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Explosive Verses Effusive
Scientists categorize types of eruptions into two extremes: explosive eruptions and effusive eruptions. As the name implies, explosive eruptions are violent and energetic, like Mount Saint Helens or Pompeii, while effusive eruptions are far quieter and less threatening, exemplified by the volcanoes of the Hawaiian islands. Not all eruptions necessarily neatly fit into one category or the other, but rather can be a blend of both types.
Viscosity and Gas
Eruption variability corresponds primarily with the composition of the volcano̵7;s magma -- including gas content -- and its resistance to flow, known as viscosity. Liquids with lower viscosity flow with greater ease, while liquids with higher viscosity flow less readily. A magma̵7;s viscosity also determines how easily gases can escape from it. Higher viscosity magmas tend to resist the expansion of gas bubbles, resulting in an increased pressure buildup. It̵7;s important to note that magmas in different volcanoes possess different amounts of dissolved gases -- mainly carbon dioxide, hydrogen sulfide and water vapor. Magmas abundant in these dissolved gases contain more explosive potential than those lacking in dissolved gases. Overall, high viscosity coupled with high volumes of gas produce the most explosive eruptions, while low viscosity and less gas produce effusive eruptions.
Temperature and Chemical Composition
Two major variables affecting the magma̵7;s viscosity are its temperature and level of silica. The lower the temperature at which a magma erupts, the greater its potential for explosiveness, and vice versa. The higher its silica content, the more viscous -- and hence, explosive -- the magma will be. Vulcanologists commonly classify magma into three main types, based on the level of silica: basaltic, andesitic and rhyolitic. Basaltic magma possesses the least silica, rhyolitic possesses the most silica and andesitic magma falls between the two extremes. Therefore, basaltic magmas are less viscous, flow more readily, and produce the quietest eruptions, while rhyolitic magmas are the most viscous, trapping a significant amount of gas and erupting with the greatest ferocity.
Eruption Frequency Variability
Unfortunately, like earthquakes, volcanic eruptions can̵7;t be predicted with a considerable degree of confidence, and the scientific understanding of the relationship between volcanoes̵7; ferocity and frequency of eruption leaves much to be desired. Generally speaking, however, smaller-scale eruptions occur more frequently than larger ones, because it takes time for the high pressure linked to powerful eruptions to build up. For example, many of the effusive-type volcanoes characteristic of the Hawaiian island chain erupt quite often. Hawaii̵7;s Kilauea is among the most active volcanoes on the planet, averaging more than one eruption every four years. Some of Kilauea̵7;s eruptions can last for years, even decades, on end. Conversely, the Earth̵7;s most violent eruptions come from long-dormant volcanoes known as rhyolite calderas. As the name implies, these volcanoes contain the most explosive type of magma, rhyolitic. Rhyolite caldera eruptions are thought to occur only every 10,000 to 30,000 years, or even less . Scientists estimate that Yellowstone -- a geologically active rhyolite caldera -- erupts on average only about once every 600,000 years, and is expected to be quite violent and far-reaching in its effects when it does.