James E Gardner
email: gardner@mail.utexas.edu

Alain Burgisser

Pete Stelling

ERUPTION AND DEPOSITION OF THE FISHER TUFF (ALASKA): EVIDENCE FOR THE EVOLUTION OF PYROCLASTIC FLOWS

Recognition that the Fisher Tuff (Unimak Island, Alaska) was deposited on the leeside of ~500-700 m high mountain range (Tugamak Range) more than 10 km away from its source played a major role in defining pyroclastic flows as momentum-driven currents. We re-examined the Fisher Tuff to evaluate whether deposition from expanded turbulent clouds can better explain its depositional features. We studied the tuff at 89 sites, and sieved bulk samples from 27 of those sites. We find that the tuff consists of a complex sequence of deposits that record the evolution of the eruption from a buoyant plume (22 km) that deposited ~0.2 km 3 of dacite magma as a pyroclastic fall layer to erupting ~10-100 km 3 of andesitic magma as scoria-rich pyroclastic falls and flows that were mainly deposited to the north and northwest of the caldera, including those in valleys within the Tugamak Range. The distribution of the flow deposits and their welding, internal stratification, and occurrence of lithic breccia, all suggest that the pyroclastic flows were fed from a fountaining column that vented from an inclined conduit, the first time such a conduit has been recognized during a large volume caldera eruption. Pyroclastic flow deposits before and after the mountain range, and thin veneer deposits high in the Range, are best explained by a flow that was stratified into a dense undercurrent and an over-riding dilute turbulent cloud, from which deposition before the range was mainly from the undercurrent. When the flow ran into the mountain range, however, the undercurrent was blocked, but the turbulent cloud continued on. As the flow continued north, it re-stratified, forming another undercurrent. The Fisher Tuff thus records the passing of a flow that was significantly higher (800-1100 m thick) than the mountain range, and thus did not require excessive momentum.

INTRODUCTION

Pyroclastic flows represent one of the most dangerous hazards of volcanic eruptions, causing some 27% of known deaths from eruptions [START_REF] Baxter | Impacts of eruptions on human health[END_REF]. Because it is impossible to see inside of a moving pyroclastic flow, we are left to interpret their flow dynamics from the deposits they leave behind. The study of pyroclastic flow deposits has thus been central in directing models. The Fisher Tuff, Alaska, played a pivotal role in developing such models, because it was one of the first to be recognized for its extraordinary deposition on the leeside of a large mountain range (Tugamak Range, Figure 1), 10-15 km away from Fisher volcano [START_REF] Miller | Spectacular mobility of ash flows around Aniakchak and Fisher calderas, Alaska[END_REF]. [START_REF] Miller | Spectacular mobility of ash flows around Aniakchak and Fisher calderas, Alaska[END_REF] argued that the Fisher Tuff was most likely deposited en masse from a relatively dense pyroclastic flow that was driven over the mountains by excessive momentum. Deposits of other pyroclastic flows were subsequently explained by similar momentum-driven deposition (e.g., [START_REF] Wilson | Ignimbrite depositional facies: The anatomy of a pyroclastic flow[END_REF]. The term pyroclastic flow thus became synonymous with high particle concentration flows. Since then, physical arguments [START_REF] Valentine | Statified flow in pyroclastic surges[END_REF] and numerical and analogue experiments [START_REF] Dobran | Numerical simulation of collapsing volcanic columns[END_REF][START_REF] Neri | Numerical simulation of collapsing volcanic columns with particles of two sizes[END_REF][START_REF] Choux | Stratification and particle sergegation in flowing polydisperse suspensions, with applicatiopns to the transport and sedimentation of pyroclastic density currents[END_REF][START_REF] Cordoba | A model of the pyroclastic flow dynamics at Galeras Volcano, Colombia[END_REF] have all shown that the vertical particle concentration is likely to vary greatly within a moving pyroclastic flow. Indeed, the current view is that particle concentration plays a central role in controlling the richness of flow dynamics [START_REF] Dobran | Volcanic processes[END_REF]. In particular, high particle concentrations developing near the ground can cause a dense undercurrent to form, the motion of which can be independent of the upper parts of the flow [START_REF] Fisher | Transport and deposition of a pyroclastic surge across an area of high relief: The 18 May 1980 eruption of Mount St[END_REF]. Field observations of compositional changes within deposits show they are likely to form by progressive aggradation of tephra from the lower boundary of density-stratified flows [START_REF] Branney | A reappraisal of ignimbrite emplacement; progressive aggradation and changes from particulate to non-particulate flow during emplacement of high-grade ignimbrite[END_REF][START_REF] Carrasco-Nunez | Progressive assembly of a massive layer of ignimbrite with a normal-to-reverse compositional zoning: the Zaragoza ignimbrite of central Mexico[END_REF].

Thus, the dynamics of the boundary layer is the key to link deposit fabric to depositional mechanisms [START_REF] Branney | Pyroclastic density currents and the sedimentation of ignimbrites[END_REF]. In recognition of the paradigm shift in thinking, the term "pyroclastic density current" was proposed to replace pyroclastic flows and surges. Here, we use the term pyroclastic flow to refer to a pyroclastic density current that travels along the ground, regardless of the particle concentration within it.

Deposits from pyroclastic flows can be either thick, valley-filling deposits that are generally massive and poorly sorted, or, when topography is irregular, thin veneer deposits [START_REF] Druitt | The physics of explosive volcanic eruptions[END_REF]. Veneer deposits vary from massive to stratified, with moderately to well-sorted layers, and often occur on topographic highs (e.g., Freundt and Schminke, 1985). Massive deposits are thought to originate from highly concentrated boundary layers in which inter-particle interactions are dominant [START_REF] Boudon | The 1984 nuée-ardente deposits of Merapi volcano, Central Java, Indonesia: stratigraphy, textural characteristics, and transport mechanisms[END_REF]. Stratified deposits probably arise from boundary layers with low particle concentration that allows particles to roll and saltate [START_REF] Sohn | On traction-carpet sedimentation[END_REF][START_REF] Burgisser | Using hydraulic equivalences to discriminate transport processes of volcanic flows[END_REF]. Thus, the interaction between the flow and topography changes the characteristics of its boundary layer and the dominant depositional mechanisms.

Little is known about the development of concentration gradients within pyroclastic flows and their relationship to undercurrents. En masse deposition implied that the thickness of a pyroclastic flow was closely related to that of the deposit. With the concept of aggradation, deposit thickness instead reflects sedimentation rate and current duration.

Thus, obstacles can no longer be used to infer the momentum of the parent flow, but they can be used to unravel relationships between variations of particle concentration and the boundary layer (e.g., [START_REF] Burgisser | Physical volcanology of the 2050 BP caldera-forming eruption of Okmok volcano, Alaska[END_REF]. We thus re-examined the Fisher Tuff in order to determine whether its interaction with the Tugamak Range left traces in the deposits, and to what extent those traces can be related to the development of concentration gradients within the parent flow.

