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Geology

Geologic Hazards

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Based on the geologic record of its past eruptions, pyroclastic flows and debris flows from Mount Shasta probably pose the greatest threats to surrounding communities. Lava flows, the growth of domes, and the blanketing of the surrounding terrain by tephra are thought to pose much lesser threats. The likely nature and scale of each of these hazards is described briefly below, and the section closes with a note on the probability of future eruptions.

Pyroclastic Flows

These hot, mobile suspensions of rock fragments in volcanic gases and superheated air can sweep down steep slopes at speeds in excess of 100 km/hr (Figure 28). Pyroclastic flows form as tephra-laden eruption columns fall back to earth or as the steep margins of domes and lava flows collapse and disintegrate. Dome growth and collapse have played major roles in each of Mount Shasta's eruptive episodes, and most of the pyroclastic flows that the volcano has produced are the results of this process.

About 9,500 years ago pyroclastic flows swept down from Shastina and its satellite, Black Butte, burning and burying the forests that stood where Weed and Mount Shasta City are today. Radiocarbon dating of charcoal from these forests dates the eruptions at about 9,500 years ago (Miller, 1978). The flow deposits have red (oxidized) tops and contain prismatically-jointed blocks, both of which indicate that they were emplaced at high temperatures (Figure 29). Similar pyroclastic flows from the Hotlum cone have travelled 10 to 20 kilometers down valleys on all sides of the mountain during the past several thousand years, and Miller (1980) estimates that the collapse of a dome high on Mount Shasta during a future eruption could create pyroclastic flows that would overrun low-lying areas up to 30 kilometers from the volcano (Figure 30).

Debris Flows

Sudden increases in runoff triggered by the rapid melting of snow and glacial ice or by heavy rains can mobilize large volumes of pyroclastic or glacial debris to produce these fast moving flows. Some debris flows have been initiated by volcanic eruptions, as hot lava or tephra melts the mountain's blanket of snow and ice. Others however, including the August 1997 debris flow in Whitney Creek, are simply the results of climatic fluctuations.

Both hot and cold debris flows have swept down canyons on all sides of Mount Shasta during the past 10,000 years, and some have travelled more than 30 kilometers from the summit. Because they need not be associated with volcanism, debris flow events are expected to occur more frequently -- and perhaps with less warning -- than eruptions. As Miller (1980) states, such flows "... are likely to cover broad areas in [a zone 20 to 30 kilometers from the summit] several times per century." Mapped hazard zones (Figure 31) suggest that towns in drainages on all sides of the mountain may be threatened by future debris flows.

Lava Flows and Domes

Because the lavas erupted from Mount Shasta are predominantly "pasty" andesites and dacites, flows on the mountain tend to move slowly and travel relatively short distances. These blocky streams and masses of molten rock pose little direct threat to people or movable property. The longest flow on Mount Shasta, for example, is the Military Pass andesite flow which extends 9 km downslope from its vent near the base of the Hotlum dome. This flow formed about 9,000 years ago, early in the most recent eruptive episode (Miller, 1980), and its modest length suggests that even flows erupted from vents low on the mountain's flanks are not likely to reach more than 15 to 20 kilometers from the summit. Perhaps the greatest hazard that both lava flows and domes pose is from the pyroclastic flows that may be formed as a result of their sudden collapse or explosive disintegration.

Tephra

Historically, Mount Shasta has produced relatively little tephra in comparison to other Cascade volcanoes. Tephras are commonly composed either of pumice (bubbly volcanic glass) or lithic fragments (bits of older, dense lava shattered by explosions) (Figure 32). The Red Banks eruption 9,600 years ago produced one of Mount Shasta's few recent deposits of pumiceous tephra. This tephra was deposited across an area of at least 350 square kilometers on the eastern side of the mountain and has a maximum thickness of about 50 centimeters (Miller, 1980). A smaller deposit of lithic tephra was formed by explosions at or near the Hotlum dome about 200 years ago. This tephra, which looks like fine yellowish-gray sand, is spread widely over Mount Shasta's northeastern flank (Christiansen and others, 1977) and has locally accumulated to thicknesses of at least a meter thick where it has been washed or blown into surface depressions.

Prevailing winds are likely to carry most tephra from future summit eruptions to the east and northeast so that large accumulations probably will not occur in the most densely populated areas on the western and southern flanks of the mountain. However, even a few centimeters of tephra might be enough to close Interstate 5, shut down services in the nearby communities, and disrupt the air traffic that uses Mount Shasta as a navigational landmark (Crandell and Nichols, 1987).

Epilogue

In light of the Mount Shasta volcanic system's nearly 600,000 year eruptive history and the continuing geothermal and seismic activity on and around the mountain today, future eruptions are considered very likely. Although predicting the exact times and natures of volcanic eruptions is notoriously difficult, two techniques are used to estimate the timing of future eruptions. First, the mountain is monitored for physical changes -- such as increased seismicity, uplift, and the emissions of heat and volatiles -- that might be associated with the rise of magma into the shallow crust. Under favorable circumstances such changes may give months to weeks of warning in advance of an eruption. Second, for a longer-term perspective, geologists map and date the mountain's ancient deposits in order to reconstruct its eruptive history. This information can then be used to calculate the average recurrence intervals for various types of events. Perhaps the best way to conclude this summary of Mount Shasta's potential hazards is with a quote from Crandell and Nichols (1987) on the chances of when its next eruption will occur:

Studies by geologists show that Mount Shasta has erupted 10 or 11 times during the last 3,400 years and at least 3 times in the last 750 years. Mount Shasta does not erupt at regular intervals, but its history suggests that it erupts at an average rate of roughly once per 250 to 300 years. If the behavior of the volcano has not changed, the chance is 1 in 25 to 30 that it will erupt in any one decade and 1 in 3 or 4 that it will erupt within a person's lifetime.
 

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