Mount Shasta, a 400 km3 volcano in northern California (United States), is the most voluminous stratocone of the Cascade arc. Most Mount Shasta lavas vented at or near the present summit; relatively smaller volumes erupted from scattered vents on the volcano’s flanks. An apron of pyroclastic and debris flows surrounds it.
Shastina, a large and distinct cone on the west side of Mount Shasta, represents a brief but exceptionally vigorous period of eruptive activity. Its volume of ∼13.5 km3 would make Shastina itself one of the larger Holocene Cascade stratovolcanoes. Its andesite-dacite lavas average 63 wt% SiO2 and have little compositional or petrographic variation; they erupted almost entirely from one central vent, although a single vent below Shastina’s north side erupted a flow of the same composition. Eruptions ended with explosive enlargement and breaching of the central crater and successive emplacement of four, more-silicic dacite domes within the crater and pyroclastic flows down its flank. Black Butte, a large volcanic dome and pyroclastic complex below the west flank of Shastina, is petrographically and chemically distinct but only slightly younger than Shastina itself, part of a nearly continuous Shastina–Black Butte eruptive episode.
Shastina overlies the widespread pumice of Red Banks, erupted from the Mount Shasta summit area and 14C dated at ca. 10,900 yr B.P. (calibrated). Shastina and Black Butte pyroclastic deposits have calibrated 14C ages indistinguishable from one another at ca. 10,700 cal. yr B.P. A cognate granitic-textured inclusion in a late Shastina lava flow yields a 238U-230Th date on zircons within error of those ages. Our conclusion that the entire, voluminous Shastina–Black Butte episode lasted no more than a few hundred years is confirmed by almost identical remanent magnetic directions of all of the lavas and pyroclastic deposits. Although extremely similar, the remanent magnetic directions do reveal a short path of secular variation through the eruptive sequence. We conclude that the entire Shastina–Black Butte eruptive episode lasted no more than ∼200 yr.
The magmas that produced the Shastina and Black Butte eruptions were separate individual bodies at different crustal levels. Each of these eruptive sequences probably represents magma approximating a liquid composition that experienced only minimal differentiation or crustal contamination and remained separated from the main central conduit for most eruptions of Mount Shasta. The probability of another rapidly developing, brief but voluminous eruptive episode at Mount Shasta is low but should not be ignored in evaluating future possible eruptive hazards.
Mount Shasta, a composite volcanic cone in the southern Cascade Range and a notable landmark in northern California (United States), is the most voluminous stratocone of the Cascade arc, at ∼400 km3 (Blakely et al., 2000). The arc, related to the Cascadia subduction zone along the coast of the Pacific Northwest of North America, extends 1250 km from southern British Columbia (Canada) to Lassen Peak 120 km south of Mount Shasta in northern California (Fig. 1). Among Cascade volcanoes, Mount Shasta, at 4322 m, is second in elevation only to Mount Rainier in Washington (4392 m). Lavas and pyroclastic materials accounting for most of Mount Shasta’s volume erupted mainly from a cluster of vents at or near the volcano’s present summit; smaller volumes erupted from scattered vents around the mountain’s flanks.
Shastina is a notable exception to this overall pattern. It is a large and distinct subsidiary cone on the western side of the main edifice of Mount Shasta, centered a little more than a kilometer from the summit of the peak (Figs. 2 and 3) and represents a very brief but exceptionally vigorous period of eruptive activity. Shastina’s voluminous lavas and pyroclastic materials, their rapid emplacement, and their distinctive magmatic evolution constitute a remarkable episode in the growth of the larger Mount Shasta. Black Butte (Figs. 3 and 4), a volcanic dome complex and related pyroclastic deposits below the western flank of Shastina, is barely younger than the Shastina cone itself and represents part of a nearly continuous Shastina–Black Butte high-rate eruptive episode. Another such rapid and voluminous eruptive episode, should one occur, could present significant hazard from the large volcanic system of Mount Shasta.
BRIEF SUMMARY OF MOUNT SHASTA’S VOLCANIC EVOLUTION
As is typical of such large stratocones, Mount Shasta has been intermittently active over a span of hundreds of thousands of years. Shasta’s intermittent growth over at least 700,000 yr has produced a group of overlapping volcanic edifices erupted from a cluster of central vents, each vent separated from the others by no more than ∼1600 m (Christiansen et al., 1977, 2017b; Christiansen and Miller, 1989; Calvert and Christiansen, 2011, 2013). The composite edifice consists mainly of five volcanic cones that appear to have grown from central vents, some of them in periods of time that were short relative to the times between major cone-building episodes. Fewer eruptions occurred between the major cone-building events, many of them from flank vents. In addition to the predominant lava flows that constitute most of Mount Shasta, there are also clastic deposits of various ages. Many of them are either debris-flow deposits or lithic pyroclastic flows that probably resulted from collapse of volcanic domes or other steep volcanic constructs.
The oldest of the volcanic segments, designated the Sand Flat cone, was perhaps the largest of all. It grew over a period from as early as ca. 700 ka to ca. 400 ka (Christiansen et al., 2017b). However, much of it was destroyed by a major sector collapse sometime between ca. 430 and 320 ka, and little is known of its original constitution, vent(s), or form. The debris-avalanche deposit produced during the sector collapse has a volume of ∼45 km3 (Crandell et al., 1984; Crandell, 1989). Nevertheless, the remaining, largely buried edifice probably has a total volume of ∼180 km3 (Blakely et al., 2000), including part of the volcano that lies isostatically depressed beneath the level of the surrounding surface, perhaps as well as part of its intrusive core. The major exposures of Sand Flat lavas on the Mount Shasta edifice occur on the southwestern side (Fig. 3), as high as ∼2100 m. The distal margins of a few flows as well as some small lava domes emerge from beneath younger lavas low on the northern, western, and southern flanks of the cone. The central eruptive vent area for the Sand Flat cone may have been removed in the sector collapse.
The next-younger cone of the composite edifice, the Sargents Ridge cone, erupted largely from a central vent now located high on the southern side of Mount Shasta. It appears to have formed in several episodes of eruptions between ca. 285 and ca. 125 ka (Christiansen et al., 2017b). The Sargents Ridge cone has a volume of ∼85 km3 (Blakely et al., 2000). Its major exposures are on the southern flank (Fig. 3), but, as with the older Sand Flat cone, the distal portions of several flows emerge from beneath younger lavas on the northern and western flanks. Several flank vents related to the Sargents Ridge cone occur along a linear zone both north and south of the central vent as well as low on the western flank of the cone.
A third major cone-building episode formed the Misery Hill cone during the Late Pleistocene, mainly between ca. 50 and 20 ka, from a vent high on the northern side of the Sargents Ridge cone. The Misery Hill cone has a volume of ∼60 km3 (Blakely et al., 2000). Lavas of this episode form much of the southeastern side of Mount Shasta and spill over the Sargents Ridge cone on the northern, western, and southwestern sides (Fig. 3). Isotopic ages show that Misery Hill flank domes occur along the same roughly north-south band of vents across the summit that formed during the Sargents Ridge episode.
