RESULTS FROM FOUR CASCADE VOLCANOES
Mount Shasta (fig. 20), with an ice volume of 4.7 billion ft3, is about 40 miles south of the Oregon California border.
Snowmelt and glacial meltwater flow from Mount Shasta in four major drainage systems that are shown on plate 6. Several intermittent creeks that are tributary to the Shasta River and to the south branch of the Klamath River drain Mount Shasta, including its most developed glacier, Whitney. Thirty-nine percent of the area covered by snow and ice is in the Klamath River watershed. The massive volcano is drained on the northeast side by several intermittent creeks, which enter a small closed depression southeast of Whaleback Mountain. Numerous creeks drain the mountain's southeast side and enter the McCloud River, which flows south to the Sacramento River. The southwest sides of Mount Shasta and the satellite cone of Shastina are in the Sacramento River drainage area, where several intermittent streams drain the region before disappearing into the porous volcanic rocks above the town of Mount Shasta.
Though the main lobe of the Hotlum Glacier is the largest in area (19.4 million ft2) and in volume (1.3 billion ft3), the thickest ice measured on Mount Shasta is the 126 ft recorded on the Whitney Glacier. Whitney Glacier is the only glacier on Mount Shasta measured successfully by ice radar, having an area of 14 million ft2 and a volume of 0.9 billion ft3 (see table 5). Isopachs for Whitney Glacier are seen in plate 6.
TABLE 5.Areas and volumes of glacier ice and snow on Mount Shasta
[Methods of determination; A, volume estimated hy uaing area correlation; M glacier thickness measured hy ice radar. Because total glacier areas are required in the application of the volume eatimation method, volumes are available by total glacier. Area measured in ft3 (x 106]; volume measured in ft3 (x 109).in the area column means no ice or snow present for glaciers at that elevation, and in the volume column it means volume by elevation not determined for that glacier]
Mount Shasta is a compound stratovolcano composed of overlapping deposits erupted during a period of several hundred thousand years. Past eruptive events have included dome building, lava flows, pyroclastic flows, mudflows, and some small volume eruptions of tephra. Similar future eruptions could occur near the present summit or could form new vents such as Shastina and Black Butte, both west of Mount Shasta (Miller, 1980, p. 28).
Unlike the valleys on Mount Rainier, those on Mount Shasta are not of great length and thus allow mudflows, lava flows, and pyroclastics to form deposits around the flanks of the mountain rather than many miles distant (Miller, 1980, p. 31).
Lava generally flows in existing valleys and, because of its viscous nature, is limited in areal extent. In Holocene time, lava was erupted most often near the summit and less often from the lower flanks of Mount Shasta. Past lava flows have been thick and blocky and have rarely extended more than 5 miles from their source, which is in the presence of some snow and ice.
Pyroclastic flows, with the potential for covering large areas of ice and snow, have been frequent in the last 10,000 years and have flowed as far as 12 miles from their source (Miller, 1980, p. 14). Though their extent depends upon the composition of gas in the eruption, it is probable they will flow over snow and ice.
Floods are common events during volcanic eruptions because of snowmelt and icemelt. Those originating on volcanoes may be more damaging than floods else where due to their high sediment concentration. Mudflows, many unrelated to eruptions, have traveled more than 16 miles from the summit of Mount Shasta in the valleys of Mud, Ash, Whitney, and Bolam Creeks and in the valleys of the McCloud and Sacramento Rivers. Figure 21 illustrates that the largest volumes of Mount Shasta's ice are perched at the tops of these drainages, on the northern and eastern parts of the mountain. Ninety-four percent of the area covered by snow and ice is above 10,000 feet of altitude. Figures 22 and 23 show ice volumes for radar-measured glaciers and ice areas by drainage as a function of altitude.
More information about geological hazards at Mount Shasta can be found in the report by Miller (1980).
Last Updated: 28-Mar-2006