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 Scoria cones have been regarded to be formed by accumulation of ballistic bombs ejected by mild eruptions. However, more recent geological investigations show that some scoria cones could be formed during explosive eruptions. Here, we demonstrate how the scoria cone of the 1986 Izu-Oshima eruption was formed during the explosive eruption. We measured particle fractionation of the cone and propose a theoretical model to explain the observation. The model considers lateral transport of particles by turbulent eddies; particles that reached characteristic column radius, L, are laterally transported to ωL where they starts to free-fall. We obtained ω = 1.2 and 2.5 for larger and smaller particles, respectively, which is consistent to the observation. In the model, extensive fallout takes place at the base of the column where it expands rapidly. We suggest that lateral particle projection and the rapid column expansion are the key processes to form the cones.
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 Scoria cones, or cinder cones, are very common volcanic landforms but the process of formation is not sufficiently known. The built-up of very coarse particles from mild strombolian eruptions to form scoria cones was proposed by McGetchin et al.  and such has been adapted in many volcanology textbooks. On the other hand, it has been argued that some cones were formed during intensive phase of eruptions [Wood, 1980; Fierstein et al., 1997; Riedel et al., 2003; Yasui and Koyaguchi, 2004; Valentine et al., 2005; Martin and Németh, 2006; Sable et al., 2006]. Some of these studies suggest that cones are principally formed by extensive fallout of finer particles from the uprising column based on the patterns of thinning rate as a function of distance from the source [e.g., Sable et al., 2006]. However, theoretical investigations on uprising column indicate that the fallout from the uprising column is very limited [Ernst et al., 1996]. Here we examine the existing model of fallout from uprising column [Bursik et al., 1992] using quantitative observation of particle fractionation of a real eruption column.
 The 1986 eruption of Izu-Oshima volcano, Japan, is an observed example of a subplinian cone-forming eruption. In this study, we obtained the total grain size distribution (TSD) of the scoria cone to verify the process quantitatively. The TSDs of the subplinian fallout [Mannen, 2006] and of the cone obtained in this study were used to calculate particle fractionation between the cone and subplinian plume. The particle fractionation as a function of particle size will be a constraint to simulate physics of eruption column.
2. The 1986 Izu-Oshima B Eruption and Its Scoria Cone
 The 1986 Izu-Oshima B fissure eruption simultaneously produced lava flows, a scoria cone and a subplinian tephra fall of basaltic andesite composition [Endo et al., 1988; Nagaoka, 1988; Sumner, 1998]. The subplinian eruption consisted of a 12 to 16 km high column [Mannen, 2006]. The umbrella spread scoria fall mainly eastward of the source vent. Around the source fissure, an irregular shaped scoria cone (CB) [Endo et al., 1988] with dimensions of approximately 1000 × 600 m elongating NW–SE direction and a height of 40 m was formed. The volume of the cone is estimated to be 6.4 × 106 m3 from morphological analysis [Nagaoka, 1988]. Here, a cone density of 790 kg/m3 is adopted based on the average field measurement ranging from 640 to 1100 kg/m3. The total mass of the cone is thus determined to be 5.1 × 109 kg.
 The cone was formed during the main phase of the eruption, which commenced in the late afternoon of 21 November 1986. The formation processes of the cone and the lava are summarized by Sumner . The rapid growth of the cone leads to the coalescence of spatters at the base and made a lubricating layer. The spread of the layer caused the failure and breaching of the cone. Some of the chunks of the scoria cone were transported by the lava flows that were issued from the base of the cone, and now recognized as scoria rafts. Such scoria rafts and the dissected central part of the cone allowed sampling of the internal sections of the scoria cone. The scoria cone is quite massive and apparent lamination or systematic change of grain size within the section was not recognized.
 Most of the lapilli-sized particles within the cone and the rafts are scoriaceous and apparently identical to the scoria of distal fallout. Particles that are generated by breakage of bombs could be recognized from its fluidal shape and vesicle-poor feature. However, such bomb origin particles are rare in the cone and in the rafts.
3. Sampling and Grain Size Analysis
 We carried out sampling and granulometric analysis on twelve sites scattering evenly in the cone (four sites) and on the scoria rafts (four sites on each two major lava lobes). Azimuth and distance to the sampling points range −31° to 161° and 0 to 1550 m respectively, from the center of the cone (Loc. 111; 34.7356°N, 139.3953°E). At the four cone sites, samples are obtained from the midmost of the sequence to omit falls of growing and waning stages of the eruption. The scoria cones and the scoria rafts exhibit welded portions. The scoria in the interior is not heavily sintered and easy to scrape. During the sampling excavation, it was necessary to break scoriaceous blocks that form the framework of the deposit. These blocks constitute the largest size class (256–128 mm) and were obtained in many localities. Particles larger than 256 mm are very rare and were excluded from the analysis.