The two youngest cones, the Hotlum and Shastina cones, are of Holocene age. Only Shastina is separated from the other recognized cone segments by a distance great enough to make it immediately evident as a separate part of the greater edifice, lying ∼1600 m west of the other, more tightly clustered cone segments (Figs. 2 and 3). The Shastina cone (Fig. 5) has a volume of ∼13.5 km3 (Gardner et al., 2013), rises to a local summit elevation of 3758 m, and would itself be one of the major cones of the Cascade arc even if it were not part of the much larger Mount Shasta edifice. Its only flank vent formed a lava flow field low on the northern side. The large Black Butte dacitic dome-and-pyroclastic complex lies low on its western flank (Figs. 4 and 5). The brief Shastina–Black Butte episode is among the most voluminous eruptive episodes in the Cascade arc during the Holocene, perhaps second only to the caldera-forming Mazama–Crater Lake eruption in southern Oregon. It is largely the point of this paper to demonstrate the great rapidity with which this episode occurred. The Hotlum cone erupted partly contemporaneously with Shastina but is largely younger.
Plinian or subplinian deposits are rare at Mount Shasta. One major pumiceous unit, however, is present as a prominent feature—Red Banks—high on the volcano (Fig. 5) and also as widespread pyroclastic fall and flow deposits on all sides of the mountain (except, perhaps, part of its western sector; Fig. 3) and more distally to the east and northeast.
SHASTINA AND BLACK BUTTE STRATIGRAPHY AND ERUPTIVE HISTORY
Following the final lava eruptions from the Misery Hill cone, the Red Banks pyroclastic eruptions and related debris flows erupted from near the Misery Hill central vent, located immediately south of the present Mount Shasta summit. Eight new 14C dates were obtained from charcoal in deposits of the Red Banks episode from several locations on Mount Shasta and to the northeast (Table 1). The total range, including 2σ error limits, of four concordant calibrated radiocarbon ages of Red Banks deposits (discussed more fully in a later section) is between 11,122 and 10,716 cal. yr B.P. The entire Shastina edifice postdates the Red Banks episode, as may a few late Misery Hill lava flows that underlie Shastina lavas near its western base (units MHad1–MHad5, Fig. 5).
All of the major Shastina cone-building flows share common petrographic and chemical characteristics, notably in having few phenocrysts more than ∼½–1 mm across. In distinct contrast to most other Mount Shasta lavas, Shastina lavas lack abundant large, generally strongly embayed plagioclase phenocrysts, cumulocrysts of plagioclase and pyroxene, or other significant evidence of chemical or thermal magmatic disequilibrium. In contrast, small, typically nonembayed or only slightly embayed plagioclase crystals are abundant, and the groundmasses commonly are wholly or partly glassy. Compositions of the lavas are generally either mafic dacite or silicic andesite, typically having SiO2 near 63%.
The stratigraphy of the individual lava flows that form the Shastina cone is not entirely evident, but a general sequence of recognized units is indicated in Figure 5.
The youngest recognized lavas of Shastina form a crater-filling cluster of volcanic domes and a field of related pyroclastic flows, somewhat more differentiated than the Shastina cone-building units. A unique dacitic dome complex—Black Butte—and related pyroclastic materials (Figs. 4 and 5) postdate the main Shastina edifice stratigraphically, but so closely that their isotopic ages are indistinguishable. The range of median calibrated radiocarbon ages on samples of four Shastina and Black Butte pyroclastic flows, discussed more fully later, is only between 11,061 and 10,587 a (Table 1). Thus, the entire history of the Shastina–Black Butte eruptive episode is brief, within no more than ∼400 yr by 14C dating.
Shastina eruptive materials can be roughly grouped into several stratigraphic packages, as suggested in Figure 5. After an initial explosive crater-forming eruption, an early group of lavas formed the main cone of Shastina; somewhat younger lavas define two groups erupted from the central-vent crater, one onto the slopes southwest of the Shastina summit crater and another to the northwest. It is possible to discern at least partial stratigraphic sequences within each of these parts of the cone, but most of the two local sequences cannot be unambiguously placed relative to each other. The cone-building period ended with dome emplacements and explosive disruptions in the summit crater and resulting pyroclastic flows emplaced toward the west. Black Butte and its pyroclastic apron at the southwestern base of Shastina include the youngest events of the eruptive episode.
Sisson Lake Crater
The oldest feature recognizable on Shastina is the remnant of a crater formed by explosive eruptions, apparently before emplacement of any of the lava flows, domes, and pyroclastic flows of the main Shastina edifice. A portion of the eastern rim of that crater remains exposed beneath the younger main Shastina cone (Fig. 6) and appears to be a remnant of a crater that originally was ∼175 m in diameter. Sisson Lake (Fig. 5) is enclosed in the space between this remnant and the younger, main Shastina cone. Only sparse outcrops of the deposits that formed the early crater can be seen because of burial by talus and pyroclastic materials, but what can be seen is a medium-grained bedded lithic-rich pyroclastic deposit (unit STasl, Fig. 5). It is highly altered by hydrothermal mineralization, its initial porosity permeated by silica and gypsum. It seems likely that the Sisson Lake crater formed by hydromagmatic eruptions, perhaps induced by the encounter of rising magma with melt waters from a glacier on Mount Shasta. Other than in this deposit, little hydrothermal mineralization is found on Shastina. Pyroclastic blocks form an apron on the eastern flank and near the southeastern base of the crater (unit STps) and may have been ejected in the crater-forming explosions. Petrographically and chemically, these blocks (e.g., sample 75SH-299, Table 2) resemble the younger Shastina lavas.
Earliest Shastina Cone Eruptions
The main edifice of Shastina comprises a fairly uniform group of high-silica andesite and low-silica dacite lava flows.
The oldest lava flows of the Shastina cone appear to be those exposed in the drainage area of Diller Canyon, on the western flank of the mountain (unit STadc, Fig. 5). It is not clear just how many flows are represented among the exposures in and adjacent to Diller Canyon. Pyroclastic flows that overlie these lavas largely bury but do not entirely hide the morphology of at least some of the flows. Their steep lateral margins, flow levees, and flow fronts are the most commonly exposed parts of these flows, and the presence of several flow-front lobes suggests that a number of individual lava flows erupted from the central vent of Shastina and now emerge locally from beneath the covering pyroclastic flows.
A relatively small exposure of early lavas on the northeastern side of Shastina (unit STae) extends from the Shastina summit-crater rim onto the steep slope above Whitney Glacier. Much of this area is isolated within a field of talus that is almost constantly replenished by rockfalls from the Shastina cone onto its northeastern flank and the surface of the adjacent glacier. Consequently, it is exceedingly difficult to sample these lavas adequately or to establish their detailed stratigraphic relations to other Shastina cone-building lavas.
Lavas Southwest of the Shastina Summit
A group of ten lava flows that are younger than those of the Diller Canyon area erupted from Shastina’s central vent and descended the southwestern flank of the cone between Diller Canyon and Cascade Gulch (Fig. 5), which separates the southern part of Shastina from the older Misery Hill cone. Not all of these flows can be placed into a single stratigraphic sequence, but groups of them can be placed relative to one another (Fig. 5). Three flows in this southwestern group appear to be the oldest of the sequence. One of them is a single flow ∼8 km long that extends from the crater rim to McBride Springs (Fig. 5) at an elevation of ∼1517 m. This flow (unit STam3) seems to be younger than the other two of this group; only a few outcrops of unit STam2 are visible, directly underlying unit STam3 along the western side of Cascade Gulch. Unit STam1 is not directly overlain by either of the other two, but its distribution, emerging from beneath several other younger flows, makes it unlikely to have been emplaced after the long flow that reaches McBride Springs (unit STam3).
