New mapping, geochemistry, and argon geochronology illuminate a brief, remarkably silicic episode set in a mafic segment of the Cascade arc. Middle Sister was constructed during a 35-k.y. episode in the late Pleistocene from mafic, intermediate, and silicic eruptions adjacent to the primarily rhyolitic South Sister. Eruptions in the Three Sisters volcanic cluster prior to 50 ka were exclusively mafic (<57 wt% SiO2), and several basaltic andesite lava flows can be traced to Middle Sister or a predecessor volcano (prior to 150 ka). Lava flows erupted 50–37 ka at Middle Sister and on its periphery were chemically diverse, with abundant basaltic andesite, a high-silica rhyolite flow, and an andesite produced from mixing of a rhyolite and mafic magma. Abundant rhyolite and rhyodacite erupted in this interval also at South Sister. Eruptive activity paused at Middle Sister 37–27 ka but continued at South Sister with large volumes of dacite and andesite lavas. Middle Sister erupted mafic, intermediate, and silicic lava flows 27–15 ka and then ceased to erupt. Calculated eruptive rates for the entire Three Sisters volcanic cluster quadrupled from ∼0.2 to ∼0.8 km3/k.y. between 50 and 15 ka, largely owing to the eruptions focused at Middle and South Sisters, and the cluster has now returned to its modest eruptive output, mainly away from the stratovolcanoes. Time–volume results for the volcanic cluster are compared to studies of other well-mapped, well-dated stratovolcanoes. Nearly all centers record similar eruptive-volume behavior with long histories of relatively constant output punctuated by short episodes of voluminous eruptions. In addition to the Three Sisters, two of these centers (Mt. Mazama, Crater Lake, Oregon, and Puyehue/Cordon Caule in the southern Andes) record significant compositional changes associated with the voluminous eruptive episodes.

Most Cascade volcanic segments host composite andesite–dacite cones (Mt. Adams, Mt. Hood, Mt. Rainier, Mt. Mazama, Mt. Shasta) with eruptive histories spanning 200–700 k.y. The Three Sisters volcanic cluster of the Cascades (Fig. 1) near Bend, Oregon, is young and compositionally diverse in comparison, containing three late Pleistocene stratovolcanoes made of basalt, andesite, dacite, and rhyolite, nestled in a volcanic segment with at least 466 Quaternary vents (Hildreth, 2007). Study of the entire history of various volcanic segments informs the underlying subduction-induced magmatic and tectonic conditions found in convergent margins. The Three Sisters arc segment is an attractive target for study because of the wide range of chemical compositions of eruptive products, access to internal portions of the stratovolcanoes due to glaciation, and the relatively high-K compositions of volcanic materials allowing high-precision geochronology.

Most of the volcanoes in the Three Sisters arc segment are mafic (<57 wt% SiO2), and many are aligned in north-south–trending arrays. At least four of these volcanoes (North, Middle, and South Sisters; Broken Top) have a long eruptive history (>10 k.y.), and others (e.g., The Husband [Fig. 2A], The Wife [6 km southeast of South Sister], Sphinx Butte [8 km west-southwest of South Sister]) may also have a protracted history, whereas most peripheral vents appear to be products of single, brief eruptions. Middle Sister (3062 m altitude), the smallest and youngest stratovolcano in the Three Sisters volcanic cluster, was constructed during a 35-k.y. episode in the late Pleistocene and is broadly contemporaneous with South Sister.

The Three Sisters stratovolcanoes are of similar elevation and appearance but are substantially different in composition. North Sister is exclusively mafic with central-vent-erupted lavas ranging from ∼120 ka to ∼50 ka. South and Middle Sisters are relatively young (<50 ka) and compositionally anomalous for this mafic portion of the Cascade arc. South Sister is composed largely of rhyolite and dacite erupted over the past 50 k.y., with andesite eruptions during brief intervals (Fierstein et al., 2011). Middle Sister is a basaltic andesite to dacite stratocone (Figs. 2A and 2B) built in narrow intervals during 50–37 ka and 27–15 ka on a glacially eroded highland of basaltic andesite. Rhyolite and rhyodacite erupted adjacent to Middle Sister but not from the central vent. This report focuses on Middle Sister, particularly on its profound late Pleistocene eruptive episode, and complements Fierstein et al. (2011), which focuses on South Sister. The resulting combination of detailed field mapping, comprehensive whole-rock geochemistry, and high-precision argon geochronology done on most eruptive units allows spatial, chemical, and temporal analysis of the volcanic cluster. The relatively short-lived episode that produced both Middle and South Sisters yielded a broad chemical range of eruptive units. Detailed investigation of this episode provides insight into the kind of episodic behavior that is common at most arc stratovolcanoes.

Early work by Hodge (1925) and Williams (1944) described the physiography and composition of the Three Sisters volcanic cluster. Prior mapping at various scales (Williams, 1957; Taylor, 1978, 1987; Taylor et al., 1987; Sherrod et al., 2004) conveys some field relations and compositional ranges, but lack of geochronology resulted in substantial overestimate of eruptive ages. Middle Sister, in particular, has a glacially ravaged east face, despite being slightly younger than the more intact South Sister. The North Sister edifice was mapped by Schmidt and Grunder (2009) as part of a petrologic study. A 1:24,000-scale geologic map (Hildreth et al., 2012), incorporating geochemistry and geochronology, established the framework for a detailed paper on South Sister (Fierstein et al., 2011) and for the present study of Middle Sister. Petrologic investigations of mafic components of the cluster include Hughes and Taylor (1986), Hughes (1990), and Schmidt and Grunder (2011), and silicic petrogenesis was studied by Hill (1991) and Stelten and Cooper (2012).