The Fisher Tuff erupted ~9400 14 C years ago, forming a broad sheet of pyroclastic deposits around Fisher caldera (Fig. 2), which collapsed during the eruption [START_REF] Miller | Spectacular mobility of ash flows around Aniakchak and Fisher calderas, Alaska[END_REF][START_REF] Stelling | Eruptive history of Fisher Caldera, Alaska, USA[END_REF]. The tuff can be divided into three parts, based on their dominant juvenile component and distributions: North Pumice, Scoria-rich, and South Pumice (modified from [START_REF] Stelling | Eruptive history of Fisher Caldera, Alaska, USA[END_REF]. The North Pumice and the South Pumice are petrologically distinct dacitic magmas, erupted from different vents [START_REF] Stelling | Eruptive history of Fisher Caldera, Alaska, USA[END_REF]. The Scoria-rich deposits range in composition from basaltic andesite to andesite, with some of that range resulting from mixing between basaltic andesite and dacite of the North Pumice [START_REF] Stelling | Eruptive history of Fisher Caldera, Alaska, USA[END_REF].

METHODS

We examined the Fisher Tuff at 89 sites, all located by GPS, up to more than 50 km away from the caldera (Fig. 1). In addition, products erupted from Fisher volcano from the Mid-Pleistocene to Recent have been examined at another 160 sites [START_REF] Stelling | Eruptive history of Fisher Caldera, Alaska, USA[END_REF], and so we feel confident in our estimates for the distributions of the units of the Fisher Tuff. Digital elevation maps used in this study are from SRTM data. At most localities, the thickness of every layer within the tuff was measured, and any internal stratifications or variations were noted. In a few cases, we had to estimate thickness because the deposit was either too thick to measure precisely or only exposed in vertical cliffs. In many places the base of the tuff is not exposed, and so those thicknesses are minimums. The coarsest lithic fragments were measured in many of the layers at 27 sites, most of the time by excavating a standard volume of each layer and measuring the long axis of the five largest clasts found. Where deposits were sufficiently thick (usually >20 cm), they were subdivided and the largest lithics from each level were measured to determine the variation in size as a function of stratigraphic height. When deposits were partly or densely welded, we outlined an area on the outcrop face and measured the largest lithic clasts that were exposed.

Bulk samples were collected at several localities to determine grain-size distributions of the deposits (Table 1). Grain size is given in φ units and mm [φ = -log 2 (mm)], and the parameters used are Inman Median Size (M d ) and sorting (σ) [START_REF] Inman | Measures for describing the size distribution of sediments[END_REF]. Those parameters are accurate to ±0.1φ. All samples were wet-sieved in the laboratory to 4φ (0.063 mm) in 0.5φ intervals. The fraction finer than 4φ was collected in water buckets and dried. Size distributions of the fine fractions were determined to 7φ (0.008 mm), using a Spectrex ILI-1000 laser particle counter. Although the counter measures sizes as small as 10φ (0.001 mm) the distilled water used to dilute the sample contained impurities smaller than 7φ, and so weight percents were normalized to sizes between 4.5φ and 7φ. Particle counter and sieve data were combined to obtain normalized grain-size distributions. The error in weights of each size fraction is less than 0.1 g, equivalent to an error of ~0.1 wt.%. Overall, M d varies from -3φ to 0.5φ (8 to 0.71 mm), and σ varies from 0.96 to 3.60 (Table 1). Following [START_REF] Walker | Grainsize characteristics of pyroclastic deposits[END_REF] and [START_REF] Cas | Volcanic successions: modern and ancient[END_REF], we call deposits well sorted if σ < 2 (very well if σ < 1) and poorly sorted if σ > 2.

Componentry of samples was obtained by manually separating and weighing a statistically significant number of the different clast types for sizes coarser than 0φ (1 mm), and by counting 500 particles per size class with a binocular microscope for the smaller sizes down to 2φ (0.25 mm). We assume that the proportions of components in the 2φ size class are the same in smaller sizes. Conversions to weight fractions for pumice, lithic, glass, and crystals were done by assuming average densities of 1000, 2500, 2000, and 2500 kg m -3 , respectively. To convert to weight fraction for scoria we used a density of 817(±242) kg m -3 , based on measurements of 116 scoria from sample FC-321E (Table 1). Errors on the normalized proportions are on the order of 0.1%. [START_REF] Stelling | Eruptive history of Fisher Caldera, Alaska, USA[END_REF] described the overall stratigraphy of the Fisher Tuff. Here, we discuss relevant details of the stratigraphy and individual eruptive layers. We divide the Fisher Tuff into three parts: North Pumice, Scoria-rich, and South Pumice, with the Scoria-rich deposits being much more voluminous than the other two. Each part contains multiple layers, which are described below.

DEPOSITS OF THE FISHER TUFF

North Pumice Deposits

The North Pumice deposits consist mostly of a pumice-rich layer distributed along a east-northeast axis (Fig. 3). It is ~3-10 meters thick on the eastern caldera wall and thins rapidly away (Fig. 4). At the eastern caldera wall the upper half of the deposit is partially welded. It also exists as a 5-cm-thick ash layer in the southern caldera wall (Fig. 5), and as one to three, 5-8-mm thick, ash layers in the Tugamak Range (Fig. 2). The pumicerich layer is well to very well sorted (Table 1), and reversely graded, with lithics increasing in size by ~75%. To the northeast there is a poorly sorted 44-cm thick ashrich layer within the pumice-rich layer (Fig. 4).

Based on its fan-shaped distribution (Fig. 3) and well-sorted nature, we suggest that the pumice-rich layer is a pyroclastic fall layer. The poorly sorted layer in the northeast probably represents a small pyroclastic flow generated during the dominantly pyroclastic fall event. Isopachs of the fall deposit indicate that the volume of tephra is ~0.5 km 3 , using the methods of [START_REF] Pyle | The thickness, volume and grainsize of tephra fall deposits[END_REF] and [START_REF] Fierstein | Another look at the calculation of fallout tephra volumes[END_REF]. Our isopachs probably cover minimal areas, and hence we have likely underestimated its total volume.

A tephra volume of ~0.5 km 3 translates to a magma volume of ~0.2 km 3 (dense-rock equivalent; DRE), assuming a bulk density of 1000 kg m -3 .

Scoria-Rich Deposits

Deposits that are rich in scoria comprise the vast majority of the Fisher Tuff. Based on their grain-size distributions, internal structure, sorting, and geographic distribution, we divide the scoria-rich deposits into pyroclastic flow deposits or fall deposits.

