Late-stage volcano geomorphic evolution of the Pleistocene San Francisco Mountain, Arizona (USA), based on high-resolution DEM analysis and 40Ar/39Ar chronology
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The cone-building volcanic activity and subsequent erosion of San Francisco Mountain, AZ, USA, were studied by using high-resolution digital elevation model (DEM) analysis and new 40Ar/39Ar dating. By defining remnants or planèzes of the volcano flanks in DEM-derived images, the original edifice can be reconstructed. We propose a two-cone model with adjacent summit vents which were active in different times. The reconstructed cones were 4,460 and 4,350 m high a.s.l., corresponding to ∼2,160 and 2,050 m relative height, respectively. New 40Ar/39Ar data allow us to decipher the chronological details of the cone-building activity. We dated the Older and Younger Andesites of the volcano that, according to previous mapping, built the stage 2 and stage 3 stratocones, respectively. The new 40Ar/39Ar plateau ages yielded 589–556 ka for the Older and 514–505 ka for the Younger Andesites, supporting their distinct nature with a possible dormant period between. The obtained ages imply an intense final (≤100 ka long) cone-building activity, terminating ∼100 ka earlier than indicated by previous K-Ar ages. Moreover, 40Ar/39Ar dating constrains the formation of the Inner Basin, an elliptical depression in the center of the volcano initially created by flank collapse. A 530 ka age (with a ±58.4 ka 2σ error) for a post-depression dacite suggests that the collapse event is geochronologically indistinguishable from the termination of the andesitic cone-building activity. According to our DEM analysis, the original cone of San Francisco Mountain had a volume of about 80 km3. Of this volume, ∼7.5 km3 was removed by the flank collapse and subsequent glacial erosion, creating the present-day enlarged Inner Basin, and ∼2 km3 was removed from the outer valleys by erosion. Based on volumetric analysis and previous and new radiometric ages, the average long-term eruption rate of San Francisco Mountain was ∼0.2 km3/ka, which is a medium rate for long-lived stratovolcanoes. However, according to the new 40Ar/39Ar dates for the last ≤100 ka period, the final stratovolcanic activity was characterized by a greater ∼0.3 km3/ka rate.
KeywordsSan Francisco Mountain DEM analysis Volcanic geomorphology 40Ar/39Ar geochronology Stratovolcanic activity Erosion Colorado Plateau
Introduction: scope and goals
We suggest that the present morphology of the cone (e.g., outer flanks) includes some portions of the original volcanic landform. By using these remnants, the latest configuration of the cone can be reconstructed.
We tie the reconstructed geomorphological elements to previously mapped geological units (Wolfe et al. 1987; Holm 1988). The chronology of the SFM eruptions is known mainly from geologic mapping, relative stratigraphy, 20 K-Ar or fission-track determinations and 1 40Ar/39Ar age that range from 2.78 ± 0.26 Ma to 91 ± 2 ka at the ±2σ level of analytical uncertainty (Damon et al. 1974; Wolfe et al. 1987; McKee et al. 1998; Morgan et al. 2004). Here, we use precise 40Ar/39Ar incremental heating methods to better constrain the sequence of late-stage volcanic events.
Digital elevation model (DEM) applications in geomorphometry are widespread (Burrough and McDonnell 1998), and DEM analysis is occasionally used in studying volcanic areas as well (e.g., Favalli et al. 1999; Székely and Karátson 2004). In our study, we used DEM and DEM derivatives (e.g., slope, aspect, and ridge maps) to quantify the volcanic geomorphology of SFM. The effect of resolution and DEM source on measurements impacts quality of DEM studies (Zhang and Montgomery 1994; Ludwig and Schneider 2006; Wright et al. 2006); our reference DEM was the best available 10 m spatial resolution US Geological Survey data base.
Constructing shaded relief images, slope and aspect maps are common products derived using GIS software. In order to emphasize the ridge and valley network, several methods (e.g., plan curvature, median difference, modified Hammond method: Hammond 1964; Dikau 1989) could be applied. Here, we used the median difference, in which the elevation difference between each pixel and the median of its 150 m neighborhood is computed by applying a moving filter. If this difference is greater than a chosen positive threshold, that pixel is classified as a ridge; if it is lower than another chosen negative threshold, it is classified as a valley.
Volumetric calculations of the volcanic edifice and the eroded material have been made by using standard GIS functions that make it possible to determine volumes between two distinct surfaces. For GIS-analysis, we used ESRI ArcView 3.2 and Golden Software Surfer 8 programs.
