Mount Shasta is a compound stratovolcano that has been built by repeated eruptions during the past 200,000 years. Although the mountain itself is relatively young, it has been built atop older basalts and andesites whose ages indicate that volcanism has been taking place at the site of the present cone for at least the past 600,000 years.
Pre-Shasta basalts form a number of shield volcanoes, such as Everitt Hill and Ash Creek Butte, that stand just south and east of the mountain Figure 16. A suite of coeval andesites, which crop out on Mount Shasta's southwestern flank, are the remnants of an earlier stratocone that stood on the site of the present mountain until sometime between about 360,000 and 160,000 years ago. The youngest rocks from this "ancestral Mount Shasta", which yield a POTASSIUM-ARGON DATE of 360,000 years (Chesterman and Saucedo, 1984), are found as blocks in the massive debris avalanche that blankets the western Shasta Valley. Mapping of the avalanche deposit by Crandell and others (1984) has shown that the avalanche flowed at least 43 kilometers northwestward from the base of the Mount Shasta and contained at least 26 cubic kilometers of material. Sedimentary rocks incorporated into the avalanche deposit (Ui and Glicken, 1986), and soft sediment injected into it along fractures, indicate that at least part of the Shasta Valley was covered by marshy lake and stream deposits when the avalanche swept across it. Following the collapse of the northern flank of ancestral Mount Shasta, olivine basalt lavas flowed from a vent between The Whaleback and Deer Mountain and spread across the eastern Shasta Valley. These basalts, which are about 160,000 years old (Christiansen and Ernst, 2001), buried the eastern part of the avalanche deposit and formed several large lava tubes including Pluto and Barnum Caves.
Modern Mount Shasta has been built atop the remains of its collapsed predecessor during four relatively brief eruptive episodes, each of which was centered at a separate vent. The locations of these vents are shown in Figure 17, and the chronology of the eruptive episodes and intervening glaciations is summarized in Table 1. The pattern of volcanic activity was similar during each episode, and began with the eruption of roughly equal proportions of two-pyroxene andesite lavas and pyroclastic flows from a central vent. The absence of erosional features or soil horizons between successive deposits suggests that each "cone-building" phase lasted no more than a few hundred to a few thousand years (Christiansen and others, 1977). These brief periods of intense eruptive activity were separated by longer intervals during which hornblende-bearing andesite and dacite domes rose to fill the earlier craters. During each of the first three episodes, the end of activity at the central vent was followed by minor eruptions of dacites or basaltic andesites on the mountain's flanks.
Sargents Ridge Cone: This oldest cone forms the southeastern part of the mountain, and a segment of its crater rim still stands above Thumb Rock near the head of Mud Creek. Although the cone has been deeply dissected during two episodes of glaciation you can imagine what it once looked like by projecting the northwest-dipping strata exposed below Thumb Rock towards the southeast-dipping ones exposed on Sargents Ridge (Figure 18).
Misery Hill Cone: This second cone grew atop the glaciated northwestern flank of the first, and makes up a large part of the present mountain. Part of its crater rim stands between the summit and Shastina (Figure 19), and the dome that fills its crater is a prominent landmark to those climbing to the summit from Avalanche Gulch. Although the main phase of Misery Hill volcanism preceded a late Pleistocene glaciation that ended about 10,000 years ago, eruption of the 9,600 to 9,700-year old Red Banks pumice post-dated this glaciation (Christiansen and others, 1977). The pumice forms a 350 square kilometer airfall deposit on the eastern side of the mountain, and a 100-meter thick arc of sintered fragments that stands as a prominent cliff across the head of Avalanche Gulch. The Red Banks pumice was erupted from a zoned reservoir, and the yellowish dacite fragments that form the cliffs at the head of Avalanche Gulch give way to dark-brown andesite fragments as one climbs stratigraphically higher through the deposit.
Shastina Cone: Shastina forms a separate peak 3 kilometers west of Mount Shasta's summit. Its early activity produced a small crater in the saddle between Shastina and the summit as well as several tongues of andesitic lava that flowed from a second, more westerly vent where the main cone later grew (Figure 20). The growth of Shastina's main cone was followed by the development of four or five small domes on the floor of its central crater (Figure 21). Explosions related to the emplacement or destruction of one of these domes caused the western side of the cone to collapse, forming Diller Canyon and triggering pyroclastic flows that buried the present sites of Weed and Mount Shasta City.
Black Butte, a complex of hornblende dacite domes that stands next to Interstate 5 between Mount Shasta City and Weed, formed during a late phase of the Shastina episode. It grew in four distinct pulses from a crater that had opened about 12 kilometers west of Shastina (Figure 22). A detailed study of dacites from Black Butte (Katz, 1997) suggests that all of the domes were fed from the same reservoir, and that the ascent of each batch of magma continued without pause for at least 8 days once it had begun. RADIOCARBON DATING indicates that the entire Shastina eruptive episode lasted no more than a few hundred years (Miller, 1980).
Hotlum cone: The eruptive products that form this fourth cone crop out mostly on the northeastern side of the mountain, and the dome that fills its crater forms the present summit (Figure 23). The earliest eruptions from the Hotlum vent occurred at the same time as those from Shastina, but most of the cone has grown since the retreat of large glaciers from the mountain about 6,000 years ago (Christiansen and Miller, 1997). Early eruptions produced several andesitic lava flows on the northern and eastern sides of the peak, including the 9 kilometer long Military Pass flow. Growth of the summit dome was followed, perhaps no more than 200 years ago, by explosions that blasted out its central part and produced an eruption cloud that spread gray lithic tephra widely across the northern flank of the mountain.
Three of Mount Shasta's major vents, as well as many smaller ones, are aligned along a north-trending zone that passes through the summit Figure 17. This linear zone parallels local faults and the trend of older rock units exposed south and west of the volcano, suggesting that bedrock structure has partially controlled the geometry of Mount Shasta's development (Christiansen and others, 1977).
Today, a field of sulfur-encrusted FUMAROLES high on the Hotlum-Bolam ridge and a small group of boiling springs just west of the summit are the main signs of thermal activity on Mount Shasta (Figure 24). Modeling of data from a magnetic survey conducted in the mid-1970s has shown, however, that the main body of the mountain is less magnetic than Shastina. Because rocks from both parts of the volcano have similar magnetic properties this difference in field strengths may reflect a weakening of rock magnetism at high temperatures beneath the Hotlum cone due to the presence of a buried body of hot rock or magma (Christiansen and others, 1977).
During the past 20 years, an average of about 5 earthquakes with magnitudes ³1 have occurred beneath Mount Shasta every year. From time to time this background seismicity has been punctuated by earthquake swarms in which many quakes with similar magnitudes have occurred during a short span of time (Figure 25). The most seismically active area beneath the mountain lies about 18 kilometers southeast of Mount Shasta City at a depth of 10 to 12 kilometers (Figure 26). A SEISMOGRAM from a small (M 2.4) earthquake that occurred in this area during the fall of 1992 is shown in Figure 27.
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