Scoria-Rich Pyroclastic Flow Deposits

The pyroclastic flow deposits vary in thickness from 10's of centimeters to >50 meters, and are found from the northeast to the southwest in a ~270° arc around the caldera (Fig. 6). From the northwest to the northeast, we found >3-10 meters near the Bering Sea coast, within valleys of the Tugamak Range, and on the slopes of Shishaldin Volcano. Further than ~26 km away to the northeast, however, we found only thin layers interspersed with scoria-rich pyroclastic fall deposits. To the east, west, and southwest, ~2-8 meters of pyroclastic flow deposits exist at the caldera rim, but are absent farther than ~2-4 km from the caldera. To the south and southeast, scoria-rich pyroclastic flow deposits are absent (Fig. 6).

Scoria-rich pyroclastic flow deposits are welded to some degree near the caldera (Fig. 2). More than 50 m of partly to densely welded tuff is found to the north, and a few meters of welded tuff exist to the east, northwest, and west (Fig. 6). From the eastern caldera wall, welded tuff thins rapidly to only a few meters thick, and is absent >4 km from the wall. The thickest welded deposits thus occur to the north of the eastern half of the caldera. At more distal sites the thicker flow deposits are often indurated, especially in their upper thirds. Gas pipe structures are common within the indurated sections.

Those gas pipes are often spatially aligned, possibly because they formed from steam percolating through the deposits from buried streams or rivers.

By far the majority of pyroclastic flow deposits occur north of the caldera (Fig. 6).

Where thicker than 50 m, the deposit is strongly internally layered, mainly with 1-to-5meter thick, massive layers inter-bedded with 1-to-5-meter thick, strongly cross-bedded and finely laminated layers. Lithics in many of the stratified layers are imbricated, and some of those layers contain accretionary lapilli. At base is >5 meters of coarse breccia that contains more than 50% lithics, many of which are larger than 1 meter. That breccia extends up to ~8 km to the north of the caldera, where it thins to ~1 m (Fig. 2). At those more northerly sites the breccia lies in the middle of the deposit, indicating that the 50 meters near the caldera is a minimum. Except for that breccia, scoria-rich flow deposits between the northern caldera rim and the Tugamak Range are massive, poorly sorted, and contain ~25-29 wt.% lithics (Table 1). No base was found, but more than 30 meters were exposed at many sites (Fig. 6).

On the north side of the main east-west ridge of the Tugamak Range Scoria-rich pyroclastic flow deposits are only 10-30 cm thick, stratified, and cap thin (3-5 mm) dacitic ash and pumice layers (Fig. 2). The ridge deposits have well-sorted unimodal upper parts and bimodal, finer grained lower parts (Fig. 7). Those two layers also have separate well-defined modes of scoria and lithics (plus glass) and, overall, contain ~22-30 wt.% lithics (Table 1). Scoria-rich deposits thicken from 3.5 m near the head of one valley within the Tugamak Range to more than 30 meters in the deeper parts of the valleys (Fig. 89). Within the valleys, deposits are relatively flat-topped and appear to have filled the valleys (Fig. 2). Those thicker deposits are poorly sorted and appear exactly like those south of the Tugamak Range, except that they are distinctly poorer in lithics (Fig. 10). On smaller ridges extending north in the Tugamak Range, only thin layers were deposited, but those are poorer sorted than the ridge layers (Fig. 9).

To the northeast of the caldera, the Scoria-rich deposits show similar stratigraphic relations as those within the Tugamak Range. Just to the east of the Tugamak Range, we find deposits that mimic those of the ridge deposits (Fig. 7). Further to the northeast, we find 3-9 meters of massive, poorly sorted deposits on the Bering Sea coast (Fig. 4) and on the flanks of Shishaldin Volcano (Fig. 6), all of which appear similar to those in the valleys of the Tugamak Range.

Scoria-Rich Pyroclastic Fall Deposits

The other unit of the Scoria-rich deposits is a pyroclastic fall layer that is distributed to the east and south of the caldera (Fig. 11). This unit is thickest near the eastern caldera wall, where it is up to ~40-50 cm thick, and caps the North Pumice fall layer to the east of the caldera. Thin scoria ash exists at the base of the eruptive sequence beneath South Pumice to the west of the caldera, whereas it caps the South Pumice in the southern half of the island (Fig. 5). Scoria-rich fall deposits are well to poorly sorted, with poorer sorting arising from inclusion of fine ash (Table 1). Given its fan-shaped distribution, sorting, coarseness, and fine ash content, we believe that the fall layer represents a mixed deposit from a relatively large buoyant eruption plume and co-ignimbrite ash from the pyroclastic flows.

Thus, during the Scoria-rich phase of the Fisher Tuff eruption, both pyroclastic flows and buoyant eruption plumes (pyroclastic falls) were generated. Isopachs of the fall layer indicate that it represents at least 0.5 km 3 of tephra (~0.2 km 3 DRE), which is a minimum because of the limited aerial extent of our measured sections. Estimating the volume of the pyroclastic flow deposit is more difficult, because its base is rarely exposed. [START_REF] Stelling | Eruptive history of Fisher Caldera, Alaska, USA[END_REF] arrived at a minimum of ~10 km 3 DRE by assuming given thicknesses of the tuff over the area that it covers on land. It is likely, however, that a significant volume went out to sea, given that the flow deposits are up to >30 meters thick near the coast.

Using the shape and collapse depth of the caldera [START_REF] Stelling | Eruptive history of Fisher Caldera, Alaska, USA[END_REF] estimated a volume of ~100 km 3 for the Fisher Caldera. It is likely that the volume of the Scoria-rich pyroclastic flow deposits is somewhere in between. This implies that the Scoria-rich fall layer represents only ~0.2-2% of the mass erupted during this phase of the eruption.

South Pumice Deposits

The South Pumice is found only to the south and southwest of the caldera, and is thickest at the southern rim (Fig. 12). It consists of multiple layers that can be grouped into three units. At base, there is a unit of 3-5 alternating layers of fines-poor, moderately sorted layers and poorly sorted layers, with the uppermost one being rich in accretionary lapilli. This unit thins from ~75 cm at the caldera rim to ~20 cm only ~6 km away. The middle unit is a coarse, moderately sorted pumice-rich layer that is found only within ~2 km of the caldera rim, where it reaches ~40-60 cm thick. The upper unit is a well to poorly sorted, pumice-rich deposit, which is the thickest and coarsest of the three units, and appears to be distributed to the south and east. Lithics in this layer are coarser in the south, but remain coarse as the layer thins to the east (Fig. 12). At the rim, the deposit is densely welded, but welding diminishes rapidly and is absent >1-2 km away.

