Classification, landing distribution, and associated flight parameters for a bomb field emplaced during a single major explosion at Stromboli, Italy L. Gurioli1, A.J.L. Harris1, L. Colò2, J. Bernard1, M. Favalli3, M. Ripepe2, and D. Andronico4 1
LMV, OPGC, CNRS, Université Blaise Pascal, 63038 Clermont-Ferrand, France Dipartimento di Scienze della Terra, Università degli Studi di Firenze, 50121 Florence, Italy 3 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, 56126 Pisa, Italy 4 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, 95125 Catania, Italy 2
ABSTRACT We propose a novel approach to studying a ballistic bomb deposit. Favorable circumstances, a unique dispersal axis, an operational thermal video camera, and application of an innovative methodology allowed estimates of volume and mass erupted, and definition of mass partitioning between bombs of various sizes. This allowed the creation of a multidisciplinary database for a single major explosion at Stromboli volcano (Italy), the type locality of Strombolian eruptions. The dispersion and direction of the deposit were consistent with a major explosion on 21 January 2010. Field data comprised 780 mapped bomb locations and sizes, and were organized into a GIS with a lidar-derived digital elevation model as its base. This allowed us to define the landing distribution and flight parameters for erupted bombs. The data defined discontinuous deposition to build a cluster-dominated bomb field, with a total deposit volume of ~10 m3, a mass of ~2 × 104 kg, and a grain size dominated by large bombs (1 m in diameter). The parameters defined here for a major eruption at Stromboli show that the Strombolian style of volcanism, and its deposits, need to be treated carefully, and a different approach is needed in the future to truly characterize and classify such small (but globally common) explosive eruptions. The recognition that sedimentation from such eruptions will be uneven leads to the important conclusion that isopachs and isopleths cannot be used to estimate eruption volumes for such explosions. INTRODUCTION Normal explosions at Stromboli are characterized by emissions lasting seconds to tens of seconds. They send jets of gas, ash, and bombs to heights of 100–200 m and emit 10–103 kg of bombs, at typical velocities of 10–50 m s–1, that land within 100 m of the vent (Chouet et al., 1974; Ripepe et al., 1993). Less frequent, major explosions are more energetic, having durations of as long as 3 min and sending bombs to distances of 1.5 km and plumes to 1000 m (Bertagnini et al., 2003; Andronico et al., 2008); more energetic paroxysms are rarer (Barberi et al., 1993). The 2003 paroxysm ejected ballistic blocks to a distance of 2 km; its plume ascended
to 4 km, fed at mass discharge rates of as much as 2.8–3.6 × 106 kg s–1, emplacing a 1.1–1.4 × 108 kg deposit (Calvari et al., 2006; Rosi et al., 2006; Ripepe and Harris, 2008). The 21 January 2010 major explosion had a dispersal direction that was not duplicated by other major explosions (Table 1); we therefore had a unique opportunity to define and sample a portion of a bomb-dominated deposit emplaced during a single event. In addition, the explosion was recorded by the permanent monitoring network of thermal, infrasonic, and seismic sensors of the University of Florence (Italy). This allowed us to provide a rare map for the landing positions of all particles across a Strombolian
TABLE 1. MAJOR EXPLOSIONS RECORDED AT STROMBOLI BETWEEN MAY 2009 AND MAY 2010 Date (dd/mm/yy)
Time (UT) (h:min:s)
Source crater
Duration (s)
Dispersion direction
Maximum particle velocity (m s–1)
Seismic displacement (×10–6 m)
Acoustic pressure (bar)
03/05/09 08/11/09 24/11/09
14:58:20 12:29:47 11:20:49 11:21:02 14:48:38 20:45:03 07:58:18
central central central central northeast central central
18 60 60 20 51 >60
NE ENE SE SE NE SSE-SW NE
130 90 150 130 72 75
7 24.4 24.6 21 15 17
0.79 0.60 0.70 1.43 0.89 0.67 0.62
10/01/10 21/01/10 12/03/10
Note: Source location and acoustic pressure are derived from infrasonic data; duration, dispersion direction, and velocity are from the thermal video data. Note that the 24 November 2009 event comprised two discrete events longer than 60 s, with a second explosion following the first after 13 s. UT—Universal Time.