The Cascade Arc in Central Oregon is exceptionally broad with abundant, distributed mafic centers and north–south alignments of vents (Hildreth, 2007). The Three Sisters lie at a complicated section of the arc where the north boundary of the Basin and Range province, the northwest-trending Brothers Fault Zone, meets the north-trending High Cascades Graben (Sherrod et al., 2004), a weakly extensional basin stretching north to Mt. Jefferson (Fig. 3).

The Three Sisters volcanic cluster as mapped by Hildreth et al. (2012) contains three stratovolcanoes, several eroded mafic shields, and numerous flank vents. Lines of mafic volcanoes are common away from the stratovolcanoes, and even the 2-ka Devils Chain (northern terminus is rdc on Fig. 2A, southern terminus 8 km south of South Sister) on the flank of South Sister forms a north–south array (Hildreth et al., 2012). The alignment of the Three Sisters and the simultaneous construction of Middle and South Sister may also reflect these favored vent alignments, which are parallel to the High Cascades Graben.

Older, eroded map units are chiefly basaltic andesite and basalt; younger units range from basalt to high-silica rhyolite. The abundance of true rhyolite (72–77.5 wt% SiO2) in this arc segment is unusual for the Cascade arc axis (Hill, 1991; Hildreth, 2007), though somewhat older rhyodacite is prevalent to the east, where Hill and Taylor (1990) described widespread silicic pyroclastic deposits erupted over the past 1 m.y., and rhyolite/rhyodacite is abundant at the great rear-arc Newberry Volcano to the southeast as well as farther east across the High Lava Plains (Figs. 1 and 3).

The Three Sisters each are strongly glacially eroded, especially North Sister with deep cirques in all sectors and Middle Sister with its well-exposed eastern flank. Glaciers are small now, but during the Pleistocene there is evidence for a broad mountain ice sheet covering the Cascade crest (Scott, 1977), and ice likely covered most of the Three Sisters for all but brief periods in the past 200 k.y. During the Last Glacial Maximum, ca. 20 ka, the ice sheet was continuous except for isolated nunataks above 1600 m elevation, and valley glaciers extended down below 1200 m elevation (Scott, 1977). Lowest elevations in the study area (Fig. 2A) are ∼1700 m. Many eruptive units interacted with ice and display glassy quenched margins, overthickening next to valleys, and lava flows confined to ridges between glaciated valleys (Hildreth et al., 2012).

This detailed report on Middle Sister grew out of geologic mapping of the Three Sisters volcanic cluster by Hildreth et al. (2012). Volcanic units were mapped and sampled during ∼1-month-long field seasons from 2000 to 2009. Map units represent similar, mappable products from single vents, or occasionally groups of vents, that typically erupted in a single episode (months to hundreds of years). Several map units (e.g., basaltic andesites of North Sister, mns, and Broken Top, mbt) represent similar-composition lavas that erupted over longer (tens of thousands of years) timespans. Major- and trace-element geochemistry and argon geochronology progressed simultaneously with field work, and those results guided subsequent mapping. Detailed map unit descriptions, chemical analyses, and geochronology were made available by Hildreth et al. (2012).


Whole-rock compositions for 743 samples from the entire Three Sisters volcanic cluster were determined by X-ray fluorescence at the U.S. Geological Survey (USGS) laboratories in Denver and the GeoAnalytical Laboratory at Washington State University (Hildreth et al., 2012). Of these, 174 analyses were obtained for Middle Sister samples. Major oxide concentrations were recalculated to sum to 99.6%. Rock names are based on SiO2 content: basalt <52 wt%, basaltic andesite 52–57 wt%, andesite 57–63 wt%, dacite 63–68 wt%, rhyodacite 68–72 wt%, and rhyolite >72 wt%. Compositions are also referred to as “mafic” to represent basalt and basaltic andesite, “intermediate” for andesite and dacite, and “silicic” for rhyodacite and rhyolite. Rock thin sections representing all map units were scrutinized to describe and correlate map units and to choose samples appropriate for argon geochronology.

Argon Geochronology

The USGS geochronology laboratory in Menlo Park analyzed the samples using 40Ar/39Ar techniques according to procedures described by Calvert and Lanphere (2006). Because most map units were glaciated, we were able to obtain dense, crystalline samples from lava flow and dome interiors. Detailed step-heating experiments on 150–220-mg groundmass separates typically yielded plateau ages with analytical errors of ±1 ka to ±5 ka (Table 1, Fig. 4), and all results satisfy field constraints. Some samples contain non-atmospheric argon contaminant; for those we interpret isochron ages as most reliable. Of the 65 eruptive units represented in Figure 2A and Table 2, we dated 43 units; 22 of the 32 Middle Sister eruptive units are dated (Table 2) and remaining units are well bracketed with age estimates listed in the “Accepted age” column. For comparison with the timing of other eruptions in the greater Three Sisters map area, dating of 73 of the 142 map units on the Hildreth et al. (2012) geologic map are discussed. Argon geochronology in this report uses standards that have been updated from those in companion papers (Fierstein et al., 2011; Hildreth et al., 2012), so listed data in this study are 1–3 k.y. older. Analytical techniques, standards, and incremental-heating data are included in Supplemental File 11.

