually cool, slow, and stop. "Slow" here means human walking speed or less. You can observe slow flows from any vantage point - front, side, even on top (in some cases). G. A. McDonald tells of a volcanologist who had observed a slow-moving "blocky" flow through a long, hot morning in Hawaii. By noon, the flow had appeared to stop, and the volcanologist decided to have lunch on the top of the flow. After sitting and eating for awhile, the volcanologist noticed that the surrounding scenery was moving past! The flow he was sitting on was still creeping along and was not "dead" after all.
How Large is Large? There is no universally accepted scale, comparable to the Richter Scale for earthquakes, for classifying the sizes of volcanic eruptions. However, one useful comparison is the volume of new volcanic rock blasted out by an eruption. Volumes of new volcanic rock are represented on this page in two ways: by the figure at right, which pictorially compares ejected volumes, and by the table below, which shows similar, numerical data. The designations of "small," "large," "major," and "great" eruptions are simply descriptive and are used only to draw attention to the enormous range in the sizes of volcanic eruptions.
* Volumes are approximate. 1 mi3 = 4.168 km3
The preceeding table was complied using data from the following sources: R. L. Smith (1979) GSA Special Paper 180, pp. 5-27; R. B. Smith and L. W. Braile (1994) J. Volc. Geotherm. Res. 61:121-187; J. J. Dvorak, C. Johnson and R. I. Tilling (1992) Sci. Am., August, pp. 46-53; J. M. Rhodes (1988) J. Geophys. Res. 93:4453-4466; F. Press and R. Siever (1974) Earth, W. H. Freeman & Co.
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