Summary of 40Ar/39Ar furnace/laser incremental heating experiments of groundmass separates of San Francisco Mountain lava flows
Total fusion age
Plateau age (age spectrum)
Age (ka) ± 2 σ
Increments used (°C)
Age (ka) ± 2σ
40Ar/36Ari ± 2σ
Age (ka) ± 2σ
595.4 ± 12.7
589.0 ± 11.1
6 of 10
290.6 ± 0.9
596.1 ± 19.5
West rim of Inner Basin
550.3 ± 24.1
556.2 ± 13.3
10 of 11
294.0 ± 1.3
563.2 ± 17.1
501.9 ± 43.8
514.3 ± 20.9
9 of 10
293.4 ± 3.3
519.7 ± 23.8
504.0 ± 10.4
505.2 ± 9.1
8 of 8
293.1 ± 6.1
508.5 ± 17.0
NE of Sugarloaf
585.3 ± 64.9
530.2 ± 58.4
8 of 11
292.4 ± 7.4
698.2 ± 356.9
Located in North Central Arizona, SFVF is one of several intracontinental, primarily basaltic volcanic fields distributed along the southern margin of the Colorado Plateau in Arizona, Utah, and New Mexico. Similar to the other fields (e.g., Mt. Taylor and Springerville Volcanic Fields), it was formed during the latest uplift of the Colorado Plateau (Wood and Baldridge 1990; Parsons and McCarthy 1995). During the volcanic activity, an area of 5,000 km2 of the underlying Permian and Triassic strata, as well as local basaltic sheet flows of late Miocene age, were covered by ∼500 km3 of lava and pyroclastic deposits (Wolfe 1984, 1990).
The majority of SFVF consists of alkali basalt lava flows ranging in age from ∼6 Ma to 1 ka that erupted from more than 600 vents, most of which are marked by scoria cones (Wolfe 1990). In addition to other vents that produced local eruptions of basaltic andesite, andesite, dacite, rhyolite, benmoreite, and trachyte (Moore and Wolfe 1987; Newhall et al. 1987; Wolfe et al. 1987), SFVF includes five long-lived composite volcanoes consisting of andesite, dacite, and rhyolite. SFM volcano is the largest of these edifices (Wolfe et al. 1987; Holm 1988).
The central location of the main andesite vents is supported by a general radial arrangement of dikes that however may have also fed flank vents (Holm 1988, 2004). These vents as well as the upper cone including possible craters and summit domes have been completely eroded, and now the center of the volcano is occupied by the elongated depression called the Inner Basin. A number of dikes exposed within the Inner Basin are found at Core Ridge (Figs. 2 and 3), a radial ridge dividing the Inner Basin into two embayments.
Previous radiometric datings show that the eruptive activity in the region of SFM took place between 2.8 and 0.1 Ma, interrupted by long repose periods (Damon et al. 1974; Wolfe et al. 1987; McKee et al. 1998; Morgan et al. 2004). The andesitic stratovolcano of SFM may have been formed between 0.9 and 0.4 Ma (McKee et al. 1998). Out of the three main eruptive stages distinguished by Holm (2004), primarily the stage 2 and 3 cone-building stages, the so-called “Older Andesites” (Holm 1988) with ∼0.7–0.8 Ma and “Younger Andesites” with ∼0.4–0.5 Ma K-Ar ages (Wolfe et al. 1987), control the present morphology of the stratovolcanic cone. According to Holm (1987), these two stages enlarged the cone to ∼110 km3 based on stratigraphy and topography. The youngest product of the volcano subsequent to the main andesitic–dacitic stratovolcanic evolution is Sugarloaf rhyolite dome; a previous K/Ar age (220 ± 40 ka by Damon et al. 1974) was proven to be too old by a recent 40Ar/39Ar dating (91 ± 2 ka, Morgan et al. 2004).