Unit STac1 descended at least in part on the northwestern side of the McBride Springs flow, where several separate lobes are tentatively correlated with one another. Overlapping the northwestern margin of unit STac1, another flow (unit STac2) has a distinct set of flow levees that mark its descent from the summit vent. About halfway down its course, the flow appears to divide into two separate side-by-side lobes that spread out onto the older lavas of the Diller Canyon area. The exact nature of this divergent pattern is somewhat obscured by younger pyroclastic flows that partly mantle unit STac2, but its general course is apparent.
Two sets of two lava flows each overlie unit STac2. On the upper slopes of Shastina south of Diller Canyon, a pair of flows (units STads1 and STads2) lies above outcrops of unit STac2. Unit STads1 has a pair of prominent flow levees and spreads out into a broad lobe at its terminus. Only two small flow-front outcrop areas of unit STads2 extend outward from beneath talus that covers the upper slopes of Shastina below the summit-crater rim. Unit STac3 is a much larger lava flow that covered much of the group of earlier flows that descended southwestward from the summit crater, directly overlying parts of each of the earlier flows and spreading to both sides of the stack of earlier flows. Unit STac4 also directly overlies the older flows and diverges into two lobes toward its lower end.
The youngest lava flow in the southwestern group (unit STac5) extends from beneath the talus on the upper slopes of Shastina to form the crest of the ridge west of Cascade Gulch (Fig. 5).
Lavas Northwest of the Shastina Summit
A somewhat more robust stratigraphic sequence can be worked out among the lava flows that cover the Shastina cone on its northwestern side (Fig. 5). All of them appear to have erupted from the central vent, although their original continuity to the crater rim is obscured by younger pyroclastic flows and talus. The largest area of this sequence is blanketed by the complex northwest lava-flow field (unit STanw), which may represent a single effusive event but has many overlapping flow lobes. Boundaries between individual lobes commonly can be mapped in the lower parts of the flow field but lose their identities as they are traced upward toward their source at the summit crater.
Other flows can be identified individually in their relative stratigraphic positions. Two sequences of flows underlie the northwest flow field: four flows (units STas1, STas2, STas3, and STas4) emerge from beneath the southern margin of flow field STanw, and four others (units STan1, STan2, STan3, and STan4) underlie the northern margin. Four flows directly overlie the northwest flow field: units STau1 and STau2 on the south, and units STauw1 and STauw2 farther east. The latter two span the upper drainage of Whitney Creek, as discussed more fully in a following section.
The only cone-building Shastina lava flows that clearly did not erupt from the central vent of the cone form an extensive flow field immediately to the north. Within this group, locally called “Lava Park” (Fig. 5), six sequential emplacement units can be recognized (units STal1–STal6, Fig. 5), all of them erupted, apparently in a continuous succession, from the same flank vent at an elevation of ∼1830 m. Although younger talus partly obscures the contact with the northwest lava-flow field, local contact relations do show that eruptions from the Lava Park vent postdate unit STanw.
Shastina Volcanism near Whitney Creek
Clear lithologic, chemical, and paleomagnetic similarity strongly indicates that units STauw1 and STauw2, exposed along the middle course of Whitney Creek and continuing along the western rim of the Whitney Creek canyon below Whitney Falls (Fig. 5 and 7), are part of the Shastina lava complex. Flows STauw1 and STauw2 overlie both the northwest lava-flow field of Shastina (unit STanw) and a lava flow and pyroclastic rocks of the Hotlum cone, derived from a vent at Mount Shasta’s present summit. The thick andesitic lava flow (unit HTawf, Fig. 5) that holds up Whitney Falls at the head of Whitney Creek canyon and underlies units STauw1 and STauw2 is similar chemically and petrographically to lavas exposed near the terminus of the Bolam Glacier (Fig. 5) that demonstrably erupted from the Hotlum cone. As detailed in later sections, the Whitney Falls flow yields a 40Ar/39Ar plateau age within error limits of the radiocarbon ages determined for Shastina pyroclastic flows that were generated by collapse of the complex of volcanic domes in the Shastina summit crater, and paleomagnetic results are similar as well. The exposure of unit STauw1 along the western rim of the canyon of Whitney Creek (Fig. 7) also overlies a thick sequence (unit HTpwc included within unit HTpf) of dacitic block-and-ash flows and a pumiceous ash flow exposed on both banks of Whitney Creek (Fig. 5 and 7). A small outlier of the northwest lava-flow field (unit STanw) crops out beneath the Whitney Falls flow (unit HTawf) high in the western bank of Bolam Creek (Fig. 5). Thus, at least part of the Hotlum cone erupted within the same short period of time as the Shastina cone.
Domes and Pyroclastic Volcanism of the Shastina Summit Crater
The youngest eruptions from the Shastina summit crater formed a series of nested domes and related pyroclastic flows. It is probable that the earliest of these domes initially filled the summit crater and partly spilled onto the uppermost northern and southern slopes near the rim, where outcrops of the original dome (unit STd1) remain (Fig. 5). The dome, however, was explosively disrupted and partly collapsed. Most of its mass was ejected westward down the slope of the cone, in part forming some of the extensive talus that mantles the upper slope, in part probably eroding the course of Diller Canyon into that slope, and partly spreading as a fan of pyroclastic debris down the canyon and across the lower slopes of the cone (Figs. 5, 8, and 9).
Although the steep uppermost slopes of Shastina are nearly completely mantled by blocky talus, it is probable that some of this debris was initially part of the pyroclastic ejecta from the earliest dome explosion. A clear boundary is evident within the talus field as a sharp change in overall color that diverges outward from near the head of Diller Canyon (Figs. 9 and 10). Much of the talus that mantles the slopes immediately below the crater rim has a reddish cast, probably reflecting the oxidized lava-flow surfaces that have spalled to form much of the debris. West of this boundary, lighter, more grayish blocky ejecta, apparently from the interior of dome STd1, are strewn across the slopes adjacent to upper Diller Canyon.
After explosive disruption of the oldest dome of the summit complex, a series of smaller domes was emplaced into the summit crater, each successive dome partly filling an explosion and collapse crater in the previous dome. The dome (unit STd2) that formed next after nearly complete removal of the initial dome rises conspicuously above the crater rim, and in views from the southeast appears nearly intact (Fig. 10). Two smaller, younger domes (units STd3 and STd4) each were emplaced within craters formed by explosive failure of the western side of the preceding dome. In each instance, much of the younger dome collapsed catastrophically and generated pyroclastic flows that spread onto the apron formed by earlier dome-collapse flows.
The pyroclastic debris that accumulated on the slopes of Shastina and spread as an apron to the west (unit STpf, Fig. 5) continued as pyroclastic flows onto the nearby lowlands and along drainages as far as 20 km from the Shastina summit, especially down the valley of Boles Creek (Fig. 3) as far as the Shasta River. These pyroclastic flows were studied and described by Miller (1978, 1980), who recognized clearly their source in the Shastina summit domes and their stratigraphic position relative to the succeeding pyroclastic flows from Black Butte. The Shastina-summit pyroclastic flows consist almost entirely of nonvesicular dacite blocks, generally with prismatic joints perpendicular to initial cooling surfaces. They are identical in lithology to the Shastina summit domes, with phenocrysts of plagioclase, two pyroxenes, and amphibole. These crystals, especially the mafic minerals, are commonly larger (1–2 mm) than those of the cone-building lava flows. Miller noted that multiple flow units, commonly each with a pinkish oxidized top, constitute these deposits; it is likely that the individual flow units relate to individual collapse episodes in the Shastina summit-dome complex.