 From each locality, samples greater than 10 kg were collected and analyzed. The samples were sieved and segregated into 13 size classes at intervals of 1 ϕ [ϕ = −log2d; d = diameter in mm] in the field and in the laboratory. Since the grain size distributions of the twelve measurements are quite similar each other, TSD of CB is represented by simple average of the twelve analyses. We also confirmed that the grain size distribution of a site where significantly hammered during sampling shows no significant modification judging from comparison to those of other site. This means that the effect of the block destruction during the sampling is limited.
 To obtain the TSD of all pyroclasts issued from the fissure, the grain size distribution of fallout of the simultaneous subplinian plume [Mannen, 2006] was integrated with the cone results of this study. The results are summarized in Table 1.
Table 1. Erupted Mass of the 1986 Izu-Oshima B Eruption for Each Grain Size and Deposit Categorya
Deposit within 0.7 to 1.4 km from the vent (square root of isopach area).
Deposit beyond 1.4 km from the vent (square root of isopach area).
−8 to −7
−7 to −6
−6 to −5
−5 to −4
−4 to −3
−3 to −2
−2 to −1
−1 to 0
0 to 1
1 to 2
2 to 3
3 to 4
4. Result and Discussions
4.1. Constituents of the Scoria Cone
 Our particle size analysis for whole CB shows that 68% of the cone is composed from non-bomb fragment (<64 mm) [Mannen, 2006]. This observation quantitatively confirms the conclusions in the previous studies that argue fine rich nature of scoria cones [e.g., Valentine et al., 2005; Martin and Németh, 2006; Riedel et al., 2003]. Since fragments of this size could not to be attributed to breakage of larger bombs, it is concluded that the scoria cone is not a simple pile of ballistic bombs that cannot be incorporate with uprising plume.
4.2. Theory of Fallout From Column Margin
 Some recent studies argue that proximal deposit in an intensive eruption can be attributed to fallout mode that were not described in the current plume models [e.g., Fierstein et al., 1997; Valentine et al., 2005; Sable et al., 2006]. Indeed, the physics of gas thrust region, where expands rapidly and the particle concentration is high, is poorly understood because analogue or numerical investigation of the region is very difficult. In present plume model, gas thrust region tentatively assumes almost same physics as that of convective region. In this study, we examined whether the observed particle fractionation of the 1986 Izu-Oshima B is consistent with fallout as predicted in the present simple models of fallout and eruption plume.
where M(ϕ, h) is the mass of particle of a given size fraction ϕ and height h, Aeff is the effective area at plume margin for particle fallout, C is the mass concentration of particle, vt is the settling velocity and u is the upward velocity of uprising column. When we consider a horizontal slice of the uprising plume with thickness of dh, these parameters are expressed as follows;
where L is the characteristic radius of eruption column defined as L2 = A and A is the cross-sectional area of the plume [Woods, 1988].
 As shown in equations 1 to 3, the fallout strongly depends on plume geometry. In this study, the plume model of Woods  for the column of the 1986B eruption is adopted. Parameters are selected to reproduce the eruption column of 14 km high (magma discharge rate is 8.1 × 105 kg/s) and initial upward velocity and water content of the column is selected based on the observation and petrological analysis (140 m/s and 1 wt% respectively) [Mannen, 2006]. Effect of particle fallout is not considered.
 The entrainment coefficient ɛ is defined as 0.09 in convective plume and 0.09√β/α in gas thrust region, where α and β are densities of atmosphere and the plume respectively. The mass change as a function of height is numerically calculated using equation 1 and terminal velocity of particle [Kunii and Levenspiel, 1960], assuming density of all particles is 1000 kg/m3.
 We also considered the effects of radial inflow due to entrainment of the uprising plume. The radial inflow velocity (ve) is expressed as a function of distance from axis of the uprising plume (r) as,
 Here, the velocity at the column margin ve(L, h) is defined as ɛU(h), where U(h) is the upward velocity of the plume at the column axis. The radial inflow affects the particle once it is released at the column margin. At certain conditions, the released particle is recycled into the column.