It is difficult to interpret the eruptive style of the South Pumice because of its limited distribution. We suggest that the lower unit represents multiple small pyroclastic fall and pyroclastic flow deposits, which is in turn overlain by a relatively coarse pyroclastic fall layer, overlain by a pyroclastic flow deposit. There is too little data to estimate the volume of the South Pumice. [START_REF] Stelling | Eruptive history of Fisher Caldera, Alaska, USA[END_REF] established that three magmas were tapped during the Fisher Tuff eruption. We have found that the resulting deposits are distributed differently around the caldera. The North Pumice consists mainly of a pyroclastic fall layer, whereas both the Scoria-rich and South Pumice deposits consist of multiple pyroclastic flow and fall deposits. Below we discuss the sequence, dynamics, and vent evolution of the Fisher Tuff eruption, and the transport and deposition of the Scoria-rich pyroclastic flows.

DISCUSSION

Dynamics and Vent Evolution of the Fisher Tuff Eruption

Where exposed, the North Pumice is always at the base of the Fisher Tuff, and is usually overlain by a Scoria-rich layer (Figs. 45). In contrast, the South Pumice underlies Scoria-rich fall at some sites, but is bracketed by Scoria-rich deposits at others (Fig. 5). Based on this sequence, we suggest that the Fisher Tuff event started with the eruption of North Pumice, after which Scoria-rich deposits erupted, during which the South Pumice erupted.

To constrain the eruption dynamics of the North Pumice, we use isopleths constructed for the largest lithic sizes at the base and top of the deposit (Fig. 13). Those isopleths indicate eruption column heights of 16 km and 22 km, respectively [START_REF] Carey | Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns[END_REF]. Such column heights would be produced by eruptive intensities of ~1-3x10 7 kg s -1 [START_REF] Sparks | The dimensions and dynamics of volcanic eruption columns[END_REF]. Although those are rough estimates, because of the amount of extrapolation needed to complete isopleth shapes, a column height of ~22 km is consistent with an erupted magma volume of ~0.2 km 3 , based on the characteristics of other sub-Plinian to Plinian eruptions [START_REF] Carey | The intensity of plinian eruptions[END_REF]. The small pyroclastic flow in the North Pumice suggests that the eruption was partially unstable.

The switch from North Pumice to Scoria-rich deposits was abrupt, as we found few deposits that contain significant proportions of both pumice and scoria. The Scoria-rich deposits consist of both flows and falls. In some cases, we find the fall layer underlying flow deposits, and in others it occupies the same stratigraphic position (e.g., Fig. 5). It is thus likely that pyroclastic flows and falls were erupted at the same time. Indeed, we note that the fall layer contains a significant fraction of fine ash (Fig. 4), which suggests that these layers represent a mixed deposit of tephra from a relatively large buoyant eruption plume and from a co-ignimbrite plume from the scoria-rich density currents.

Such simultaneous eruption of flows and falls has been recognized in other eruptive sequences (e.g., [START_REF] Fierstein | The plinian eruptions of 1912 at Novarupta, Katmai National Park, Alaska[END_REF][START_REF] Scott | Pyroclastic flows of the June 15, 1991, climactic eruption of Mount Pinatubo[END_REF][START_REF] Wilson | The Bishop Tuff; New insights from eruptive stratigraphy[END_REF].

Lithic sizes in the Scoria-rich fall layer are typically larger than those in the underlying North Pumice (Fig. 13). This indicates that the column height of the Scoriarich plume (from which the large lithics likely originate) reached higher than 22 km.

Mass flux of the Scoria-rich phase of the Fisher Tuff (density currents and falls) was thus at least ~10 7 kg/s, and probably much greater, considering that >98% of the erupted mass went into the flows. There is too little data to constrain intensities for the South Pumice phase of the eruption, but we note that it wavered in character before appearing to increase in intensity.

The distribution and maximum lithic sizes of the North Pumice suggest that it vented from the eastern half of the caldera (Fig. 3). Welding of the upper part of the fall layer on the eastern caldera rim, even in the absence of any overlying flow deposit, suggests that its vent was very near the eastern rim (e.g., [START_REF] Sparks | Welded air-fall tuffs, in Chapin and Elston, Ash Flow Tuffs[END_REF]. The distribution and maximum lithic sizes of the Scoria-rich fall layer suggest that it also vented from the eastern half of the caldera (Fig. 11). The thicknesses and welding of the Scoria-rich pyroclastic flow deposits suggest that they vented from near the northern caldera wall (Fig 6). The relatively thin yet welded scoria-rich tuff to the west (Fig. 5) may, however, have vented from that area. The South Pumice erupted from the western half of the caldera [START_REF] Stelling | Eruptive history of Fisher Caldera, Alaska, USA[END_REF]. Welding of the South Pumice is, in fact, restricted to the southwest margin of the caldera, implying a vent very near that area. South Pumice overlies debris avalanche deposits (Fig. 5), which may record partial collapse of either the caldera or the source vent of the South Pumice. The only other potential evidence for the timing of caldera collapse during the Fisher Tuff eruption is the lithic breccia within the Scoria-rich pyroclastic flow deposits.

Asymmetric Fountaining of the Scoria-Rich Eruption

To the north and northwest of the caldera Scoria-rich pyroclastic flow deposits tend to be >20 meters thick (Fig. 6). Outside of that region, those deposits are thinner and more sporadic. In fact, further than ~2 km away from the caldera to the east, southeast, and south margins of the caldera, only North Pumice, South Pumice, and Scoria-rich fall layers are found. It seems unlikely that thick pyroclastic flow deposits would erode from those areas, while much thinner fall layers remained intact, and so we believe that the asymmetric distribution of Scoria-rich pyroclastic flow deposits reflects their original deposition (Fig. 6).

In other asymmetric pyroclastic flow deposits, surrounding topography probably channeled pyroclastic flows in certain directions, such as the 1912 eruption of Katmai [START_REF] Hildreth | The compositionally zoned eruption of 1912 in the Valley of Ten Thousand Smokes, Katmai National Park, Alaska[END_REF], the Laacher See ignimbrite (Schumacher and Schminke, 1990), and the 1980 eruption of Mount St. Helens [START_REF] Rowley | Pyroclastic-flow deposits[END_REF]. In the case of the Fisher Tuff, however, most large pre-caldera volcanic features were actually in the northwest and west areas of the caldera, with essentially none to the southeast [START_REF] Stelling | Eruptive history of Fisher Caldera, Alaska, USA[END_REF].

That argues against the Scoria-rich pyroclastic flows being funneled to the north by large topographic barriers.

