9.10: Volcanic Hazards
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When comparing the two volcano types, shield and composite, it is obvious that although the shield volcanos are more massive (see Figure 9.9), they are far less dangerous to the population than the smaller composite volcanoes. Shield volcanoes produce basaltic lavas that may fountain at the vent, due to gases, but end up flowing passively down the flanks of the volcano (Figures 9.3 and 9.12). Other than property damage, anyone living on or near a shield volcano is not likely to perish due to a volcanic eruption. This is not the case for the composite volcanoes; explosive eruptions produce a lot of volcanic fragments, called pyroclastic debris or tephra, that range in size from dust and ash to large blocks (or bombs) of volcanic material (Figure 9.13). Pyroclastic debris at first travels high up into the atmosphere into an eruption column, which then spreads outward along with the prevailing wind direction, but during the explosive phase of the eruption, the central vent widens as the rocks around the vent are also blown during the eruption; this widening of the vent results in less upward momentum of the eruption column. As a result, the pyroclastic material travels down the flanks of the volcano as a pyroclastic flow, a very dangerous mixture of hot volcanic material and noxious gases.
Although pyroclastic flows are extremely dangerous, most deaths associated with composite volcanoes are from mudflows, called lahars. Many composite volcanoes are capped by snow and ice, and even a small eruption can result in meltwater running down the sides of the volcano. This water can easily erode the ash and other volcanic debris on the flanks of the volcano and result in a fast-moving slurry of mud and larger material such as trees and boulders. Lahars move swiftly through river channels and can endanger any town or city that is built in the low lying areas downstream from the volcano (Figure 9.13). Lahars can also be generated by large amounts of rainfall in the area. Several ancient mudflow deposits are recognized in the Cascade area of Washington and Oregon. Because of the proximity of major cities and many towns in the vicinity of these dormant volcanoes (and the recently classified “active” Mt. St. Helens), volcano monitoring systems are in place. For example, seismic activity is monitored, as earthquakes are generated by the upward migration of magma beneath the volcanic structure, and GPS technology is used to monitor any changes in slope, as magma may push the sides of the volcano outwards and increase the angle of the volcano’s slope. Also monitored are the stream valleys on the volcano to detect lahars that can happen at any time, regardless of the active or dormant status of a composite volcano.