Is Montana at Risk?
Identified Hazards for the State of Montana
Basic Disaster Information:
VOLCANIC FALLOUT
There may be no natural event that can cause greater damage to its surrounding area than the eruption of a volcano. However, eruptions can be mild, almost passive events that have little effect outside their immediate surroundings. The range of destructive capability is a function of the type of volcanic activity. Montana is bordered on two sides by areas of volcanic activity. To the west are the volcanoes of the Cascade Range (Mt. St. Helens, Mt. Rainier, Mt. Hood) and to the south is the Yellowstone Caldera.
The degree of destructiveness of a volcanic eruption is dependent on many factors. The frequency, magnitude, and duration of the eruptions, the nature of ejected material, the pre-existing topography, and the weather conditions interact with such variability that a precise assessment of potential hazard is impossible. The risks associated with each type of eruption can be discussed.
There are two major types of volcanos, shield volcanos and composite volcanos. Shield volcanos are found in the ocean and are typical of those in the Hawaiian Islands. Composite volcanos are found on the continents and are typical of those in the Cascades. They are very explosive and often send huge clouds of ash and other volcanic debris far up into the atmosphere. The ash clouds can cover large areas and can even drop several inches of as thousands of miles from the volcano. The damage caused by these volcanos is extensive and widespread. It is this type of volcano that affects Montana.
The distribution of ash from a violent eruption is a function of the weather, particularly wind direction and velocity, and the duration of the eruption. As the prevailing wind in the mid-latitudes of the northeastern hemisphere is generally from the west, ash is usually spread eastward from the volcano. Exceptions to this rule do occur, however. Ash fall, because of its potential widespread distribution, offers some significant volcanic hazards. It can affect our health, structures, transportation, agriculture and even the climate.
Although there are no active volcanoes in Montana at present, there have been several eruptions in the recent geologic past (between 30 and 100 million years ago.) There are many well-exposed remnants of this latest volcanic activity in several counties throughout Montana; however, there is no reason to believe these become active gain anytime soon. Our biggest concern is the fallout that will occur from eruptions of nearby volcanos and we have experienced this type of event numerous times.
The May 1980 eruption of Mt. Saint Helens resulted in the deposition of up to three inches of uncompacted ash in western Montana, tapering to near zero in eastern Montana. It is estimated that the ashfall cost Missoula nearly $6 million in cleanup and lost work time. The statewide cost has been estimated at between $15 and $20 million.
Another areas of volcanic activity that has affected Montana is the past and could pose a serious threat in the future is the Yellowstone Caldera in northwestern Wyoming and northeastern Idaho just south of the Montana border. A caldera is a term for a large volcanic crater. The Yellowstone Caldera is 45 miles across at its greatest diameter. The eruptive center has migrated along the axis of the present Snake River Plain over the past 10 million years, with periodic eruptions generating massive pyroclastic flows. The last three eruptions, about 2.1, 1.3 and .6 million years ago, occurred in the Island Park/Yellowstone region of southwest Montana and northwest Wyoming. Fortunately for mankind, an eruption comparable in magnitude with those of Yellowstone has not occurred during recorded history.