It thus seems that the Scoria-rich flows were directed rather than being channeled to the north and northwest. One explanation for why the flows could have been directed that way is that the eruptive jet generating the flows was forced to collapse that way by strong winds. Such winds would have to differ in direction from those at ~20 km altitude that blew the major fall deposits to the east (Figs. 3,11). Wind velocity often differs with height (e.g., [START_REF] Sarna-Wojcicki | Areal distribution, thickness, mass, volume, and grain-size of air-fall ash from the six major eruptions in 1980[END_REF], and so upper level winds could have been to the east, while lower level winds (where collapse is expected to occur) were to the north.

It has been shown that weak eruption columns can be bent if wind speed greatly exceeds their rise rate [START_REF] Carey | Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns[END_REF]. A collapsing fountain, however, occurs in the jet regime, before the erupted mass becomes buoyant. By estimating the amount of bending caused by strong winds of the momentum-driven jet, we can assess whether it collapsed entirely to the leeside of the wind. How much such a jet can be deflected by wind depends on the ratio (r) of jet momentum to wind momentum:

w j w j U U r ! ! = (1)
where ρ w and ρ j are densities of wind and the jet, and U w and U j are velocities of wind and the jet [START_REF] Muppidi | Study of trajectories of jets in crossflow using direct numerical simulations[END_REF]. The minimum upward speed of the jet is given by

5 / 1 ) 17 . 0 09 . 3 ( M n U j ! = ( 2
)
where n is the mass fraction of gas and M is mass flux [START_REF] Kaminski | Marginal stability of atmospheric eruption columns and pyroclastic flow generation[END_REF], and the density of the erupted mass in the jet is

g p j n n ! ! ! + " = 1 1 (3)
where ρ g and ρ p are densities of gas and particles [START_REF] Woods | The fluid dynamics and thermodynamics of eruption columns[END_REF]. The trajectory of the bending jet can be described by its horizontal displacement caused by the wind (x) at each height (z):

15 . 0 2 33 . 0 4 05 . 0 3 2 2 ! " # $ % & + ! " # $ % & ' r d r x r z ( ) ( 4
)
where δ is the wind boundary layer and d is vent diameter [START_REF] Muppidi | Study of trajectories of jets in crossflow using direct numerical simulations[END_REF].

We scale both x and z by conduit diameter, and assume that ρ w = 1.2 kg m -3 , n = 0.05, ρ g = 0.9 kg m -3 , ρ p = 2500 kg m -3 , and δ = 200 m (for a smooth surface; [START_REF] Turner | Workbook of atmospheric dispersion estimates[END_REF]. We also assume that z/d = 10 because fountaining has been calculated to restrict the jet propagation to ~10 vent diameters above the crater [START_REF] Bursik | Buoyant, superbuoyant and collapsing eruption colums[END_REF][START_REF] Neri | Numerical simulation of collapsing volcanic columns with particles of two sizes[END_REF]. We let vent diameter vary between 0.2 and 9 km (i.e. medium size conduit to caldera diameter), wind speed vary between 50 and 100 m/s, and mass flux vary between 10 6 and 10 10 kg/s (Fig. 14). We find that the horizontal displacement of the jet at its fountaining height is less than one conduit diameter for high mass fluxes (e.g., 10 9 kg/s), regardless of wind speed and vent size. At low mass fluxes (e.g., 10 6 kg/s), however, the jet can be deflected significantly, up to 20 conduit diameters.

Assuming that deflection must exceed 2-3 vent diameters to cause unidirectional fountaining, winds would have to have blown in excess of ~60-70 m/s during the Fisher Tuff eruption (Fig. 14). In fact, much stronger winds would likely need to have blown, because mass flux during the Scoria-rich phase was likely higher than our estimate, as discussed above. Moreover, displacements given by ( 4) are maxima, because U j is likely a minimum and because we neglect any possible overpressure of the jet. Although such winds are not unreasonable, we do not believe this likely, because those high winds would need to have died down to allow the South Pumice flows to travel to the south, and yet restart just when the Scoria-rich phase began again. Wind can be highly unstable, but it seems too fortuitous that it would blow only when scoria erupts.

Instead, we believe that the Scoria-rich pyroclastic flows were directed to the north because the conduit that fed the column was inclined to the north (Fig. 15). Because the jet would be inclined, its gravitational collapse would be asymmetric. In the case of the Fisher Tuff, fountaining was displaced to the north, and hence the thickest deposits, internal stratification, lithic breccia, and greatest welding are all found to the north of the caldera. The inclined conduit may have resulted from asymmetric block faulting as the caldera collapsed, which is supported by the complex evolution of vents.

Inclined conduits have been recognized in single-vent, small-volume eruptions (e.g., Stromboli, [START_REF] Chouet | Source mechanisms of explosions at Stromboli volcano, Italy, determined from moment-tensor inversions of very-longperiod data[END_REF]; Unzen, [START_REF] Umakoshi | Volcano-tectonic seismicity at Unzen volcano, Japan, 1985-1999[END_REF]Montaña Blanca, Folch and Felpeto, 2005). But, except for cases where topography acts as channels, caldera-forming eruptions often produce relatively axisymmetric deposits (e.g., Okmok, [START_REF] Burgisser | Physical volcanology of the 2050 BP caldera-forming eruption of Okmok volcano, Alaska[END_REF] Kos Plateau Tuff, [START_REF] Allen | Reconstruction of a major caldera-forming eruption from pyroclastic deposit characteristics: Kos Plateau Tuff, eastern Aegean Sea[END_REF]Tambora, Sigurdsson and Carey, 1989;Pinatubo, Scott et al., 1996), which suggests they were fed by vertical conduits. To our knowledge, the Fisher Tuff is the first example in which a large eruption involving caldera collapse was fed by an inclined conduit.

Transport and Deposition of the Fisher Tuff Pyroclastic Flows

A key to understanding the mechanics of the Scoria-rich pyroclastic flows is their deposits on the main ridge of the Tugamak Range and on its eastern side. (Fig. 7). We doubt the thin deposits are the eroded remnants of thicker flow deposits, because it seems unlikely that only thick deposits would be eroded, whereas thin layers, including underlying dacite fall layers, survived. We also would not expect erosion to leave a consistent stratigraphy across the ridges and to the east (Fig. 7). Although their sorting is similar to fall layers, the ridge deposits are as coarse as the fall layer at the caldera wall and as rich in lithics as flow deposits nearer the caldera (Fig. 10). Both of those characteristics argue against them being fall deposits (Plinian or co-ignimbrite). We thus interpret the layers on the ridges of the Tugamak and on its eastern border as veneer deposits from pyroclastic flows.