Domes and Pyroclastic Volcanism of Black Butte
The last event in the Shastina–Black Butte eruptive episode was emplacement of the large and lithologically distinct volcanic dome complex of Black Butte, below Shastina’s southwestern flank (Figs. 3, 4, and 5). Although, as noted earlier, the lava flows and domes of the main Shastina edifice all share many major lithologic characteristics, the Black Butte domes and related pyroclastic materials are uniquely characterized by conspicuously large hornblende phenocrysts, no pyroxene phenocrysts, and small plagioclase phenocrysts (McCanta et al., 2007). The aggregate volume of the Black Butte dome complex itself is ∼2.4 km3, and that of its pyroclastic apron is at least as great or greater (Gardner et al., 2013). The emplacement history of the Black Butte sequence was complex, and some aspects of its earliest events remain unclear. The sequence entirely postdates the domes and related pyroclastic deposits that erupted from the Shastina summit crater, although no visible gullying or significant soil formation occurred between pyroclastic deposits from the Shastina summit and the Black Butte deposits.
The outermost part of the Black Butte complex is a pyroclastic field (parts of unit BBsh) that surrounds the domes and appears in part to have preceded emplacement of the presently exposed domes. This part of the field may perhaps be best described as a complex pyroclastic avalanche deposit—that is to say, hot, partly gas-mobilized avalanches that slid and tumbled off from growing dacite domes. Miller (1978, 1980) described these deposits as pyroclastic flows and noted their hummocky morphology and the locally disrupted and chaotic internal structure of some of the hummocks. The easternmost part of the pyroclastic field, in particular, stands slightly higher than most of the rest of the field and consists mainly of megablocks with little or no evident matrix. This easternmost area is bounded on its western side, the side facing the present domes, by a low, irregularly arcuate scarp, at the base of which are exposed older pyroclastic flows of the Misery Hill cone (Fig. 5), suggesting a crater-like depression now partly filled by younger elements of the Black Butte complex.
The presently exposed edifice of Black Butte consists of four dacite dome segments, the oldest and largest of which (unit BBd1) extends on its eastern side nearly to the remnant of the arcuate scarp just described. The dome complex is surrounded by deposits that appear to represent one or more pyroclastic avalanches. Dome BBd1, like those of the Shastina summit, was explosively disrupted and failed catastrophically in a hot avalanche that produced a partly surrounding apron of prismatically jointed dacite blocks, both vesicular and nonvesicular, in a matrix that was mobilized by hot gases (much of unit BBsh). This matrix continued to escape as farther-traveled pyroclastic flows (unit BBpf) that rafted large blocks of the pyroclastic landslide. Miller (1978) noted that these pyroclastic flows comprise multiple flow units. To the north of Black Butte, the pyroclastic materials overrode pyroclastic flows from the Shastina summit.
The three later dome segments (units BBd2, BBd3, and BBd4), lithologically identical to the remaining earlier and larger dome segment, were emplaced successively into the growing Black Butte crater complex, each partly filling an open-sided crater formed by partial disruption and collapse of the preceding dome (Fig. 4). Interpretation of breakdown rims on the hornblende phenocrysts by experimental calibration indicates steady emplacement of the dome sequence with no opportunity for any early-emplaced magma to stall, chill, and be incorporated into the later-rising magma (McCanta et al., 2007).
Hot talus tumbled off of the dome complex during successive emplacements, leaving a mantle of blocks on the lower slopes of Black Butte, many of them having conspicuous prismatic joints perpendicular to their commonly reddish, oxidized broken surfaces (unit BBch). Younger, normal rockfall talus has mantled much of the upper slopes of the dome complex (Figs. 4 and 5).
PETROGRAPHY AND CHEMISTRY
As noted earlier, the lithologies of the main cone-building lavas of Shastina are generally quite similar. In contrast to typical cone-building lavas of the other constituent cone segments of Mount Shasta, their mineral assemblages and textures are fairly simple. Shastina phenocrysts are generally small plagioclase crystals—typically on the order of a millimeter or less—and similar-sized or only slightly larger crystals of orthopyroxene and less common small cumulocrysts of orthopyroxene and clinopyroxene. The groundmass within which the crystals are embedded typically is largely glassy, although the interiors of flows may be partly to completely crystallized. Crystallized groundmass contains the same minerals as are found as phenocrysts in addition to Fe-Ti oxides. Because of the generally glassy matrix, especially near lava-flow surfaces, many of the rocks, when freshly broken, are quite dark in color.
The Shastina summit domes contrast in appearance with the more common cone-building lavas, being generally lighter in color and coarser in texture. They commonly have larger and somewhat more abundant phenocrysts—up to several millimeters—including phenocrysts of amphibole as well as both orthopyroxene and clinopyroxene.
The rocks of Black Butte are distinctive, consisting of a generally devitrified plagioclase-rich groundmass with abundant large well-formed phenocrysts of hornblende, commonly a centimeter or more, and less-abundant, smaller plagioclase and small Fe-Ti oxides. The hornblende phenocrysts invariably have fine-grained opaque-oxide rims of substantially equal width, representing reaction during eruptive ascent of the Black Butte magma (McCanta et al., 2007).
The chemical composition of Shastina cone-building lavas is broadly homogeneous (Tables 2 and 3; Fig. 11; also see Supplemental Item 11, Tables S1 and S2). They range in SiO2 content from high-silica andesite of ∼62 wt% to low-silica dacite of ∼65 wt% SiO2. It is notable that K2O increases linearly with SiO2 in the Shastina andesites but generally decreases in the dacites (Fig. 12). Thus, while the Shastina andesites and other Holocene Shasta lavas all lie clearly within the medium-K field of Gill (1981), the dacitic Shastina lavas and those of Black Butte span into the low-K field (Fig. 12).
In the nomenclature of Miyashiro (1974) for subalkalic andesites and dacites (Fig. 13), the lavas of Mount Shasta define a calc-alkalic series; all lie within the low-Fe field of Arculus (2003). As with some other oxide constituents, FeO*/MgO (where FeO* is FeO + Fe2O3 recalculated as FeO) of Shastina lavas differs slightly from corresponding trends in the other Holocene lavas; whereas Red Banks, Black Butte, and Hotlum compositions are either flat or tend to increase very slightly in FeO*/MgO with increasing SiO2, Shastina values are nearly constant at an average of 1.16. The Shastina cone-building lavas generally are slightly lower in SiO2 than the Shastina summit-dome materials and related pyroclastic-flow deposits, which in turn are less silicic than the Black Butte lavas and pyroclastic deposits. These differences generally are small—with average SiO2 within the three groups of ∼63.3, 64.5, and 65.3 wt%, respectively, but the ranges of SiO2 overlap.
Mount Shasta lavas as a whole resemble other lavas from this same sector of the Cascade arc. In an overall sense, the mainly Holocene lavas of the Red Banks and Hotlum episodes rather closely resemble those of the older cone-building episodes of Mount Shasta. The Shastina and Black Butte lavas, however, tend to have lower contents of Cs, Rb, Th, U, Nb, Ta, Ti, and Y, and they are notably depleted in heavy rare-earth elements (Figs. 14A and 14B). The most extreme compositions in these regards are those related to Black Butte.