GEOLOGY, May 2013; v. 41; no. 5; p. 559–562; Data Repository item 2013156
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bomb field. Detailed maps of particle landing positions have been produced for more violent events (Minakami, 1941; Self et al., 1980; Adams et al., 2006; Houghton et al., 2011), and ballistic parameters have been measured for mapped bombs (e.g., Lorenz, 1970; Fudali and Melson, 1972; Self et al., 1974; Steinberg, 1976; Fagents and Wilson, 1993; Mastin, 1995; Ripepe and Harris, 2008; Kilgour et al., 2010; Pistolesi et al., 2011). Our data add a resource for understanding the relation between emission dynamics and plume sedimentation for ballisticdominated Strombolian eruptions. METHODOLOGY During June 2010 and June 2011 (6 and 18 mo after the January 2010 eruption) we mapped a bomb field that opened southeast to southwest from Stromboli’s central crater (Fig. 1). Across our mapped area, maximum surface slopes were 27°–30° (Fig. 1B). The bombs (characterized by ellipsoid shape and similar thickness) were all flattened on impact, and stuck to the impacted surface without breaking, preserving their pristine landing shape and position. In total, 780 bombs were mapped (Fig. 2A; Item DR1 in the GSA Data Repository1). No other bombs were apparent within the crater area and beyond the southern edge of the mapped area (Fig. 1, solid red line), but others were observed on the steep slopes of Sciara del Fuoco (Fig. 1, dashed red line). For each bomb, we recorded GPS location (with a Garmin eTrex), and measured its long and intermediate axes (a and b, in millimeters) parallel to the ground surface, the shortest axis being perpendicular to the surface (i.e., bomb thickness; Fig. 3B). Scaled, vertical digital photos of 229 bombs were also manually taken from a height of 1.2 m. These were used to draw the bomb perimeter and to obtain the “footprint” area using the shape 1 GSA Data Repository item 2013156, Item DR1 (photographic survey of the Stromboli’s Crater Terrace between the six major explosions of 2009-2010), Item DR2 (complete bomb field data set), and Item DR3 (full image series during the 21 January 2010 eruption of Stromboli volcano), is available online at www.geosociety.org/pubs/ft2013.htm, or on request from
[email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
Published online 18 March 2013
© 2013 Geological America. For permission to copy, contact Copyright Permissions, GSA, or
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Figure 1. A: Plan view of shaded relief map of Stromboli volcano (Italy) summit area derived from August 2010 lidar data; yellow dots indicate mapped bomb locations. Source vent, central crater, and thermal camera (station ROC, Roccette site) locations are indicated by large yellowfilled circles. Solid red lines indicate walked margins of the bomb field; dashed red lines are projected limits. Note that within this zone all bombs were mapped, so that any gaps in coverage are real. Contours indicate travel distances reached by bombs launched with indicated velocities. B: Cross-section view showing topographic profiles along A-A′ and B-B′; yellow highlighted areas define bomb field.
preferred orientation software of Launeau and Robin (1996). The best fit between bomb area and axes ab was bomb area = 0.7502(ab)0.9787 (average error 23%). We also derived a best fit between bomb volume and ab based on an analysis of 53 bombs of known weight and density (average measured density 1810 kg m–3, range 1370–2300 kg m–3, as sampled along the longitudinal and horizontal dispersal axis; Fig. 2A). The best fit for bomb volume = 0.2786(ab)1.3676 had an error of 25%, much lower than the 150% error that we obtained from estimating bomb volume by multiplying average bomb thickness (6.5 cm) by bomb area. We applied the best fit relation to derive the volume for all other bombs across the field. For the 53 sampled bombs, mass was measured in the laboratory. For all unsampled bombs, we calculated mass from multiplying volume by the average density. To allow comparison with sieving grain size analyses (Inman, 1952), we used the b axis dimension to obtain φ ( = −log2b). The full data set is given in Item DR2. Bomb locations were projected onto a digital elevation model generated from an August 2010 lidar survey (Fig. 1), and converted to spatial distributions (Fig. 2) using a smoothing kernel estimate (Silverman, 1986). To eject a bomb to a horizontal distance d and an elevation Δh, launch velocity (v0) and angle (θ) (under dragfree conditions) are: v0 =
Figure 2. A: Bomb locations plotted by size (size of circle is function of bomb diameter; purple—24 November 2009; yellow—21 January 2010; red—sampled bombs) over crater area slope map. Dashed line is tourist track. B: Areal bomb densities, in terms of number (no.). C: Areal bomb densities, in terms of weight. In B and C, bombs associated with 24 November 2009 event have been excluded, and black dots indicate 21 January 2010 bomb locations. Diameter in key corresponds to average diameter.