Erupted Volumes

Lava flow volumes were compiled using field-thickness estimates and graph paper overlays on the 1:24,000-scale maps for areal extent. Exposed volumes were calculated and original extents were estimated for all 142 units on the Hildreth et al. (2012) geologic map so that we could assess the volumetric contribution of the 50–15-ka eruptive episode. Because most of the map units have been eroded, there is significant uncertainty in these estimates. Volumes for eruptive units represented in Figure 2A are listed in Table 2, and volumes for all units mapped by Hildreth et al. (2012) are listed in Supplemental File 2 (footnote 1).

Eruptive units from the Three Sisters are medium-K rocks that range from 49–77 wt% SiO2; Middle Sister units span 52–77 wt% SiO2 (Fig. 5). In and around Middle Sister (Fig. 2A), there is not much true basalt but abundant basaltic andesite, andesite, dacite, and rhyolite. Similar to other eruptive units throughout the Oregon Cascades, these basalt and basaltic andesite units are highly variable in alkalis (K20 + Na2O = 3.8–5.7 wt%) and MgO (3.0–9.1 wt%), and, like the nearby Crater Lake/Mazama system and other Cascade volcanoes (Bacon et al., 1997; Bacon and Lanphere, 2006; Schmidt et al., 2008), these regional lavas reflect a heterogeneous mantle source. All Three Sisters map units have Mg#s (100 × Mg / (Fe + Mg)) under 50, so they are not primitive. North Sister lavas are similar in composition to the regional basalts and basaltic andesites (Fig. 5). Middle Sister, South Sister, and Broken Top contain substantially more-evolved rocks. Broken Top analyses are limited but have a compositional range from basaltic andesite to rhyodacite. South Sister is composed of andesite, dacite, and abundant rhyolite. Glacial cirques offer the deepest exposure on South Sister and commonly expose rhyolite, so Fierstein et al. (2011) inferred that the bulk of its unexposed core is rhyolite. Middle Sister consists of abundant basaltic andesite similar to regional units as well as andesite and dacite, and low on its northwestern flank there is a high-silica rhyolite coulee. This rhyolite unit, the Rhyolite of Obsidian Cliffs (roc), contains the highest silica content of any Quaternary rock in the Oregon Cascades.

Eruptions at Middle Sister and in most of its immediate periphery occurred in three pulses (Fig. 2B): (1) early mafic through silicic eruptions 49–37 ka, (2) one intermediate composition unit (alc) ca. 27 ka as South Sister was erupting andesite/dacite lavas (Fierstein et al., 2011; Hildreth et al., 2012), and (3) the bulk of the volcano 25–19 ka, including the 20 ka, ∼1.2 km3 basaltic andesite of Middle Sister (mms) that built the upper half of the edifice.

Middle Sister is flanked by older basaltic andesite map units to the west, north, and east (Hildreth et al., 2012). These lavas are similar to products from monogenetic and mafic polygenetic volcanoes north and south along the High Cascades. The basaltic andesite of The Husband (mth), the underlying basalt south of The Husband (bsh), and the basaltic andesites of Little Brother (mlb) and North Sister (mns) were not included in volume calculations for Middle Sister because they are separated from Middle Sister and are interpreted to reflect regional volcanism. Lavas from the andesite of Obsidian Creek (aoc) and from the basaltic andesite of North Fork (mnf) were included in volume calculations because they are temporally and/or spatially associated with the Middle/South Sister episode.

Pre-50 ka Mafic Eruptions

The basaltic andesite of The Husband (mth) is a 5- by 8-km mafic volcano (Fig. 6) with multiple vents and numerous dikes and intrusions (Sherrod et al., 2004; Taylor et al., 1987) located 4 km west of Middle Sister. Lavas range from 52 to 57 wt% SiO2 and are phenocryst-poor. The Husband overlies the less-silicic basalt south of The Husband (bhs), typically containing more olivine and ranging from 51 to 53 wt% SiO2. A flank vent of The Husband with dikes that intrude and vents that overlie other mth lavas has a 40Ar/39Ar age of 151 ± 5 ka, providing a minimum age for the stratovolcano. North Sister lies less than 2 km north of Middle Sister and is also a glacially eroded, polygenetic basaltic andesite volcano (Taylor, 1987; Schmidt, 2005; Schmidt and Grunder, 2009; Hildreth et al., 2012). The basaltic andesite of North Sister (mns) includes hundreds of similar lava flows erupted between 47 ± 6 ka to at least 121 ± 6 ka (Hildreth et al., 2012). We were not able to find fresh, crystalline textures for geochronology of lavas at the deepest stratigraphic levels to assess the older mns dates (140–500 ka) obtained by Schmidt and Grunder (2009). The basaltic andesite of Broken Top (mbt) is an assemblage of undifferentiated mafic lava flows, erupted from the 300–150-ka Broken Top stratovolcano (Hill, 1991) southeast of the map area. Small-volume mafic map units are typical of central Oregon Cascades volcanism and are exposed east and west of Middle and North Sister. The basaltic andesite of Demaris Lake (mdl, 184 ± 21 ka) is a clinopyroxene–olivine–plagioclase basaltic andesite largely buried by younger lava flows and glacial deposits. The basaltic andesite of South Fork Whychus Creek (msf) is a crystal-poor lava flow and tuff erupted 169 ± 16 ka. Both mdl and msf may have erupted from beneath the later site of Middle Sister. The basaltic andesite of Soap Creek (msp, 131 ± 6 ka) is a small scoria cone exposed east of Middle Sister that produced two phenocryst-poor lava flows exposed 2–4 km downstream. The basaltic andesite of Linton Spring (mls, undated), west of Middle Sister, is also likely to fall within this age range. It underlies Middle Sister lavas but is not well bracketed by stratigraphy and is undated due to glassy textures. This cliff-forming crystal-poor unit has a cube-jointed texture that suggests ice-contact emplacement, and it is likely to have erupted during Marine Isotope Stage (MIS) 6 glaciation, ca. 190–130 ka. Lavas interpreted to have erupted from Middle Sister prior to 50 ka total ∼0.4 km3 (Table 2).