On the basis of geologic mapping (Holm 1988), the Older Andesites (Fig. 3) constitute mainly the west part of the volcano (vicinity of Agassiz Peak and west rim of the Inner Basin) and most of the lower slopes of the north wall of the Inner Basin. Occasionally, they can be found at the north, northeast, and east foot of the volcano. The Younger Andesites are present on all flanks of the volcano; specifically, there is a large, well-defined lava flow series on the west flank of Agassiz Peak (Fig. 3). Dacites (that are related in time both to Older and Younger Andesites) can be found on all slopes of the volcano, principally on the lower flanks; in addition, they occur in some places on the southern rim of the Inner Basin (Fig. 3). A stratigraphically important, young dacite flow, called Lockett Meadow dacite (Fig. 3) is interpreted to be post-Inner Basin age by Holm (2004). It originated from around Reese Peak (Fig. 3) and lies on debris-flow deposits that appear to be part of the northeast debris fan deposited during the formation and/or subsequent erosion processes of the Inner Basin. If this interpretation is correct, then the age of this flow gives a minimum date for the formation of the Inner Basin. Of the small patches of rhyolites, the most pronounced is the above-mentioned, 91 ka Sugarloaf dome (Fig. 3), which was emplaced in the outlet of the Inner Basin but much later than the formation of the basin.
Unsolved problems of cone evolution
Despite accurate geologic mapping, the original constitution of the volcanic cone and formation of its present morphology are still debated. The creation of the Inner Basin and its rim (∼3,200–3,500 m high) has been explained by different ideas from an earlier hypothesis of erosion caldera formation (Robinson 1913) through intense glacial erosion (Updike 1977) to modern sector-collapse-related explanations (Holm 1988, 2004; Duffield 1997). According to the latter authors, toward the end of cone evolution, large sector collapses occurred to the NE. Unfortunately, no debris avalanche deposits have been preserved, but they may be present at ∼350–400 m depth according to the analysis of a borehole NE of the Inner Basin (Holm 2004). The elongated Inner Basin, the aligned centers of O'Leary Peak (Fig. 1), Sugarloaf, and SFM, as well as some of the longest dikes of Core Ridge (Holm 2004), and a well-defined magnetic low, are all colinear and thus may have formed under the influence of a common structural control (Tanaka et al. 1986; Wolfe 1990).
The orientation and the half-open mouth of the Inner Basin are in accordance with its primarily sector collapse origin. On the other hand, at least a part of its present shape can be attributed to glacial resculpturing. The depression hosted a number of glaciers (Updike 1977) whose cirque walls and moraines are still visible. Due to the obvious post-collapse erosion, Holm (2004) considered the Inner Basin as an “erosion caldera,” although it lacks the typical fluvial erosional dissection and modification defined for erosion calderas by Karátson et al. (1999). However, the contribution of erosional modification as well as the role of glacial vs fluvial erosion in forming the present-day Inner Basin has not been clarified.
Another unresolved problem is the original summit morphology of the stratocone. Holm (1987, 2004), assuming a point-like summit, reconstructed a 4,700 m highest (pre-collapse) elevation for the volcano. According to Holm’s reconstruction, this volcano (corresponding to the Younger Andesites) may have superposed an older cone (of Older Andesites) with unknown elevation. A more precise configuration (i.e., positions, heights) of these landforms as well as precise age constraints distinguishing the Older and Younger Andesites were not presented.
Cone volume and cone-building lava flows
Based on our DEM analysis, the actual volume of SFM above the boundary is 70.5 km3. Every further 50-m-thick underlying basalt/basaltic andesite or intrusive substrate would give an ∼10 km3 increase in volume, i.e., the error of ±50 m base level is ±15% in terms of volume.
At present, the overall morphology of the cone reveals an eroded, degraded surface rather than the original lava-flow surfaces. However, delineation of lava flows on the lower slopes is possible in many cases with the help of DEM-derived shaded relief images and especially slope maps (Fig. 4). We consider a linear, tongue-shaped form with steep scarps and an orientation radial from center of the cone to be a remnant of an original lava flow. The result of lava flow delimitation (Fig. 4a) is certainly in accordance with the geologic map (Holm 1988), but makes further refinements possible. Sometimes, even the original lobes and flow structures (e.g., pressure ridges) of individual flows can be reconstructed. This is the case with Agassiz Peak lava flows (Fig. 4a), where the geologic map indicates a large undifferentiated lava flow: on the basis of the slope map, a number of individual flow units can be delimited, and even the approximate position of their source can be inferred. Some dacite lava flows (e.g., on the north slopes) are even more pronounced than andesite flows. The huge dacite flow east of Doyle Peak (Fig. 3) seems to have a source not at Doyle Peak (i.e., on the southern rim) but inside the Inner Basin. Similarly, dacite flow lobes southwest of Doyle Peak and those north of Reese Peak (Fig. 4a) point to the central part of the Inner Basin. This way, slope map analysis gives a rough indication for locating the eruption center.