There are some fairly clear chemical distinctions among lavas erupted from each of the Holocene eruptive episodes of Mount Shasta. Notably, Sr is generally higher in Shastina and Black Butte lavas than in most other Holocene eruptive materials, although one subgroup of Hotlum lavas has Sr as high as some of the Shastina materials (Fig. 15). Most of the highest-Sr lavas of Mount Shasta—not only those erupted during the Shastina–Black Butte episode, but also a few of the oldest Mount Shasta eruptive materials with similarly high Sr contents—have the lowest ratios of 87Sr/86Sr (Fig. 16A). These same lavas also tend to have high values of εNd (Fig. 16B).
The significantly higher values of Al2O3 in Black Butte than in Shastina lavas (Fig. 11) may well reflect the much greater abundance of hornblende in the Black Butte material.
Despite the restricted compositional variation in cone-building lavas of Shastina, some limited trends can be discerned (Fig. 17). The initial Shastina eruptions, those associated with the explosive Sisson Lake crater (unit STasl), were high-silica andesite and low-silica dacite of about the middle range of Shastina compositions. The earliest recognized cone-building lavas of the main edifice (unit STadc) are mainly high-SiO2 andesites having generally higher K2O (Fig. 17A) and somewhat lower Sr (Fig. 17B) and Ba (Fig. 17C) than those of the Sisson Lake crater.
Succeeding eruptions from the summit crater onto the northwestern side of the cone produced both andesites and low-silica dacites. K2O and Ba increase with SiO2 in the andesites but in the dacites decrease abruptly with SiO2 (Figs. 17A and 17C). Sr in these lavas of the northwestern sector tends to define two groups, respectively higher and lower in Sr; the lower-Sr group generally rises in Sr content with increasing SiO2 (Fig. 17B). Sr and Ba together define fairly distinct groups in each part of the northwestern flow complex, but there is little correlation between them on the whole (Fig. 17D). The flow complex of Lava Park (units STal1–STal6) comprises mainly dacites that closely resemble lavas of the northwest flow field.
Eruptions from the Shastina summit crater onto the southwestern sector of the Shastina cone produced only dacitic lavas; they show abrupt decreases in K2O and Ba with increasing SiO2 (Figs. 17A and 17C) but corresponding increases in Sr (Fig. 17B), just as the northwestern dacites do. For the series of Shastina andesites and dacites as a whole, Sr and Ba are generally correlated inversely (Fig. 17D).
The domes and related pyroclastic flows that erupted into the summit crater at the climax of Shastina cone formation are dacites having the highest SiO2 of any eruptive products of the Shastina edifice. Most of them are lower in both K2O and Ba than earlier units but about the same in Sr (Fig. 17). The youngest dome, however, and pyroclastic flows related to it are slightly less SiO2 rich but higher in K2O and Ba and lower in Sr than the other units of the dome complex. It appears, thus, that the more differentiated compositions that produced most of the crater-filling dome complex and related explosions were nearly exhausted before the last eruptive event of the crater sequence.
The Red Banks eruptions that preceded those of Shastina as well as the succeeding Hotlum eruptions were compositionally similar to each other and closely resembled many earlier lavas of Mount Shasta (Figs. 11–13). Lavas of the Shastina–Black Butte eruptive episode, however, are higher in Sr and lower in Ba than is typical of either the earlier or later lavas. The last-erupted Shastina summit eruptions are the most silicic compositions on the Shastina edifice, with SiO2 ≥64% and relatively low K2O and Ba but Sr generally about at the mean concentration for Shastina.
As noted above, the Red Banks Plinian eruption vented near the present summit of Mount Shasta; pumiceous fall was deposited primarily to the east and northeast while pyroclastic flows were directed to most sectors of the volcano. Red Banks deposits underlie Shastina lava flows on the northern flank and nowhere overlie any Shastina flows, and thus appear to predate the entire Shastina episode. Where both are exposed together, Black Butte pyroclastic flows overlie Shastina-summit pyroclastic flows with no visible gullying or intervening soil. Black Butte postdates growth of the Shastina edifice, but the difference in their ages is less than the resolution of available geochronologic data.
Nine radiocarbon dates were obtained in the 1970s by C.D. Miller and D.R. Crandall (Miller, 1980) from wood and charcoal in Red Banks, Shastina, and Black Butte deposits using scintillation techniques (Supplemental Item 2, Table S3 [footnote 1]). As the reported analytical errors were large (±200–350 yr), eruption ages were difficult to resolve. Recently, seven archived splits of those samples were recovered and reanalyzed using modern accelerator mass spectrometry (AMS) techniques, along with five new samples. All of the AMS dates yield smaller probable errors (±25–35 yr). Radiocarbon ages discussed below are calibrated using the latest implementation of CALIB software (Stuiver and Reimer, 1993), revision 7.1.0. Using measured, 13C-corrected radiocarbon ages and the IntCal 13.14c calibration data set (Reimer et al., 2013), the CALIB program reports calibrated age ranges that fit the measured results, as well as a median age within the calibrated range distribution. All ranges are reported here at the 2σ level in calibrated years before present (cal. yr B.P.). AMS and scintillation counting data are included in Supplemental Item 2, Tables S1 and S2 (footnote 1); only the AMS data (Table 1) are discussed here.
Two carbon samples were collected from a sequence of Red Banks pyroclastic flows and mudflows in Mud Creek (Fig. 5) on the southeastern sector of Mount Shasta, including charcoal (sample 77-18-1) from the base of a pyroclastic flow and a scorched tree knot (sample 77-18-3b) within a tree-trunk cast in a pyroclastic flow. AMS results (Table 1; Fig. 18) yield median ages of 10,927 and 10,953 cal. yr B.P., respectively. Charcoal from a log (sample 77-56) burned in a pyroclastic flow deposit along U.S. Highway 97 on the northwestern flank of Mount Shasta yields a median age of 10,929 cal. yr B.P. Four samples of charcoal (S-105-G, CG-2, S-105-F, and CG-1) collected from a mixed pyroclastic flow and fall unit southeast of The Whaleback (Fig. 3) are discordant as a group and with calculated ranges that do not overlap. One of them, sample S-105-G, yields an AMS age of 10,956 cal. yr B.P., concordant with samples 77-18-1, 77-18-3b, and 77-56. The other three from southeast of The Whaleback yield AMS ages of 11,248, 11,302, and 11,628 cal. yr B.P. and calculated ranges that do not all overlap. A single AMS determination on charcoal from a fall deposit (sample 77-19) in Mud Creek yields a median age of 12,281 cal. yr B.P.
We interpret the four concordant AMS results (Fig. 18) as dating the Red Banks eruptive episode. These samples were collected from pyroclastic deposits in three sectors of the volcano and yield age ranges of 10,716–11,122 cal. yr B.P.; their median ages range between 10,953 and 10,927 cal. yr B.P. Three of the four discordant ages were collected from a pyroclastic flow and fall sequence on the southeastern flank of The Whaleback. The section contains alternating pyroclastic flow and fall intervals with no depositional evidence of time breaks. Charcoal sample S-105-G, one of the four concordant samples, was collected from the deepest portion of the section. The three charcoal fragments yielding older apparent ages come from higher in the section and may have come from one or more 500–800-yr-old trees entrained in the deposit. The oldest AMS date (sample 77-19) was collected from the middle of a 25 cm fall unit in Mud Creek in the southeastern sector of the volcano. It was surprising to find charcoal in this fall deposit as the tephra should not have been hot enough to burn wood, and the charcoal yields a puzzling age. Possibilities include: (1) the unit is misidentified as Red Banks and belongs to an older fall unit not identified elsewhere on the volcano, (2) the fall unit entrained older material as inferred for the Whaleback section above, or (3) the charcoal was contaminated by magmatic CO2 from an unidentified buried source.