Mafic and Silicic Units in Immediate Periphery (50–37 ka)

In the 50–37 ka time interval, multiple eruptive units vented from and adjacent to Middle Sister, particularly in its northwest sector (Figs. 6 and 7). The basaltic andesite of Little Brother (mlb) is a ∼2-km diameter, mafic (52–53 wt% SiO2) edifice at the northwest toe of North Sister. Lavas from the vent extend over 6 km to the west and northwest. The edifice consists of hundreds of thin lava flows containing 1–5% plagioclase, 3–5% olivine, and abundant olivine–plagioclase and all-plagioclase crystal clots. Several comagmatic dikes and a small plug cut the edifice. A lack of unconformities between lava flows suggests Little Brother was a short-lived eruption, and two 40Ar/39Ar plateau ages from the unit overlap at 48 ± 6 ka and 49 ± 4 ka, within analytical error of two out of three age determinations by Schmidt and Grunder (2009).

The basaltic andesite of North Fork Whychus Creek (mnf) is a set of thin lava flows exposed through glacial till along and north of the North Fork of Whychus Creek. It contains 5–10% plagioclase, 1–3% olivine, and sparse clinopyroxene. The mafic flows are the earliest identified lavas erupted from Middle Sister, dated at 49 ± 10 ka.

The andesite of Obsidian Creek (aoc) is an extensive apron of crystal-poor andesite lava flows (58–63 wt% SiO2) that overlie Little Brother (mlb) lava flows (Figs. 6 and 7). The unit, aoc, appears to have vented from Middle Sister and is as thick as 120 m at medial exposures and far thicker at its terminus 7 km from Middle Sister. Rare phenocrysts include commonly rounded 0.5–1.5-mm plagioclase and rarer <0.5-mm olivine, which is typically resorbed and has reaction rims. Geochemically, with mixing trends between basaltic andesite and rhyolite (such as the younger roc, described below), samples from aoc are unique in the map area (Fig. 8). Major and trace elements (Hildreth et al., 2012) from aoc are distinct from all other andesite units in the Three Sisters cluster, and the inferred silicic component is the oldest rhyolite at Middle Sister. Two 40Ar/39Ar plateau ages from the unit are 49 ± 2 ka and 45 ± 2 ka, and an isochron age from a third sample yielded 50 ± 4 ka. These eruptions are coeval with early cone-building rhyolite eruptions at South Sister (Fierstein et al., 2011).

The roc coulee emerges from beneath younger Middle Sister lavas ∼3 km northwest of the summit and extends another 3 km northwest (Figs. 2A, 6, and 7). The aphyric, high-silica rhyolite (76.4–76.7 wt% SiO2) is the most silicic Quaternary lava in the Oregon Cascades. The coulee is glacially scoured and is 1 km wide and 120 m thick near Obsidian Falls and 135 m thick at its distal flow front. A syneruptive lapilli-fall deposit containing pumice, obsidian, and felsite is preserved on the west ridge of Little Brother. Dense felsite from roc yielded a 40Ar/39Ar plateau age of 38 ± 2 ka. Additionally, a small circular rhyolite lava dome (r58, 74 wt% SiO2, 41 ± 1 ka) lies ∼9 km northwest of Middle Sister (well beyond area mapped in Fig. 2A) (Hildreth et al., 2012).

The basaltic andesite of Hayden Glacier–Diller Glacier cleaver (mhd) is a stack of several mafic lava flows and intercalated scoria exposed in the east face of Middle Sister (Fig. 9). Phenocryst abundance is variable: 7–15% plagioclase and 1–3% olivine. A flow on the east end of the cleaver yielded a 37 ± 9 ka 40Ar/39Ar plateau age. The basaltic andesite of the headwaters of Linton Creek (mhl) is a small window of glacially scoured lava flows that are chemically identical to mhd and inferred to be coeval. The underlying dacite of Linton Creek (dlc) apparently also erupted in this interval. Unit mhd overlies ∼45 m of stratified proximal andesite ejecta, the andesite tephra fall at the base of the east face of Middle Sister (aef), comprising two sequences separated by an angular unconformity and consisting of 2–15-cm clasts, coarse ash, and small (<5 mm) lapilli. The unconformity likely represents a small change in vent location or eruptive style rather than significant time. Sampled clasts are compositionally indistinguishable (62.6–63.1 wt% SiO2) and lithologically similar to andesites erupted subsequently at South Sister. Although undated and difficult to access, its preservation suggests aef erupted shortly before mhd and is considered to represent the earliest erupted silicic andesite in the Three Sisters cluster. During 50–37 ka, ∼2.1 km3 of lava and tephra erupted from Middle Sister (Table 2).

South Sister Eruptive Pulse (37–27 ka)

Volcanism shifted southward for ∼10 k.y. and built South Sister (Fig. 6). Eruptive units, summarized here, are described in detail by Fierstein et al. (2011). The rhyolite of South Fork Whychus Creek (rsf) erupted from a flank vent ∼2 km northeast of South Sister and ∼3 km southeast of Middle Sister. The glacially scoured coulee is ∼1 km wide, 150–300 m thick, and composed of three compositionally identical flow units (73.8–74.4 wt% SiO2). This rhyolite yielded a 40Ar/39Ar plateau age of 36 ± 2 ka.