Reconstructing the original volcanic cone
As suggested by earlier workers, the present-day volcano is but a remnant of an original cone of SFM (e.g., Holm 1988, 2004). Detailed analysis of DEM, with the help of new 40Ar/39Ar ages, allows us to precisely reconstruct the original volcanic landform and date its cone-building activity.
Under semihumid to dry climates, remnants of original cone surfaces may be found as planèzes or flatirons (Ollier 1988; Karátson et al. 1999). These are polygonal, preferably triangular flank segments that remain relatively undissected between the pathways of fluvial (or glacial) erosion (Cotton 1952; Ollier 1988). Although wet climates (i.e., high valley density and rapid cone erosion) may obliterate planèzes in a short time, fortunately the relatively high precipitation at San Francisco Mountain has been snow rather than rain. Today, there is only 600 mm of annual rainfall and more than 2,000 mm of snow in Flagstaff (S foot of the mountain). During Ice Ages, higher precipitation fed glaciers and only minor fluvial erosion may have occurred; also, during the interglacials and in the Holocene, the high altitude of the volcano has promoted periglacial rather than fluvial processes (e.g., Updike 1977). Thus, some planèze landforms are expected to have survived (i.e., remained undissected) in spite of the relatively old age of the cone surface.
Results of cone circle fitting to planèze points
Circle (as in Fig. 7)
Fitted Points Elevation (m)
Distance (m) of fitting point to the fitted circle at different planèzes
3 910 261
3 910 322
3 910 907
3 910 889
3 911 012
3 910 970
In fact, the geometric centers of the two fitted circle series determine two centers of possibly two upper cones. Those of the Older Andesites are shown in red circles and those of Younger Andesites are in blue triangles. The northern cone is determined by five planèzes distributed roughly evenly around the cone using four to five fitting points per circle. Their centers are located within a circle with 80 m radius. The horizontal error between the fitted points and fitted circles on planèze surfaces (Table 2) may differ up to 150–180 m, but in most cases, it is less than 30 m. The southern cone has only three points per circle, and two fitted circles along three planèzes and a point-like spur on the SE flank, which are unevenly distributed (because the north flank became covered subsequently by Younger Andesites). Therefore, the uncertainty can be considered larger, nevertheless the distance between the two resulting centers is only 75 m.
The projected centers of both cones fall within the Inner Basin. The circumscribed white circles correspond to possible vent regions (large crater(s) and/or summit lava domes) with an ideal 700 m diameter each, i.e., a minimum size of broad summit at many present-day active volcanoes (e.g., Wood 1978). The two projected vent regions happen to plot, respectively, on diorite and monzodiorite intrusions, that were considered as principal conduits of the Older and Younger Andesite (Holm 1988, 2004).
In order to determine the original volume of the cone, we constructed a new DEM in which all valleys were infilled up to the level of the dividing ridges. Starting from the ridge map (Fig. 5), the infilling was made by connecting the ridges by the Natural Neighbor interpolation method of Surfer 8 software. The volume difference between the infilled cone and the present cone is 4.5 km3, which may have resulted primarily from glacial and subordinately from fluvial erosion. (Of this volume, 2.7 km3 falls within the Inner Basin, and 1.8 km3 within the outer valleys). The volume difference between the infilled cone surface and the reconstructed original cone surface is 4.9 km3, which is the result of the disappearance of the upper cone mostly due to the flank failure. This way, the original volume of SFM might have been 70.5 + 4.9 + 4.5 = 79.9 km3. This figure is significantly smaller than the previous estimate of 110 km3 by Holm (1987).
In order to geochronologically constrain the cone-building volcanic activity, four andesitic lavas and one dacitic lava in key positions were dated (for sample locations see Fig. 3). The andesitic samples (two each from Older and Younger Andesites) originate from the highest point of the rim of the Inner Basin, and for stratigraphic reasons are expected to reflect the latest effusive periods within each andesite group. The sampled dacitic lava is from Lockett Meadow dacite that is supposed to postdate the formation of the Inner Basin.
The 40Ar/39Ar plateau age determinations of the selected samples range from 589 ± 11 to 505 ± 9 ka (Table 1). The age spectra are concordant with >80% of the gas comprising the age plateau for each sample. Sample 106, however, yields a weakly saddle-shaped spectrum with a single step comprising 10% of the gas that gives an age slightly lower than the mean age, thereby increasing the mean squared weighted deviate to greater than five (Table 1). Despite this, the plateau age for this sample is consistent with those of the other samples, thus we accept it as the eruption age.