As expected, such veneer deposits are difficult to reconcile with a model that relies on deposition from only a dense, momentum-driven flow [START_REF] Miller | Spectacular mobility of ash flows around Aniakchak and Fisher calderas, Alaska[END_REF][START_REF] Legros | On the ability of pyroclastic flows to scale topographic obstacles[END_REF]. In particular, such a model cannot explain the absence of thick, massive deposits near the eastern border of the Tugamak Range nor, for that fact, their absence from the ridge on the Tugamak Range. Instead, we favor a model of densitystratified flow fed by an asymmetrically fountaining eruption (Fig. 15). As discussed earlier, grain-size distributions and stratigraphic relations indicate that the fountaining eruption produced both a buoyant plume and pyroclastic flows, but we note that >98% of the mass went into the flows. Internal depositional layering (cross bedding and lithic imbrication) and the coarse lithic breccia near the caldera rim are signs of highly unsteady deposition. Tephra falling from a fountaining eruption is either re-entrained upwards by the jet or fed into a lateral pyroclastic flow [START_REF] Neri | Numerical simulation of collapsing volcanic columns with particles of two sizes[END_REF][START_REF] Todesco | Pyroclastic flow hazard assessment at Vesuvius (Italy) by using numerical modeling. I. Large-scale dynamics[END_REF]. Re-entrainment creates a highly unstable recirculation zone that causes irregular feeding of the flow. As the flow traveled north away from the feeding zone, it would develop a concentration gradient and stratify into a dense undercurrent and an over-riding dilute turbulent cloud (e.g., [START_REF] Valentine | Statified flow in pyroclastic surges[END_REF][START_REF] Fisher | Transport and deposition of a pyroclastic surge across an area of high relief: The 18 May 1980 eruption of Mount St[END_REF][START_REF] Druitt | The physics of explosive volcanic eruptions[END_REF][START_REF] Burgisser | Reconciling pyroclastic flow and surge: the multiphase physics of pyroclastic density currents[END_REF]. The denser boundary layer at the base would produce massive, poorly sorted deposits (Fig. Model).

It is thus reasonable to assume that when the scoria-rich pyroclastic flow reached the Tugamak Range it was consisted of a dense undercurrent and an over-riding turbulent cloud. When the flow ran into the Tugamak Range, however, the ridge profoundly modified the highly concentrated boundary layer. The stratified current is unlikely to have maintained its integrity while ramping up the mountain range, because this would most likely have caused backflow of the high-concentration part [START_REF] Fisher | Transport and deposition of a pyroclastic surge across an area of high relief: The 18 May 1980 eruption of Mount St[END_REF]. This plus the absence of erosion on the ridge top, the abrupt loss of lithics across the ridge, and the presence of well-sorted veneer deposits all suggest that the vertical concentration gradient was significant and that the dense boundary layer was completely stripped off (Fig. 15).

The division between the dilute transport system, which is unaffected by the ground, and the undercurrent, where most deposition occurs, disappeared when the Tugamak Range blocked the undercurrent and let the turbulent cloud continue over the ridge.

On the ridge, the relatively coarse, well-sorted nature of the veneer deposits suggests they formed by aggradation of suspended tephra that could no longer be supported by the most energetic eddies of the turbulent cloud [START_REF] Burgisser | Physical volcanology of the 2050 BP caldera-forming eruption of Okmok volcano, Alaska[END_REF]. The stripped flow thus had a distribution of particles strongly controlled by the turbulent gas, and deposition on the ridge was from suspended load fallout [START_REF] Burgisser | Physical volcanology of the 2050 BP caldera-forming eruption of Okmok volcano, Alaska[END_REF]Bergantz, 2002, Druitt et al., 2002). The veneer deposits east of the Range could either suggest that the Tugamak Range also deflected undercurrents away from its sides or that the edges of the flow coincided with the eastern margin of the Range.

The ridge veneer deposits have well-defined coarse modes for scoria and lithics (Fig. 7). Assuming those modes result from suspended load fallout, we can calculate the average speed of the turbulent cloud as it passed over the ridge, following the method of [START_REF] Burgisser | Physical volcanology of the 2050 BP caldera-forming eruption of Okmok volcano, Alaska[END_REF]. Briefly, that model uses a combination of two dimensionless numbers (the Stokes number and what we call the Stability number) to determine speeds and sizes of supporting eddies. The Stokes number (S T ) is the ratio of pyroclast response time (U T /g, where U T is the particle terminal fall velocity and g is gravitational acceleration) to eddy rotation time, and the Stability number (Σ T ) is the ratio of U T to eddy rotation speed [START_REF] Burgisser | Reconciling pyroclastic flow and surge: the multiphase physics of pyroclastic density currents[END_REF]. For each size mode (scoria and lithic), pyroclasts with sizes within one standard deviation of the mode are assumed to be deposited by supporting eddies in which S T =1 and Σ T =1 (i.e., eddies with speeds equal to the terminal fall speed of that size). We find that coarser upper parts of the veneer deposits yield current speeds (~3.2 times supporting eddy speed; [START_REF] Burgisser | Reconciling pyroclastic flow and surge: the multiphase physics of pyroclastic density currents[END_REF] of 78 to 142 m/s, whereas the lower parts yield speeds of 43 to 82 m/s (Fig. 16). Uncertainties, estimated from differences in speeds for the two modes, are 10-38%. Particle aggregation or slower speeds may account for the secondary mode of fine ash in the lower parts of the veneer deposits (Fig. 7). Overall, the veneer deposits indicate that eddies depositing the upper layer were 30 to 320 m tall, and those for the lower layer were 16 to 200 m tall. We thus find that the Scoria-rich deposits record the passing of a pyroclastic flow that was significantly taller than the Tugamak Range.

As the current continued north, it stratified anew, producing another undercurrent, and thus deposits become thicker and richer in fine ash until eventually appearing like deposits south of the ridge (Fig. 8). Generation of this second undercurrent from a dilute, light-component-enriched part of the original flow is consistent with the deposits north of the main ridge being distinctly poorer in lithics compared to those to the south (Fig. 10).

Our model is thus of a density stratified turbulent flow that travels north from the caldera and interacts with the Tugamak Range (Fig. 15). The Campanian Tuff [START_REF] Fisher | Mobility of a large-volume pyroclastic flow -emplacement of the Campanian ignimbrite, Italy[END_REF], the Laacher See Tephra (Schumacher and Schminke, 1990), and Okmok Caldera [START_REF] Burgisser | Physical volcanology of the 2050 BP caldera-forming eruption of Okmok volcano, Alaska[END_REF] are other examples where topography strongly controlled deposition of density stratified flows. In the case of the Campanian Tuff, the flow was partly blocked by high ridges, and much thicker deposits are found in valleys than on local ridge tops, because of secondary flowage off of the ridges [START_REF] Fisher | Mobility of a large-volume pyroclastic flow -emplacement of the Campanian ignimbrite, Italy[END_REF]. In the Tugamak Range, we also find significantly thicker valley deposits compared to those on ridges, which may indicate some gravity-driven flowage. At Okmok caldera, veneer deposits found on ridge tops have grain-size distributions that match the coarsest half of nearby valley-filling deposits [START_REF] Burgisser | Physical volcanology of the 2050 BP caldera-forming eruption of Okmok volcano, Alaska[END_REF]. That relationship, similar to that at Fisher (Fig. 8), also led to the conclusion that veneer deposits originated from suspended load fallout.