Cone-building eruptions at South Sister were principally rhyolite prior to 36 ka, followed by voluminous andesite and dacite 33–27 ka (Fierstein et al., 2011); Middle Sister did not erupt again until the end of this interval. Rhyolite and dacite erupted from South Sister summit 33–31 ka, then andesite and dacite erupted from South Sister’s Hodge Crest flank vent 29–27 ka. One of these South Sister andesite map units, the andesite west of Lost Creek Glacier (alg), forms an apron of uniform phenocryst-rich lava that is remarkably similar to the Middle Sister-sourced andesite of Linton Creek (alc), a 120–450 m thick stack of lava flows. The alc unit locally has jointing patterns indicating ice-marginal emplacement and appears to have flowed along the northeast margin of a large Linton Creek valley glacier. Dates of the two map units are indistinguishable; alg yielded a 27.6 ± 1.1 ka plateau age, and two samples from alc yielded 27.7 ± 1.0 ka and 27.3 ± 1.2 ka plateau ages. While tempting to lump alc and alg into a single eruptive unit, topographic constraints require that alc was erupted from a vent on the Middle Sister edifice. These coeval South and Middle Sisters eruptions signal a shift in eruption locus to Middle Sister. The volume of alc, the only Middle Sister lava erupted in this interval, is ∼0.74 km3 (Table 2).

Middle Sister Eruptive Pulse (27–15 ka)

The 12-k.y. Middle Sister eruptive pulse has been divided into two time intervals (27–21 ka and 21–15 ka) in Figure 6. In the saddle between South and Middle Sisters, two ca. 26-ka units (rsc and dcl) mark the shift of volcanism toward Middle Sister (Fig. 6). The rhyolite of Separation Creek (rsc) is a 1-km-diameter dome equidistant (2.5 km) from South and Middle Sisters summits (Fig. 10) that yielded a 40Ar/39Ar plateau age of 25 ± 1 ka. The dome appears contemporaneous with the neighboring dacite of Chambers Lakes (dcl), which is composed of orange and black agglutinated tephra fallout that vented from the Chambers Lakes depression (Fig. 10). The dacite, which is widely exposed in the saddle, is crudely stratified and eroded into ledges with variable dips as much as 25°, suggesting buried preexisting topography. The main texture is loose or tack-welded pumice and scoria grading to vitrophyre. On the ridge crest between the Chambers Lakes, the deposit was locally thick enough to spread southeastward as rheomorphic flows, one of which is dated at 26 ± 2 ka.

Several dacite and andesite units on the western flank of Middle Sister appear to have erupted from Middle Sister during this time period, but their vents are obscured. The dacite of Sister Spring (dss) is a 60–80 m thick lava flow (Fig. 7), dated at 25 ± 3 ka and overlying roc. Andesite lava flows in the west and northwest sectors (Fig. 7) are similar in chemical composition, variable in phenocryst assemblage, and yield ages or brackets between 25 and 22 ka. Several have evidence of interaction with ice and water during emplacement. These include the (1) andesite of the southwest slope of Middle Sister (asw), a small window of crystal-poor andesite surrounded by the younger mms; (2) andesite west of Collier Glacier (awc), a fan of numerous shingled lava flows, mainly 5–25 m thick but with a 70 m thick flow apparently ponded against an expanded Collier Glacier; (3) andesite west of Renfrew Glacier (awr), a fan of thin (2–5 m) and thick (15–40 m) moderately porphyritic lava flows; (4) >70 m thick andesite of headwaters of Linton Creek (ahl); and (5) andesite west of Middle Sister (awm), a fan of lavas that are 2–50 m thick and that are lithologically similar to awr but are chemically distinct with higher Fe, Mg, and Ca and lower K. The andesite of Demaris Lake (adl), of similar age and composition, is exposed 4 km east of Middle Sister. Its source is uncertain, but small windows of this lava project toward Middle Sister. Also in this interval, basaltic andesite lavas encircle Middle Sister at low elevations: The voluminous and well-exposed basaltic andesite of Camp Lake (mcl) vented low on the southeastern slope of Middle Sister and overlies adl; the basaltic andesite south of Scott Pass (mps) erupted north of North Sister; the basaltic andesite north of Hayden Glacier (mnh) and the andesite agglutinate north of Hayden Glacier (anh) erupted between Middle Sister and North Sister; the basaltic andesite of Renfrew Glacier (mrg) and basaltic andesite of Montague Memorial (mmm) likely erupted on the northwestern flank of Middle Sister, but their vents are obscured by younger eruptive units; and the basaltic andesite of Separation Creek (msc) vented low on the southwestern flank of Middle Sister and drapes flows from The Husband and South Sister. Units mps and anh are the southernmost units of the Matthieu Lakes fissure, 2−10 km of North Sister, described by Schmidt and Grunder (2009).

The summit sequence at South Sister was coeval with these cone-building Middle Sister lavas between 23 ka and 22 ka. The thick dacite of Lewis Glacier headwall (dlg) immediately preceded the dacite ejecta of South Sister summit (des), the dacite north of Teardrop Pool (dnt) and, the youngest South Sister edifice unit, the andesite of Teardrop Pool (mtp) (Fierstein et al., 2011).