The new 40Ar/39Ar age determinations of the andesitic cone-building activity coincide with the field-based stratigraphic division. The Older Andesites gave ages of 589 ± 11 and 556 ± 13 Ma, while the Younger Andesites 514 ± 21 and 505 ± 9 Ma. The 589 ± 11 ka age of the Older Andesite on Agassiz Peak is younger than a previously reported K-Ar age of 660 ± 160 ka (also with ±2σ analytical error, i.e., big uncertainty) obtained at the northern side of the peak (McKee et al. 1998). In contrast, the new 505 ± 9 ka age of the Younger Andesite on Humphreys Peak (the youngest lava flow apparently truncated by the collapse) is older than the previously reported 430 ± 60 ka K-Ar age of the same flow (McKee et al. 1998) and defines the termination of the andesitic volcanism. This way, according to the new ages, the two andesitic stages represent a much shorter final cone-building activity (<100 ka) than suggested by previous work (i.e., 0.8–0.4 Ma).
The 40Ar/39Ar age of the dated dacite is 530 ± 58 ka. It significantly improves a previous dating that gave 410 ± 320 ka (Damon et al. 1974) for the same dacite (on the east flank of Reese Peak). The obtained new age is not definitely younger than the latest andesites, but with respect to its relatively big analytical uncertainty, it is indistinguishable from the 514 ± 21 and 505 ± 9 ka Younger Andesites. Given the assumption of being post-Inner Basin, the age of the dacite would place the collapse and formation of “proto”-Inner Basin at 530 ± 58 ka, but the eruption of the youngest andesite (505 ka) and its error (±9 ka) suggest a more likely post-514 ka age. Anyway, the obtained ages of Younger Andesite and Lockett Meadow dacite are consistent with a rapid series of volcanic and collapse events at ∼530–505 ka.
The DEM analysis allowed us to restore the adjacent stage 2 and 3 cones of SFM. Our results imply that the lower flanks of both cones are represented in the actual relief, in spite of the late-stage collapse and subsequent erosion of the cone.
The method we applied, however, started from the assumption that both cones had perfect, symmetric morphology. We should also consider the possibility of other configurations, above all a less regular morphology of a single upper cone and crater. A (long-lived) stratovolcano may erupt lavas that overflow at the lowest point on the crater rim, which becomes higher by the end of the eruption, and the next time the lava flows down a different flank. Hence, as the volcano grows, the lava flows gradually and alternately cover all flanks. With regard to our reconstruction, this scenario raises the alternative that the Older and Younger Andesites at SFM may have come from different sides of the same crater (or vent region).
The centers of Older and Younger Andesites are <1 km from each other. Their respective center points obtained from the fitted circles are not scattered, but grouped in two limited areas, making the existence of two vents more likely than two points on opposed rims. However, their distance is not large enough to exclude a single crater that issued lava flows to different directions. To address this question, first we discuss the chronology of the two andesite groups.
The new 40Ar/39Ar ages support the distinction between the Older and Younger Andesites. The two dated andesites of each unit are arranged in subsequent order, and may have been separated by a dormant period (within analytical uncertainty). According to the new data, the eruptive center of the Older Andesites is ∼30–50 ka older than that of the Younger Andesites. Based on our elevation profile analysis, the elevation of the eruptive center of the Older Andesites was 4,350 m. The time span between eruption of the Older and Younger Andesites, assuming erosion during a dormant period, argues for a considerable degradation of the older vent (crater) by the time the eruptions of Younger Andesites began. Thus, eruption of the Younger Andesites may have begun from a center below 4,350 m a.s.l. Distribution of the Younger Andesites argues against the alternative configuration (i.e., eruption of both the Older and Younger Andesites from opposite sides of a single large crater). Given that their projected center is a small dot in the north, it would be difficult to explain how the southerly lavas flowed from the northern single rim. Therefore, it is more logical to assume that the Younger Andesites erupted from a vent shifted slightly to the north, then buried the eroded vent of the Older Andesites. Alternatively, a part of the Younger Andesites may have come from fissure vents on the south flanks. Based on our elevation analysis the younger vent eventually achieved an elevation of 4,460 m.