Shastina and Black Butte
Two concordant AMS determinations on charcoal with median ages of 10,645 and 10,717 cal. yr B.P. (Table 1; Fig. 18) were obtained from pyroclastic-flow deposits that erupted from the Shastina summit crater, now exposed in a quarry along Boles Creek, west of the town of Weed (Fig. 3). Age ranges are 10,567–11,061 cal. yr B.P. Three concordant determinations on Black Butte pyroclastic deposits were published in Miller (1978); AMS reanalysis of material from two of them yields median ages of 10,686 and 10,698 cal. yr B.P., with age ranges of 10,587–10,768 cal. yr B.P.
We attempted to date Shastina and Black Butte lavas by 40Ar/39Ar techniques (Table 4), but were unable to attain high-resolution results because of their young ages and a lack of erosional incision to expose materials particularly favorable for dating. We attempted to date seven samples: a pre-Shastina flow from south of Diller Canyon (unit MHad2); four Shastina samples, including one from Cascade Gulch (unit STam3), one from along the railroad right-of-way north of Black Butte (unit STas1), and two from Whitney Creek (unit STauw1); a dense block from Black Butte (BBch); and a lava flow in the Whitney Creek stratigraphy (unit HTawf) that erupted from the Mount Shasta summit and is intercalated with Shastina lavas. Individual analyses are discussed below at 1σ analytical probable error. Fully propagated errors at 95% confidence are also listed in Table 4 for more direct comparison with radiocarbon and U-series results. Analytical data are given in Supplemental Item 2, Tables S3 and S4 (footnote 1).
The pre-Shastina sample from the lower Diller Canyon area (unit MHad2) was analyzed four times, yielding plateau ages of 14.0 ± 3.2, 13.2 ± 1.8, 13.2 ± 1.9, and 9.3 ± 1.7 ka with a weighted mean of 12.0 ± 1.0 ka. The Shastina lava from Cascade Gulch (unit STam3) yields a 6.7 ± 2.7 ka plateau age, and the railroad right-of-way sample (unit STas1) yields a 6.7 ± 2.8 ka plateau age. At the 95% confidence level, these ages are within analytical error of radiocarbon ages on Shastina pyroclastic flows that are stratigraphically younger and have similar remanent paleomagnetic directions, as noted in the following section on paleomagnetism. The Shastina flow in Whitney Creek (unit STauw1) overlies the andesite lava of Whitney Falls (unit HTawf) and an underlying sequence of ash flows, both of which erupted from the Mount Shasta summit. An initial sample from the Whitney Falls flow yielded a plateau age of 8.2 ± 2.3 ka, appearing to be younger than Shastina pyroclastic-flow radiocarbon ages. This young apparent age led us to obtain additional geochronologic and paleomagnetic data to test whether some Shastina lavas erupted during more than one time interval. Two subsequent analyses of samples from the Whitney Falls flow yield plateau ages of 10.3 ± 2.3 and 10.1 ± 2.7 ka, leading us to reject that likelihood. Four fractions of unit STauw1 were analyzed from a single location stratigraphically above Hotlum pyroclastic flows and yield a weighted-mean isochron age of 8.5 ± 5.2 ka. A second sample of unit STauw1 collected above Whitney Falls yields somewhat disturbed, inconsistent, and unreasonably old apparent ages, seemingly reflecting excess argon. The Black Butte sample (unit BBch) yields negative apparent ages and a 17.2 ± 11.1 ka isochron age.
In summary, 40Ar/39Ar results from the pre-Shastina lava near Diller Canyon, Shastina lavas, the summit-derived Whitney Falls flow, and Black Butte are indistinguishable from radiocarbon results on Shastina and Black Butte pyroclastic deposits at 95% confidence.
U-Th Disequilibrium Dating
We performed 238U-230Th dating of euhedral zircon crystals separated from a meter-sized granitoid block that was included within Shastina flow STauw2. The zircon analyses were conducted using the Stanford University–U.S. Geological Survey sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) (Stanford, California), with individual crystals embedded in indium metal so that the outer few micrometers of a zircon’s crystal face could be sampled. A detailed description of the analytical techniques, interpretation, and tabulated results are included in Supplemental Item 2, Table S4 (footnote 1). Using the initial 230Th/232Th ratio apparent from U-Th isotope analyses of Shastina lavas (Wende et al., 2015), the zircons from the Shastina granitoid yield model crystallization ages with the youngest crystals forming an apparent majority population at 9.1 ± 2.6 ka (n = 25/33, MSWD [mean square weighted deviation] = 1.4, 95% confidence), and a minority of crystals with dates of ca. 35, 80, and 100 ka when deconvolved using the mixture model of Sambridge and Compston (1994) (Fig. 19; see Supplemental Item 3, Tables S4 and S5 [footnote 1]). A single zircon is in 238U-230Th secular equilibrium, indicating a crystallization age >300 ka. This majority population of zircon crystal faces yields an isochron date of 10.1 ± 2.6 ka (MSWD = 1.4, 95% confidence), also assuming the initial 230Th/232Th for Shastina lavas from Wende et al. (2015). These U-Th dates represent the crystallization age of the included granitoid block rather than eruption of the host lava; the older zircon crystals are likely to represent recycling of antecrysts. Nevertheless, the ages for the final crystallization of the granitoid, apparent from the youngest population of zircons, overlap the radiocarbon ages for Shastina deposits at 95% confidence.
Discussion of Geochronology Data
Four concordant 14C dates on the pre-Shastina Red Banks pyroclastic deposits yield a range between 11,122 and 10,716 cal. yr B.P., and their median ages suggest a preferred age for the Red Banks pyroclastic deposits of 10,934 cal. yr B.P. 14C dates on the youngest Shastina cone deposits, pyroclastic flows related to emplacement of the summit domes, range between 11,061 and 10,567 cal. yr B.P. 14C dates on Black Butte pyroclastic deposits that postdate the Shastina-summit units lie in the range 10,768–10,587 cal. yr B.P. The 14C ages of the culminating Shastina and Black Butte pyroclastic deposits, thus, are analytically indistinguishable (Fig. 18), and the radiocarbon age ranges permit less than ∼200 yr between the main Shastina and Black Butte eruptions. Median ages of the Shastina and Black Butte deposits are ∼250 yr younger than the preferred age for the Red Banks pyroclastic eruption. Dating of Shastina and Black Butte lavas by 40Ar/39Ar yields ages consistent with the 14C dates but with significantly less precision. Similarly, 238U-230Th dating of zircon crystals from a granitoid block included in a young Shastina lava flow yields a result consistent with the other dates within analytical precision.
Taken together, these results show that the >13.5 km3 Shastina edifice grew entirely within a span of no more than 250 yr. Paleomagnetic data, described below, were examined to further refine this result.