Two dacite units, the dacite of Lane Plateau (dlp) dated at 22 ± 2 ka and the less-precisely dated dacite north of Separation Creek (dsn) at 19 ± 4 ka, were emplaced on the western and southwestern flanks of Middle Sister shortly before the cone-building eruption described below. Map unit dlp (Fig. 7) intruded into and then apparently flowed over a glacier to feed the Lane Plateau coulee. Intricately jointed columns are abundant in the vent area and there is a conspicuous gap between the vent and the bulk of the lava flow. The dlp coulee sits atop a ridge suggesting it was bounded by 100–300 m thick ice during emplacement.

The voluminous mms inundates older eruptive units in the south and west sectors (Figs. 7 and 10) of the Middle Sister volcano and is well exposed (Figs. 6 and 9) on its steep eastern face, where it makes up the upper half (∼225 m) of the exposed cone. It consists of thin flows of vesicular, porphyritic mafic lava flows with intercalated blocky rubble. The lava flows contain 25–35% plagioclase and 5–10% olivine; most contain 52.2–52.9 wt% SiO2, but several late flows and dikes contain 53.3–54.1 wt% SiO2 (Fig. 6). One of these dikes fed the basaltic andesite intrusion of Middle Sister (mmi), which cut through early-erupted mms flows and is well exposed high above the Hayden Glacier (Fig. 9). Discontinuous patches of mafic tuff exposed low on the southwestern flank of Middle Sister are mapped as the basaltic andesite tuff of Middle Sister (mmt) and are interpreted as surges and/or lahars, coeval with mms, perhaps initiated by water produced by lava–ice interaction. Minor undated remnants of mafic lava flows (basaltic andesite of Middle Sister summit, mss) and silicic lava flows (dacite of Middle Sister summit, dms) are exposed at the summit of Middle Sister (Fig. 9). Dating the mms eruptive episode was difficult because of the glassy, vesicular textures. 40Ar/39Ar dating of two lavas and a dike yielded large errors: 17 ± 12 ka and 21 ± 19 ka for the flows and 18 ± 4 ka for the intrusive unit (mmi), but precise dating of bracketing lava flows (Fig. 6) constrain the episode to ca. 20.5 ka. Fourteen units that directly underlie mms were dated, the youngest two being dlp (22 ± 2 ka) and dsn (19 ± 4 ka) (Fig. 7). Four units directly overlying mms were dated, the oldest two being dhr, the dacite of Hayden Glacier–Renfrew Glacier col (19 ± 2 ka), and dbh, the dacite of Black Hump (18 ± 2 ka). The dacite domes southwest of Middle Sister (dsw) and the dacite of Irving Glacier (dig) erupted at 17 ± 15 ka and 15 ± 3 ka, respectively (Fig. 10). Chill joints at the terminus of dig indicate ice was at least 200 m thick in the saddle between South and Middle Sisters at 15 ka. During 27–15 ka, ∼3.7 km3 of lava and pyroclastic material erupted, of which ∼1.2 km3 is the widespread mms (Table 2).

Regional Volcanism and Holocene South Sister Rhyolite Flows (<15 ka)

Following the unusually productive episode of Middle and South Sisters construction between 50 ka and 15 ka, volcanism in the Three Sisters cluster waned (Fig. 6), and, except for two late Holocene rhyolite flank eruptions south and northeast of South Sister, was no longer focused at the central vent volcanoes. Two eruptive units located mostly north (acc) and mostly south (rdc) of Figure 2A were dated at <15 ka. The rhyolite of Devils Chain (rdc) erupted at ∼2 ka to the northeast, east and southeast of South Sister (Fig. 2); the related, but slightly older, rhyolite of Rock Mesa (rrm) coulee (Scott, 1987; Stelten and Cooper, 2012) erupted south of South Sister (beyond the limits of Fig. 2A). The andesite of Collier Cone (acc) is a map unit of the mafic periphery that erupted 1.5 ka and is exposed several kilometers northwest of the North Sister summit (Fig. 2A, upper left). Nothing erupted from Middle Sister in this <15-ka interval (Table 2).

Volcanic rocks around the Three Sisters are as old as 600 ka (Hildreth et al., 2012). Eruptive volumes were estimated for each map unit (Supplemental File 2 [footnote 1]) using Hildreth et al.’s (2012) geologic map, though estimates for units older than 150 ka are considered minima because of erosion and burial by younger units. The eruptive rate from 600 ka to 50 ka appears to have been consistent at ∼0.1–0.2 km3/k.y., and from 200 ka to 50 ka, at ∼0.2 km3/k.y. (Fig. 11). Middle Sister is a small stratovolcano and appears to have erupted only a negligible pre-50 ka volume. Starting at 50 ka, the eruptive composition of the volcanic field and its rate of eruption both changed dramatically. In the interval 50–15 ka, Middle Sister grew to ∼7 km3 and South Sister grew from ∼5 to ∼20 km3. Both composite volcanoes erupted significant amounts of andesite, dacite, and rhyolite in this episode. Maximum estimated eruptive rates for the entire cluster in the 50–15-ka interval increased to ∼0.8 km3/k.y., four times the 200–50-ka rate. Following the 15-ka cessation of volcanism at Middle Sister, no central vent eruptions have occurred in the cluster, and the regional eruptive rate appears to have returned to its modest long-term average of ∼0.2 km3/k.y. Approximately half of the total volume of the Three Sisters cluster erupted since 200 ka was produced in the 35-k.y. episode (50–15 ka).