Our model suggests that the Scoria-rich pyroclastic flows segregated in less than 15 km from the caldera, and probably even less than 6 km (Fig. 15). At Okmok caldera, the distance between the first ridge top and the caldera rim suggests flows segregated in less than 10 km [START_REF] Burgisser | Physical volcanology of the 2050 BP caldera-forming eruption of Okmok volcano, Alaska[END_REF]. That same distance is only a couple kilometers at Laacher See (Schumacher and Schminke, 1990). Fast stratification of fountaining-generated pyroclastic density currents thus appears common. But, can turbulent clouds stripped of their undercurrents re-stratify as fast? At Montserrat, much smaller pyroclastic flows took ~3-5 s and ~28 s, respectively, to form dense undercurrents that were able to sediment massive deposits [START_REF] Druitt | Small-volume, highly mobile pyroclastic flows formed by rapid sedimentation from pyroclastic surges at Soufrière Hills Volcano, Montserrat: an important volcanic hazard[END_REF]. At Fisher, the distance between the veneer deposits and the thick, poorly-sorted deposits in the valleys of the Tugamak Range is only 1-2 km (Fig. 8). Using our estimated speeds of the dilute cloud at the ridge (Fig. 16), we calculate that a dense undercurrent was re-generated in 7 to 26 s, on a time scale is similar to that at Montserrat. It thus appears that segregation is only a weak function of the total volume of material transported by the currents.

Stratification of flows can also be assessed using veneer deposits deposited after the flows interacted with a body of water. In the1883 Krakatau eruption [START_REF] Mandeville | Sedimentology of the Krakatau 1883 submarine pyroclastic deposits[END_REF], pyroclastic flows developed such strong vertical concentration gradients that they split at the shoreline into a denser subaqueous part and a more dilute subaerial part. The subaerial current remained dilute (less dense than water) for up to 40 km from source, when it reached land and left veneer deposits [START_REF] Carey | Pyroclastic flows and surges over water: An example from the 1883 Krakatau eruption[END_REF]. There is no indication of whether it reformed a dense boundary layer. At Kos, relatively dilute flows that crossed water did produce valley-filling deposits on the Turkish coast, and thus they reformed a dense boundary layer in less than 12 km [START_REF] Allen | Transport of pyroclastic flows across the sea during the explosive, rhyolitic eruption of the Kos Plateau Tuff, Greece[END_REF]. In contrast, at

Okmok extensive veneer deposits left by the flow after it crossing an 8-km strait between islands suggest the flow lost its undercurrent to the water, but was unable to recreate one before running out after more than 20 km inland [START_REF] Burgisser | Physical volcanology of the 2050 BP caldera-forming eruption of Okmok volcano, Alaska[END_REF].

The emerging picture is that pyroclastic flows segregate rapidly, regardless of their size (from <0.1 km 3 to >10 km 3 ) and fountaining geometry. If the dense boundary layer is thick enough, it forms an underflow that strongly reacts to encounters with topography or the sea. If the underflow is blocked, then the current generally continues its course with a weak concentration gradient and low depositional rate. Segregation, however, does not always recur in such stripped flows. In some cases, the concentration gradient completely fails to reestablish before run-out, but in others, like the Fisher Tuff, the undercurrent regenerates in a matter of seconds. Given that veneer deposits in the Fisher Tuff are much coarser than for example those at Montserrat, but have similar grain sizes as those at Okmok caldera, we suggest undercurrent generation is independent of the grain-size distribution of the stripped flow and its eruption size. Further study is needed to determine the controls of secondary segregation.

CONCLUSIONS

The stratigraphy of the Fisher Tuff suggests a complex eruptive sequence that started with a Plinian column of 18-22 km height that was blown east. This partly unstable column produced the ~0.2 km 3 DRE North Pumice deposits. The eruption then switched abruptly to a mixed basalt-dacite composition, producing Scoria-rich deposits. At some point during that phase of the eruption, a separate batch of dacite erupted from the western half of the caldera, producing the South Pumice. The Scoria-rich deposits consist of co-erupted pyroclastic flows and falls. The buoyant plume reached more than 22 km in height and was blown east, whereas most pyroclastic flows traveled to the north and northwest.

In the absence of topographic control, we suggest the strongly asymmetric distribution of the Fisher Tuff around the caldera implies that Scoria-rich pyroclastic flows were directed to the north. As this directed eruption cannot be explained by wind effects, we suggest that the conduit that fed the column was inclined to the north. This explanation makes the Fisher Tuff the first example of an inclined conduit feeding a >10

km 3 DRE eruption involving caldera collapse, and suggests subsurface geometry of the conduit controls the area affected by an eruption as much as does surrounding topography, regardless of eruption size.

On their way to the north, the pyroclastic flows crossed over the 500-700 m high Tugamak Range, ~15 km away from the caldera. Changes in deposits before, on, and after the Tugamak Range are best explained by a flow that was stratified into a dense undercurrent and an over-riding dilute turbulent cloud. Deposition before the Tugamak Range was mainly from the undercurrent. When the current ran into the Range, the ridge blocked the undercurrent, but let the turbulent cloud continue on over the ridge.

Deposition on the ridge was thus directly from the lower part of the turbulent cloud as suspended load fallout. As the flow continued north, it stratified anew, creating another undercurrent in 7-26 s. This interpretation implies that the Fisher Tuff records the passing of a flow that was able to quickly segregate twice. We believe this conclusion, which differs from that of [START_REF] Miller | Spectacular mobility of ash flows around Aniakchak and Fisher calderas, Alaska[END_REF], reflects recent developments in our views of the inner workings of pyroclastic flows, as well as the importance of finding thin remnants of the passing of the dilute cloud. (Table 1). NP=North Pumice. which thicken to the east, overlain by one or two scoria-rich deposits that are either well sorted or bimodal. Grain-size distributions and componentry are shown (Table 1). the Scoria-rich deposits with distance from caldera rim. Sizes are in φ units and in mm (Table 1). Open symbols are valley-filling deposits, closed symbols are veneer deposits, and the gray area indicates the main E-W ridge of the Tugamak Range. A dual lobed distribution (dashed lines are approximate isopachs) is suggested by the extent of welding to the south and west. Dash-dot line outlines Fisher Caldera.

a

  locations in Fig.1, except AFI-10, which is to the northeast on the Bering Sea coast. b NP=North Pumice, Scoria= Scoria-rich deposits, and SP=South Pumice. c All data in φ and (in mm) except total lithic content in wt.%. M d = median size; σ = sorting. Mode and second are coarse and fine modes of grain sizes.