A study of paleomagnetic secular variation was carried out on rocks from both the main Shastina edifice and Black Butte as well as older and younger bounding units in order to discern whether there were any discrete temporal groupings of lavas or pyroclastic deposits within the overall Shastina sequence; to attempt to discriminate among any such groups independently of field-geologic, chemical, and isotopic-dating criteria; and to test relations to the bounding units. Standard methods used for the paleomagnetic study are described in Supplemental File 3 (footnote 1).
We sampled 12 groups of rocks for paleomagnetic study: five groups from the younger parts of the Misery Hill sequence, one group from the Red Banks pyroclastic sequence, a group of lavas erupted from the main Shastina edifice, a group of Shastina pyroclastic flows that postdate these lavas, a Black Butte group, and a mixed lava and pyroclastic sequence in Whitney Creek probably related to the Hotlum Cone, and two additional groups of Hotlum lavas that were sampled and measured but are not discussed further here (listed only in Supplemental Item 3, Table S6 [footnote 1]). Where multiple samples were collected, each of these groups has tightly clustered remanent magnetic directions, suggesting that rocks in each group took no more than a few decades to erupt and cool.
A characteristic direction of remanent magnetization for each site was calculated using Fisher statistics on data from a blanket level of alternating field treatment or line fits of data on vector-component diagrams. For each discernible grouping of samples, site-mean directions of magnetization are presented in Table 5 and are illustrated in Figures 20 and 21 as partial equal-area projections (lower hemisphere) of mean directions with ovals of 95% confidence (α95).
Misery Hill and Red Banks Eruptive Units
The Misery Hill units sampled for paleomagnetic measurements were found to include two distinct groups of pyroclastic flows. One group has a distinctive direction of remanent magnetism that is significantly different from that of all other studied samples, with a northwesterly declination of ∼350° and a fairly steep inclination of ∼60° (group 1 Misery Hill pyroclastic flows of Table 5; MHpf-1 of Fig. 20). Because of this marked divergence from the apparent range of secular variation defined by all of the other studied samples, this group may reasonably be presumed to be older. The other Misery Hill pyroclastic flows have a more easterly declination, with mean directions similar to those of most of the other sampled units (mean declination of 9° and mean inclination of 57°: group 2 Misery Hill pyroclastic flows of Table 5, MHpf-2 of Fig. 20). That general direction appears to be one from which magnetic secular variation wandered slightly during the short eruptive episode just before, during, and shortly after growth of the Shastina cone.
Five young-looking lava flows emerge from beneath younger Shastina lavas and pyroclastic flows in the vicinity of Diller Canyon (Fig. 5). Although no Red Banks pumice is found on them (nor on other, even older units nearby), their chemistry and petrography clearly associate them with Misery Hill and not Shastina compositions (Tables 2 and 3). Three were sampled paleomagnetically. The oldest (unit MHad1) has a direction much like the common direction just noted, but the successively younger ones have progressively shallower inclinations and slightly more westerly declinations (though with somewhat larger probable errors), such that the youngest of the flows (unit MHad5) has an apparent declination that is nearly due north and an inclination of 42° (Fig. 20).
Red Banks pyroclastic deposits all have directions of remanent magnetization that are close to the direction common to so many of the sampled Shastina and youngest Misery Hill units, a declination of ∼5° and a 54° inclination (Fig. 20).
Shastina-Edifice and Black Butte Eruptive Units
The lavas and pyroclastic flows of the main Shastina edifice all share very similar unit-mean directions of magnetization (Table 5), with individually measured inclinations of 52°–58° and declinations of 5°–10°. Average rates of Holocene secular magnetic variation (Champion and Shoemaker, 1977) suggest an eruptive time span for all of them together of less than about two centuries. The mean direction of the Shastina-cone lavas is 54° inclination and 8° declination with an α95 of 1.6°. The limiting 14C data suggest that the small range of individual measurements could include a coherent path of slight magnetic secular variation.
Magnetic directions of Shastina pyroclastic flows (mean inclination 58°, mean declination 7°), which postdate the cone-building lava flows, are steeper than those of the Shastina-cone lavas. The pyroclastic flows erupted from Black Butte, stratigraphically younger than all of those from the main Shastina edifice, also have a tight group of remanent magnetic directions, at 64° inclination and 15° declination (Table 5; Fig. 20). The mean direction for this group is distinctly steeper and somewhat more easterly than any of the Shastina-cone mean directions but only by a few degrees, seeming to confirm that, together, the eruptions of the main Shastina edifice, a brief inter-eruptive hiatus, and the Black Butte eruptions probably spanned only a short interval of time.
Distorted Directions Northwest of Black Butte
Paleomagnetic results on a few Black Butte and Shastina pyroclastic deposits generated scattered or distorted remanent magnetic data (Table 5; Fig. 21). A sample from the south end of the Black Butte pyroclastic field, near Lake Siskiyou, was from a laharic runout of a pyroclastic flow, too distal and too cold to have recorded even a partial thermal-remanent magnetization (Table 5, unit BBw, Cold Creek).
More confusing was a group of sites in two excellent outcrops, one in Union Pacific railroad cuts north of Black Butte near Summit Lake, and the other a little farther northeast near the Black Butte railroad siding (Fig. 5). At each of these localities, a seemingly intact stratigraphy of Black Butte pyroclastic flows concordantly overlies Shastina-summit pyroclastic flows. The paleomagnetic results, however, show considerable remanent magnetic dispersion, represented by much larger α95 ovals (Fig. 21), and mean directions different from those of the same units documented elsewhere (Figs. 20 and 21). Figure 21 suggests that segments of intact vertical stratigraphy have rotated about more or less vertical axes. The outcrop at Summit Lake, northwest of the Black Butte dome complex, rotated ∼70° counterclockwise, while the outcrop at Black Butte siding, north of the dome complex, rotated ∼100° clockwise. Such rotations indicate a dispersive but more or less coherent spreading motion, suggesting that these outcrops slid short distances outward, away from Black Butte, after thermoremanence of both the Shastina and Black Butte pyroclastic flows had been acquired. Preservation of the vertical stratigraphy in each of the rotated domains indicates an underlying subhorizontal slip surface; internal deformation caused clast-to-clast rotations within the pyroclastic layers and greatly enlarged the measured remanent-magnetic uncertainties, particularly within the deeper Shastina pyroclastic deposit (Fig. 21).
Hotlum Eruptive Units
Remanent magnetic directions of the oldest recognized volcanic materials that erupted from the Hotlum cone at Mount Shasta’s summit record a direction similar to that preserved in the main Shastina-cone units. These Hotlum units, part of the sequence exposed in the canyon of Whitney Creek, include pyroclastic deposits consisting of several conformable lithic pyroclastic flows and an overlying pumiceous pyroclastic flow (Fig. 7). The pyroclastic deposits appear to be overlain by the andesite lava flow that forms Whitney Falls (unit HTawf), in turn intercalated with the youngest Shastina lavas. The magnetic direction recorded in the Hotlum pyroclastic materials (mean inclination 52°, mean declination 5°), is indistinguishable from that of the Shastina-cone lavas (Table 5; Fig. 20).
INTERPRETATIONS AND CONCLUSIONS
Mount Shasta, like most large stratovolcanoes, shows evidence for short intervals of high eruptive output separated by long quiescences. An episode including the latest Misery Hill, Red Banks, Shastina, and Black Butte phases appears brief based on lack of erosion between emplacements of these units. Combining the results of 14C dating with paleomagnetic study indicates a highly active eruptive period of a few hundred years during which the youngest Misery Hill lavas and the Red Banks pyroclastic deposits were emplaced, followed by an interval of no more than ∼250 yr before growth of the entire Shastina cone of >13.5 km3, then within a few decades, by the Black Butte dome and pyroclastic deposits.