The 50–15-ka eruptive episode coincided with cold temperatures and thick ice, best recorded by oxygen isotope variation (Fig. 11) in marine sediments (Lisiecki and Raymo, 2005). Timing of ice thickness in the Oregon Cascades may not be exactly synchronous with trends in marine oxygen isotopes, but abundant ice-contact features in lavas erupted 50–15 ka in the Three Sisters volcanic cluster do reflect the gradual cooling from MIS 3 to MIS 2 globally. The coincidence of this 35-k.y. magmatic episode with this cooling trend before and during the Last Glacial Maximum glaciation is intriguing. Many studies (Miller and Smith, 1987; Singer et al., 1997, 2008; Bacon and Lanphere, 2006; Huybers and Langmuir, 2009; Watt et al., 2013; Rawson et al., 2015) suggest increased eruptive rates associated with deglaciation at arc volcanoes. This study reveals a well-mapped and dated example that shows the opposite. Peak eruptive behavior was coincident with the buildup to peak glaciation of the Last Glacial Maximum (MIS stage 2) (Lisiecki and Raymo, 2005) and ended immediately prior to deglaciation (Fig. 11).

Middle Sister is a relatively young stratovolcano that grew during an anomalous episode in the Three Sisters volcanic cluster’s history. From 600 to 50 ka the Three Sisters segment of the Cascade volcanic arc was characterized by eruptions of basalt and basaltic andesite, both from discrete vents and from composite cones (Hildreth et al., 2012). Peripheral units that include basaltic andesite of The Wife, basalt of Sphinx Butte, basaltic andesite of the Husband, and basaltic andesite of North Sister typify the cluster prior to 50 ka, and rhyodacite (rhyodacite southwest of Golden Lake and rhyodacite of Upper Chush Falls) and dacite (dacite of Todd Lake) associated with either Broken Top or with rhyodacite lavas near Tam McArthur Rim (Hildreth et al., 2012; Hill, 1991) erupted east of the Three Sisters cluster. During this interval, several voluminous silicic eruptions inundated lowlands to the east and southeast from vents in the Tumalo volcanic field, 20 km east of Middle Sister (Sherrod et al., 2004). Our map focuses on South, Middle, and North Sisters, so volumes tabulated here do not include appreciable amounts of these voluminous silicic deposits or Broken Top lavas. Using estimated volumes from 200 to 50 ka yields ∼0.2 km3/k.y. productivity rates across the Three Sisters map extent. The eruptive rate appears to have been somewhat lower prior to 200 ka; however, erosion and inundation by younger eruptive units renders volume estimates uncertain.

Beginning at 50 ka, the eruptive character at Middle Sister and in the cluster as a whole changed from mafic to bimodal mafic/felsic, and the eruption rate approximately quadrupled (Figs. 6 and 11). No rhyolite has been identified along the High Cascades crest older than 50 ka. Mafic eruptions, the high-silica rhyolite roc, and the chemically mixed aoc, interpreted as a mixture of basaltic andesite and rhyolite, erupted adjacent to and from Middle Sister coincident with inferred silicic eruptions at South Sister (Fierstein et al., 2011). In the time interval 37–27 ka, volcanism waned near Middle Sister and flared up at South Sister. Abundant andesite and dacite lava flows (Fierstein et al., 2011) covered South Sister, and several rhyolite flows erupted on its flanks. Copious amounts of phenocryst-rich andesite erupted at 27 ka at South Sister and one of these eruptive packages, alg, is identical in composition and time to andesite alc erupted from Middle Sister. Andesite/dacite composition lavas erupted at Middle Sister 26–23 ka. The cone-building mms eruption that built the upper half of Middle Sister is bracketed between 21 and 19 ka. Several small dacite eruptions (dig, dsw, dhr, dsn, dms, and dbh) at Middle Sister overlie mms and are waning eruptions of the 50–15-ka episode.

The temporary quadrupling of the eruption rate (Fig. 11) and introduction of andesite/dacite compositions are a profound departure from the productive, but consistently mafic, earlier eruptive history of the Three Sisters volcanic cluster. The mafic character of the field prior to 50 ka implies that there was neither a magmatic system undergoing fractional crystallization producing variable-composition lavas nor enough magmatic input to cause melting of wall rocks or antecedent mafic intrusions. The eruption of rhyolite starting ∼50 ka and the apparent mixing of mafic material with rhyolite in aoc heralds a significant increase in intrusive inputs. Previous study of rhyolite at South Sister (Hill, 1991) attributes rhyolite to partial melting of amphibolitic crust; however, the subsequently abundant dacite and andesite erupted at Middle and South Sister imply development of a fractionating magmatic system that waxed during 50–30 ka, culminated during 30–20 ka, then waned by 15 ka. The 50–15 ka episode that built Middle and South Sisters has no precedent on the arc axis here and appears to have ended at 15 ka. Eruption of rhyolite ∼2 ka south and east of South Sister may have introduced a new eruptive episode or it may have been a cryptic rejuvenation of residual rhyolite magma beneath South Sister. Notably, zircons separated from rrm and rdc yielded ages as old as 80 ka, although most yielded ages between 20 ka and 50 ka, when South Sister was most active (Stelten and Cooper, 2012), so at least some of the rhyolite appears to be related to the 50–15-ka eruptive episode.

Previous workers have pointed out that the silicic volcanism at the Three Sisters is at the western end of a 10-m.y. rhyolite sweep (Fig. 3) across the High Lava Plains of Oregon (MacLeod et al., 1976; Jordan et al., 2004; Fierstein et al., 2011). Arguments over the cause of this sweep range from westward spreading of the eastward-propagating Yellowstone hotspot (Draper, 1991; Jordan et al., 2004), to crustal-block rotation (Carlson and Hart, 1987), to asthenospheric flow related to the rollback of the downgoing Cascadia slab (Ford et al., 2013), to lithospheric extension at the edge of the Basin and Range province (Fitton et al., 1991). This study does not resolve this controversy, but it does document the late Pleistocene introduction of rhyolite to this formerly mafic portion of the Cascade Arc—arguably the leading edge of the propagating anomaly.