 Using the given equations, the mass and trajectory of particles that are released from a certain height are calculated for the whole size range and height from the vent to the neutral buoyancy height [640 and 10000 m above sea level respectively]. For numerical calculation of particle trajectory, the Euler method was used with a step size for vertical axis of 5 m. When particle that released at a given height (hd) is re-entrained into the plume in the lower height (hr), particle release at hd is not counted to simplify the calculations. Bias arising from the omission does not affect the calculations since hd and hr are similar in most cases. Since the area of CB is roughly 0.5 km2, the calculated particle size distribution of the cone is represented by the composition of particles settled within 400 m from the vent. Firstly, we calculated for the two cases, with and without considering the radial inflow effect (model 1 and 2 respectively).
4.3. Particle Fractionation of CB
 The calculated particle fractionation of the cone is shown in Figure 1. The observed particle fractionation shows broad agreement with the prediction of model 1, however, it shows excessive fallout of lapilli sized particles. The finer particles rather appear to be consistent with the model 2. In other word, the finer particles seem to be less affected by lateral inflow. This observation is inconsistent because finer fraction is affected by weaker lateral inflow. Partial column collapse or some discontinuity of the uprising plume such as ‘radially suspended flow’ [Suzuki et al., 2005] may cause such fractionation. However, such have never been observed in the eruption. Here we would like to attribute particle transport within the plume.
 The present model for eruption column discusses the mass and heat balance of the ascending plume to introduce ‘control volume’ dimension of which is L2dh. The characteristic column radius L does not correspond to the ‘visible’ column radius (Lv). It is noteworthy that the material released at L can be transported by the turbulent eddies to some distance up to Lv (>L). Here, we assume that smaller particles released at L drift laterally to Ld (<Lv) of the same height loss their support by upward gas flow and start to fall out due to the effect of atmospheric radial inflow (model 3). In this paper, the coefficient defined as Ld/L is given as ω. The maximum ω correspond to Lv/L and can be obtained from the numerical result and the observation. In Figure 2, the increase rates of L and Lv as a function of height are shown. From numerical analysis L/h is known to be 0.14 and from photographic analysis maximum Lv/h is obtained to be 0.35. Using them, maximum ω is assumed to be 2.5. In the model 3, ω is thus assumed to be 1.2, 1.4 and 2.5 for particles of >8 mm, 8-4 mm and <4 mm, respectively. The model 3 seems to be consistent with the observation (Figure 1).
4.4. Fallout of Cone-Forming Column
Ernst et al.  argued that fallout from the uprising column is very limited and major fallout takes place at the transition between the column and the umbrella region. It is also argued that the particles that fallout at the transition are swept back to form near vent fall deposits. The model 1 and 3 in this study also predict that the fallout from the column margin is nearly absent in almost the whole range of the column height (Figure 3). However, our calculation indicates that the contribution of fallout from the column-umbrella transition is minor while the fallout from the base of the uprising column is most significant. Extensive fallout at the base of the column is attributed to the slope of column edge.
 We may derive a conceptual criterion for particle re-entrainment to understand the reason of extensive fallout at the base of the column. The slope of the column edge is expressed as dL/dh. Provided the settling velocity and air inflow velocity are constant, the condition for re-entrained particles is expressed as dL/dh < ve/vt. This relationship qualitatively indicates that rapid expansion of column as a function of height tends to avoid particle re-entrainment.
 In the case of the 1986B eruption, our calculation predicts dL/dh = 0.38 for h < 500 m and 0.15 for > 500 m (Figure 3). Since the heat transfer from the pyroclasts to the entrained air is significant, rapid expansion takes place at the bottom part of the column [Woods, 1988]. Such rapid expansion is not predicted in the classical column model based on scaling approach and was not reproduced by analogue experiments that do not replicate great heat transfer from magma to air [Sparks, 1986; Ernst et al., 1996]. The effect of rapid expansion to extensive fallout near the source vent has not been considered in previous studies. In addition to the rapid expansion, high particle density is also essential for extensive fallout as known from equation .
 Scoria cones seem to be developed on relatively flat ground. As such, the occurrence of scoria cones may be attributed to this ideal condition for particle fallout as predicted in the model presented here. In certain situation such as the source vent located in the bottom of a large crater, the plume may sufficiently expand until it rises to the crater rim. Indeed, we often observe plume that is issued from very small sources vent occupies entire width of the large crater. In such case, significant fallout of uprising plume beyond the crater rim could be absent. Construction around the source vent could be impeded due to vigorous mixing within the crater or the steepness of the slope of the crater prevents cone formation.
 Cone-forming eruptions could be hazardous in certain situations since they may generate pyroclastic flows [Yamamoto et al., 2005], however, physics of basal part of the eruption column is poorly understood. Quantitative observation of particle fractionation as shown in this study will be one of a few observational constraints to the modeling such as high resolution 3D calculation [e.g., Suzuki et al., 2005], which is a promising approach to understand processes in gas thrust region.