The rapid eruptive growth of the Shastina cone and associated lavas may provide some insight into an interesting short period of paleomagnetic secular variation. Although the youngest Misery Hill units; all of the Red Banks, Shastina, and Black Butte units; and the oldest Hotlum units have similar directions of remanent magnetism, there appears to have been a path of magnetic secular variation during eruption of those units that began in a small area of direction space, and migrated from that centroid first toward somewhat shallower inclinations, then during the Red Banks pyroclastic eruptions and Shastina cone-building eruptions, back to somewhat steeper inclinations (Fig. 20). Paleomagnetic directions of the culminating pyroclastic flows from the Shastina summit domes are again somewhat steeper than those of the cone-building lavas, and that of the overlying Black Butte deposits is steeper yet. Variations throughout this directional sequence are small, but they are greater than the purely statistical variability recorded in the α95 ovals of confidence.
The small differences within this apparent path of secular variation and its tendency to wander through a centroid of directions are consistent with only very small net differences in ages among the entire group of volcanic units that was emplaced in the brief eruptive episode. The ∼10° of local mean-field magnetic variation, from the youngest Misery Hill and Red Banks deposits, to the Shastina lava flows, to the Shastina pyroclastic flows, to the Black Butte pyroclastic flows, appears to represent a continuous path of secular variation that probably required no more than ∼500 yr, based on average Holocene rates of paleomagnetic variation (Champion and Shoemaker, 1977).
The especially rapid growth of Shastina is a profound example of Mount Shasta’s episodic eruptive nature. Petrologic differences between lavas of Shastina and the other major lavas of Mount Shasta may suggest something of the magmatic processes that distinguish brief, voluminous periods that erupt multiple cubic kilometers of magma from ongoing effusive periods with orders-of-magnitude-smaller eruptions.
Most lavas of Mount Shasta, like those typical of large andesitic to dacitic stratocone volcanoes, notably feature abundant fairly large (1–5 mm) phenocrysts of the major constituent minerals, mainly plagioclase, pyroxenes, and possibly other mafic phases, especially amphibole. Each of these phenocryst types may have complex compositions and zoning. Glomerocrysts of multiple pyroxenes or pyroxene and plagioclase are typical, and plagioclase crystals commonly are strongly embayed. All of these features at Mount Shasta indicate magmatic disequilibrium and complex mixing of phases crystallized under different conditions, likely within a complicated mush column beneath the central conduit, and entrained during ascent.
By contrast to such typical andesitic stratocones, Shastina’s generally fine-grained phenocryst-poor lavas have simpler compositions and textures. Shastina lavas seem likely to more nearly represent liquid magmatic compositions. Such an interpretation is consistent with the relatively high Sr contents and low 87Sr/86Sr ratios characteristic of Shastina lavas, which, along with the relatively high εNd values, suggest that they were largely unaffected by interaction with continental lithosphere. These observations, together with the location of Shastina’s vent, suggest that Shastina magma bypassed Mount Shasta’s central-vent conduit. Intercalation of summit-derived andesite with Shastina lavas suggests, nevertheless, that there was significant communication between the long-lived Mount Shasta magmatic system and the ephemeral Shastina system. An alternative interpretation of Shastina’s distinctive chemistry could be the involvement of mafic and ultramafic ophiolitic crustal basement rocks in melt generation. The lower values of heavy rare-earth elements in most of the Holocene lavas, but especially the Shastina summit dacites and the Black Butte dacites, might be indicative of magmas derived from parental melts generated in garnet-enriched source materials below the continental lithosphere.
It appears that magma with a small range of compositions remained available for eruption throughout the short period of Shastina activity, with more andesites erupted in the earlier stages of eruption and a singular episode of somewhat more silicic dacite dome emplacement and explosive activity marking the climax of cone building. These summit dacite domes suggest a short episode of high-level differentiation within the Shastina magma body.
The Black Butte eruptions appear to represent a separate magma batch, probably from a deeper source (McCanta et al., 2007). The Black Butte magma may too approximate a liquid magmatic composition. Speculatively, a Black Butte magma body might have been stimulated into eruption by rapid pressure changes associated with the preceding Shastina eruptions from an overlying body of magma.
Many details of the origin of the Black Butte complex are not clear. The crater-like form east of the later domes and the avalanche-like character of the pyroclastic deposits suggest the possibility that an early dome largely collapsed following explosive disruption, with the later, now-visible domes having grown within the resulting crater, obscuring any older dome remnants, and each later dome successively filling scars of the preceding explosive disruptions. Paleomagnetic evidence suggests that part of the first and largest pyroclastic avalanche, along with portions of the underlying pyroclastic deposit of the Shastina summit-crater eruptions, spread more or less as a unit, radially away from the growing dome complex during or shortly after consolidation of the avalanche materials. The section near Summit Lake on the western side of the slide (to the left of Black Butte in normal map view) rotated counterclockwise, and the Black Butte siding section farther northeast (to the right) rotated clockwise. This spreading slide presumably accompanied an episode of gravitational instability on the northern side of Black Butte during a chaotic series of dome emplacements and collapses.
The Black Butte complex included repeated episodes of dome growth, explosive destruction, and regrowth. Miller (1978) described several faults that offset both Black Butte and underlying Shastina pyroclastic flows by a few meters, exposed in new highway roadcuts in the decade of the 1970s but no longer discernible. He noted evidence from the nature of weathering profiles on these deposits that the faulting was more or less contemporaneous with deposition of the flows. These localized faults are probably effects of internal sliding within the pyroclastic avalanches during various stages of the Black Butte sequence of events.
The Shastina stratocone was, in itself, one of the largest Holocene volcanic constructs in the Cascade arc; the Shastina–Black Butte eruptive episode produced the most voluminous Holocene Cascade eruptions except for the climactic Mount Mazama eruptions that formed Crater Lake caldera in Oregon. Nevertheless, Shastina and Black Butte grew within a period of no more than about two centuries, and possibly less. It is exceedingly difficult, if not impossible, to calculate a meaningful probability that another such eruptive episode might occur on the larger compound stratovolcano of Mount Shasta, but the possibility of such an event—however low its probability—should not be ignored.
An analysis of potential volcano hazards from Mount Shasta was published by Miller (1980), based on available information about the distribution of volcanic and laharic materials around the volcano. The remarkably short but voluminous Shastina–Black Butte eruptive episode possibly adds further insight into Mount Shasta’s potential hazards, suggesting that there might be little advance warning for opening of a major new vent somewhere on the volcano. At the beginning of Shastina’s growth, only a small-volume phreatic dacitic explosion presaged initiation of an andesite-dacite stratocone of >13 km3. This could be taken as a first-order example of a low-probability but high-consequence potential hazard for Mount Shasta that could initiate a period of nearly continuous eruptive activity over a period of decades or a century or more.
The manuscript of this paper benefited greatly from a careful technical review by Wes Hildreth. Additional helpful comments were provided by the editor of Geosphere, Shan de Silva, and by Brian Jicha and Michael Ort. Jack McGeehin located and retrieved splits of 14C samples from the U.S. Geological Survey (Reston, Virginia, USA) warehouse and prepared new targets for AMS analysis. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.