Comprehensive eruptive histories for active volcanoes are time-consuming endeavors and have been completed at relatively few volcanoes, mainly in the Cascades (Hildreth and Lanphere, 1994; Hildreth et al., 2003a; Bacon and Lanphere, 2006), Chilean Andes (Singer et al., 1997, 2008; Hora et al., 2007), Mexico (Frey et al., 2004), and Alaska (Hildreth et al., 2003b; Jicha and Singer, 2006). Despite being compositionally diverse and active over varied timescales, all the studied volcanoes display some degree of episodic behavior, typically with tens of thousands of years of modest output punctuated by brief episodes (1–20 k.y.) of high output. A subset of these detailed time–volume studies are compiled in Figure 12.

Eruptive rates calculated for the Three Sisters cluster average 0.2 km3/k.y. before and after the 50–15-ka uptick (to 0.8 km3/k.y.) in volcanism, yielding a long-term average of ∼0.3 km3/k.y. since 200 ka and likely similar back to >500 ka, though units older than 250 ka are poorly preserved because of extensive erosion and exposures are limited because of deposition of younger eruptive units. Compilations of various well-dated centers (Bacon and Lanphere, 2006; Jicha and Singer, 2006) identify long-term average eruption rates that vary from ∼0.1 km3/k.y. for Tartara–San Pedro complex (Singer et al., 1997) and the Mt. Baker volcanic field (Hildreth et al., 2003a) to 0.8 km3/k.y. for Mt. Katmai (Hildreth et al., 2003b). Short-term eruptive rates for typical stratovolcanoes commonly are 0.5–1 km3/k.y. but can range as high as 32 km3/k.y., such as at Klyuchevskoy (Fedotov et al., 1987), and even higher for other, caldera-forming eruptions. Several of these studies identify compositional variation with time. Mt. Mazama/Crater Lake was an andesite/dacite stratovolcano until ∼27 ka, when rhyodacite began to dominate (Bacon and Lanphere, 2006). The Puyehue/Cordon Caule complex produced basaltic andesite early in its history and rhyodacite and basalt in the past 40 k.y. (Singer et al., 2008). The Three Sisters transition from monotonous basaltic andesite to rhyolite is similar to the Puyehue/Cordon Caule complex; however, with no prior rhyolite emplaced along the arc front, the 50–15-ka eruptive episode at the Three Sisters produced high-silica rhyolite and then extruded andesite/dacite at four times the earlier eruptive rate.

All the compared eruptive histories document that arc magmatism is focused at individual stratovolcanoes for hundreds of thousands of years, but eruptive flux can vary over short periods by an order of magnitude or more. The form of several of these detailed eruptive histories are remarkably similar: Seguam Island, the Volcán Ceboruco, and the Three Sisters produced ∼80 km3 over 400 k.y. at rates that accelerated in the late Pleistocene (<150 ka). The selection of these stratovolcanoes may be biased toward modest-sized edifices that are clearly constructional and active today yet are small enough to present tractable field projects. Other, larger centers, with eruptive-history studies in progress (e.g., Mt. Shasta, Mt. Veniaminof, and Newberry Volcano [Calvert, 2015]), display episodic volcanism at times different from those in Figure 12. Erosion and inundation by younger eruptive units clearly impacts exposure, so sampling and volume estimation is challenging, particularly for deposits older than the end of the penultimate glaciation (MIS 6, ∼135 ka).

Middle Sister is the product of a profound 50–15-ka eruptive episode that also built South Sister and abundant lava flows in their periphery. At 7 km3, Middle Sister’s eruptive volume is modest, but its diverse chemistry and its sudden onset and abrupt end are intriguing. Built on an older basalt and basaltic andesite platform, this episode produced volcanism ranging from basaltic andesite to high-silica rhyolite at four times the previous or current volumetric eruption rates in the Three Sisters cluster. In studies of other arc volcanoes, Hildreth and Lanphere (1994), Bacon and Lanphere (2006), and others have discussed intense eruptive periods succeeded by long periods of quiescence. The Sisters are notable because detailed mapping and high-resolution geochronology show that two adjacent stratovolcanoes were concurrently active over the same short, but measurable, interval. The episode epitomizes and informs similar episodic behavior at many other stratovolcanoes.

We thank Ed Taylor for sharing mapping and insight into the volcanic field. Andy Ouimette carefully prepared geochronology and geochemistry samples, Tim Debey handled irradiations, and James Saburomaru oversaw argon analysis. Willie Scott, Brian Jicha, Mariek Schmidt, and Science Editor Shan de Silva provided helpful reviews. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

1Supplemental Files. 1: 40Ar/39Ar tabulated data 2. Estimated eruptive volumes for all map units on the geologic map of the Three Sisters volcanic cluster (Hildreth et al., 2012). 3. Individual parts of Figure 6. Please visit or access the full-text article on to view the Supplemental Files.
Figure 6 is interactive when viewed with Adobe Acrobat. View age-separated maps and plots by hovering mouse over time-interval buttons. Individual parts of the figure are also accessible in Supplemental File 3 (text footnote1).
Science Editor: Shanaka de Silva
Gold Open Access: This paper is published under the terms of the CC-BY-NC license.