The further erosional history of the central part of SFM, i.e., the enlargement of the Inner Basin after collapse of the upper cones, seems to be governed by Core Ridge. Based on its geologic mapping (Holm 1988), Core Ridge represents an emergent part of the initial stratovolcano. Subsequently, it was buried by the stage 2 and 3 upper cones and interlaced and strengthened by their feeder dikes and intrusions as well as propylitic alteration (Wolfe et al. 1987; Holm 1988). After the collapse that destroyed a significant portion of both cones, the exhumed, resistant rocks of Core Ridge may have controlled the location of glacial erosion. At present, the west part of the Inner Basin contains two cirques north and south of Core Ridge (Fig. 2) that fed considerable glaciers (Updike 1977, see below). These cirques may have formed in the less resistant basal portion of stage 2 and 3 cones, whereas in between, Core Ridge became progressively more exhumed. Glacial erosion occurred during the Illinoian (most extensive) and Wisconsin glacier advances (from ∼300 to 20 ka: Updike 1977; Bezy 2003), which, however, may have overprinted the record of older glacials (pre-Illinoian B and A, from ∼450 to 300 ka: Ehlers and Gibbard 2004; Gibbard et al. 2007).
Finally, the growth rates of SFM are addressed using the volumetric reconstruction of the original cone and new radiometric dates. The reconstructed volume of SFM is 80 km3, and its lifetime is ≤ 400 ka. Therefore, the time-averaged eruptive flux of SFM is 0.2 km3/ka. This rate is similar to that obtained for a number of stratovolcanic complexes such as Volcán Tatara, Chile (0.24 km3/ka: Singer et al. 1997), Mt. Adams, OR, USA (stratocone, 0.38 km3/ka; volcanic field, 0.24 km3/ka; Hildreth and Lanphere 1994), and Seguam, Aleutians, USA (0.25 km3/ka: Jicha and Singer 2006). Other volcanoes, however, show higher average rates up to 1 km3/ka or even higher (for examples and comparisons, see Jicha and Singer 2006; Hora et al. 2007; Singer et al. 2008). Based on our new 40Ar/39Ar ages and taking a significant ∼50 km3 volume (Holm 1987) of stage 1 into account, the average eruptive rate for the subsequent cone-building stages 2 and 3 (∼100 ka interval) is higher, ∼0.3 km3/ka. Following these stages and the final flank collapse, only sporadic, peripheral, mostly dacitic and rhyolitic eruptions occurred at a much reduced eruptive rate.
The original volcanic cone, including the lobes of many lava flows, can be reconstructed by the DEM analysis of the present-day landform. Using planèzes as remnants of the lower flanks and assuming regular, symmetric cones, we propose a two-cone model. The two restored, adjacent (but not coeval) cones, with original maximum elevations of 4,350 and 4,460 m, a.s.l., respectively, are the result of the migration of volcanic activity from S to N.
New 40Ar/39Ar results are in agreement with the twofold andesite stratigraphy, namely, the succession of the Older and Younger Andesites that may correspond to the S and N centers, respectively. The obtained ages fall in two distinct age groups of 589–556 and 514–505 ka, supporting two stages possibly separated by a dormant period. The youngest age (505 ± 9 Ma) places the final cone-building activity of SFM ∼100 ka earlier than suggested by previous K-Ar determinations. This age is geochronologically indistinguishable from the final cone-building dacitic activity as well as the collapse of the late-stage cone.
At the end of stage 3, the volume of the original volcanic edifice was ∼80 km3, of which ∼7.5 km3 is the volume removed by the flank failure and subsequent erosion within the Inner Basin, and ∼2 km3 is the volume eroded from the outer flanks.
On the basis of original volume and radiometric ages, the average eruptive flux of SFM over its lifetime was ∼0.2 km3/ka, whereas the latest cone-building stage may have been characterized by a higher rate of ∼0.3 km3/ka.
The research work was initiated under a 2004/2005 Fulbright Grant to DK at Flagstaff, Northern Arizona University (NAU), AZ, USA, and greatly supported by cooperating scientist Michael Ort. We thank Richard Holm (NAU) for thorough discussions and for providing specimens for 40Ar/39Ar dating. Financial support for 40Ar/39Ar dating by the Hungarian Fulbright Committee is gratefully acknowledged. We thank Wendell Duffield (also at NAU) for his comments on stratigraphy, Steve Schilling, Scott Rowland, and Ed Wolfe for helpful reviews, and Michael Clynne for editorial comments and improvements.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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