FIGURE CAPTION Figure 1 .

 CAPTION1 FIGURE CAPTION

Figure 2

 2 Figure 2. a: Aerial photograph looking south at the Tugamak Range and Fisher Caldera.

Figure 3 .

 3 Figure 3. Thicknesses (in cm) of North Pumice deposit. Filled circles are sites where the

Figure 4 .

 4 Figure 4. Representative stratigraphic sections of sites east and northeast of Fisher

Figure 5 .

 5 Figure 5. Representative stratigraphic sections of sites west and south of Fisher Caldera

Figure 6 :

 6 Figure 6: Thicknesses (in m) of Scoria-rich pyroclastic flow deposits. Filled circles are

Figure 7 :

 7 Figure 7: Correlation of stratigraphic sections across the main east-west ridge of the

Figure 8 :

 8 Figure 8: Topographic variations across the middle part of the Tugamak Range, with

Figure 9 :

 9 Figure 9: Topographic variations across the eastern part of the Tugamak Range, with

Figure 10 :

 10 Figure 10: Variations in lithic content (in wt.%) and medium grain size (and sorting) of

Figure 11 :

 11 Figure 11: Thicknesses (in cm) of Scoria-rich pyroclastic fall deposit. Filled circles are

Figure 12 :

 12 Figure 12: Thicknesses (upper number, in cm) and lithic sizes (lower number when

Figure 13 :

 13 Figure 13: Lithic sizes (in mm) of North Pumice (lower number) and Scoria-rich (upper

Figure 14 :

 14 Figure 14: Minimum horizontal displacement (curved lines, in fractions of conduit

Figure 15 :

 15 Figure 15: Model for the generation and evolution of Scoria-rich pyroclastic flows. The

Figure 16 :

 16 Figure 16: Reverse modeling of velocities of pyroclastic density currents that deposited

  Figure 2

Table 1 : Grain-size data of Fisher Tuff

 1

	Sample a	Unit b	M d	c	σ c	Mode c	Second c	Lithics c Location Description
	AFI-03B NP	-3.25 (9.5)	1.15	-3.0 (8.0)			~2.5 km east of caldera wall
	AFI-03C Scoria -2.40 (5.3)	1.29	-2.5 (5.66)			~2.5 km east of caldera wall
	AFI-08B NP	1.04 (0.38)	2.60	3.5 (0.09)		10.7	~27 km northeast of caldera on Bering Sea Coast
	AFI-08D NP	0.97 (0.51)	2.52	-0.5 (1.41)		13.1	~27 km northeast of caldera on Bering Sea Coast
	AFI-10A NP	0.19 (0.88)	0.96	0.5 (0.71)			~36 km northeast of caldera on Bering Sea Coast
	AFI-10C Scoria -0.33 (1.26) 1.82	-0.5 (1.41) 4.0 (0.06)		~36 km northeast of caldera on Bering Sea Coast
	AFI-13A Scoria -0.51 (1.42) 3.31	-1.0 (2.0)		28.6	~2 km north of caldera wall
	AFI-13B Scoria -1.02 (2.03) 2.35	-1.5 (2.83)		24.9	~2 km north of caldera wall
	AFI-21D SP	-3.67 (12.7) 2.04	-3.5 (11.3)			~1.5 km from southern caldera wall
	AFI-21E SP	-3.76 (13.6) 1.82	-4.5 (22.6)			~1.5 km from southern caldera wall
	AFI-27A NP	-3.41 (10.6) 1.86	-4.0 (16.0) 4.5 (0.04)		~2 km from southeastern caldera wall
	AFI-27B Scoria -2.24 (4.72) 2.85	-3.0 (8.0)	3.0 (0.13)		~2 km from southeastern caldera wall
	FC-161B Scoria -1.50 (2.83) 3.42	-2.5 (5.66) 5.5 (0.02) 27.8	next to eastern boundary of Tugamak Range
	FC-162A Scoria -2.42 (5.35) 1.04	-2.5 (5.66)		25.6	on main ridge of Tugamak Range
	FC-162B Scoria -1.50 (2.83) 3.06	-2.0 (4.0)	5.5 (0.02) 30.4	on main ridge of Tugamak Range
	FC-317A Scoria -1.02 (2.03) 2.85	-1 (2.0)	4.5 (0.04) 21.9	on main ridge of Tugamak Range
	FC-317B Scoria -2.96 (7.78) 1.04	-3.5 (11.3)		22.1	on main ridge of Tugamak Range
	FC-320D Scoria -1.09 (2.13) 3.60	-2.5 (5.66) 4.5 (0.04)		at head of valley in Tugamak Range
	FC-321B Scoria 0.34 (0.79)	2.64	1.0 (0.5)		14.1	in valley of Tugamak Range
	FC-321C Scoria 0.00 (1.00)	3.11	-1.5 (2.83)		8.7	in valley of Tugamak Range
	FC-321D Scoria 0.15 (0.90)	2.97	-1.0 (2.0)		9.7	in valley of Tugamak Range
	FC-321E Scoria -0.04 (1.03) 2.66	-1.5 (2.83)		17.9	in valley of Tugamak Range
	FC-326A Scoria 0.50 (0.71)	2.54	2.5 (0.18)		13.9	on small ridge in Tugamak Range
	FC-327A Scoria -0.79 (1.73) 2.62	-1.5 (2.83)			in valley of Tugamak Range
	FC-330A Scoria 0.44 (0.74)	2.91	1.5 (0.35)		27.5	between Fisher Caldera and Tugamak Range
	FC-338B Scoria -1.07 (0.48) 2.60	-1.0 (2.0)			between Fisher Caldera and Tugamak Range
	FC-339B Scoria -0.06 (1.04) 2.96	1.5 (0.35)			between Fisher Caldera and Tugamak Range

ACKNOWLEDGEMENTS

This research was generously supported by the Alaskan Volcano Observatory and a research grant to J.G. from the National Science Foundation (EAR-0401784). We thank clast densities, where Σ T is the ratio between particle terminal fall velocity and eddy speed. We used 1059 kg/m 3 for scoria density (median value +1σ) because of their low vesicularity at those sites. Other variables used are dynamic viscosity (1.5x10 -5 Pa s) and kinematic viscosity (3x10 -5 m 2 s -1 ) of hot air (assumed to be at 350 °C).

John Eichelberger, Terry Keith, and Tom Murray for their support of our work at Fisher Caldera. The manuscript was greatly improved by thorough reviews by Peter Kokelaar and an anonymous reviewer.

FC-