gd 2 , d sin(2θ) − 2 Δh cos2 θ
(1)
where g is the gravitational constant. The minimum velocity required to reach any point on our map is now obtained at optimum launch angle θ of 45° + α/2, α being the angle between the launch and landing points [α = arctan(Δh/d)]. All bombs will also be subject to drag force (FD) and so will undergo deceleration (aD), given by: aD =
ρ AC v 2 FD = − air d , m 2m
(2)
where m is mass, A is cross-sectional area, Cd is the shape-dependent drag coefficient (0.47 for a large, spherical particle traveling at tens of meters per second; we assume that particles attained elliptical shapes upon landing), ρair is air density, and v is the bomb instantaneous velocity. This means that a higher v0 is required to throw a bomb the same distance as under drag-free conditions, and bomb mass, area, and shape are required to reconstruct bomb launch angles and velocities. Keeping all else constant, we see that drag deceleration increases as velocity increases, or as bomb mass decreases. EXPLOSION Six major explosions occurred at Stromboli between May 2009 and May 2010 (Table 1); all except one were from the central crater. They involved seismic displacement and acoustic
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Figure 3. A: Thermal image at onset of 21 January 2010 eruption of Stromboli volcano (Italy) (full image series is in Item DR3 [see footnote 1]), showing bomb spray that emplaced smaller bombs (white dotted line is leading edge), and second burst that emplaced larger bombs (red dotted line is leading edge). Emission can be separated into two directions (one south-southeast and one southwest). B: Left: example of bomb in the field, with long and intermediate axes (a and b) parallel to ground surface, and shortest axis, c, perpendicular to surface. Right: fractional number (no.) of bombs and bomb weight percent by class size φ. C: Plot (solid black square) of bomb size σφ (standard deviation) against Mdφ (median diameter) with areas defined by Walker (1971) for fall and pyroclastic flow deposits. A third field needs to be defined for 21 January 2010 bomb deposit.
pressures higher than those recorded during normal explosive activity, the latter having mean values of 4 × 10−6 m and 0.2 bar, respectively (Marchetti et al., 2010). All events produced dilute convecting plumes of ash and gas with maximum bomb velocities (derived from thermal video analysis) of 150 m s–1 (Table 1). The deposit in question was dispersed SSESW of the central crater (Fig. 1). This means that the major explosion of 21 January 2010 was the source of the bombs, because it was the only explosion of the series with such a dispersal direction (see Item DR1). To the east is the bomb field of 24 November 2009, which had a southeast axis (Andronico and Pistolesi, 2010); the majority of the purple bombs of Figure 2A are considered part of this field, based on their larger size and on photos taken before the 21 January explosion. Thermal video taken from station ROC (Fig. 1) of the 21 January event (Item DR3 and Video DR1), show an emission lasting 51 s and consisting of three main phases. Phase 1 comprised two simultaneous bursts, which sent bombs to the SSE and southwest. Each burst comprised two components: a leading spray of smaller bombs, quickly followed by emission of larger bombs that attained lower heights and fell closer to the vent than those of the first spray. Finger jets could be identified in the plume (Fig. 3A). An associated cloud of gas and finer material was observed, mostly associated with the southwest-directed burst and directed at an ejection angle of 45°. The second phase began after 7 s and involved emission of 2 or 3 plumes of gas and fine particles, lacking bombs. Phase 2 was over within 20 s and was followed by a third phase marked by a series of weakening gas-rich puffs that lasted 24 s.
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BOMB FIELD The thermal video reveals bombs moving away from the camera, indicating a southward dispersal direction opening at an angle of 180° (Fig. 1). Within this expected landing distribution, we mapped the SSE- and the SW-directed bursts (Fig. 1). However, for the volume estimation we extrapolate the dispersion, as mapped for the SSE lobe, across the unmapped southwest lobe on the Sciara del Fuoco (Fig. 1). The general shape of the south-directed burst is lobate and elongated to the SSE, with bombs landing at maximum distances of 429 m from the vent (Fig. 2A). Plan-view bomb areas total 91 m2, or 0.12% of the mapped area (75,000 m2). We find bomb-free zones (50–100 m in diameter), zones of scattered bomb coverage, and clusters of large and small bombs side by side (Fig. 2). The bombs are dominantly composed of fresh juvenile scoria with minor lithics of old scoria (1%–2%). Summary statistics for measured and calculated parameters for each bomb are given in Table 2. Long axis diameters were 7–459 cm, and only 3 scoria lapilli were found. Thus the fine components were either dispersed beyond the bomb field, and/or scarce and incapable of leaving a distinct deposit. Volumes for individual
bombs in the south sector ranged from 24 to 1.0 × 106 cm3, for a total volume of 10.24 m3. Masses ranged from 43 g to 1.9 × 106 g, for a total mass of 1.8 × 104 kg. Assuming the same mass for the unmappable southwest-directed burst, the total erupted bomb mass was 3.6 × 104 kg. The main phase of emission lasted 1–2 s. Bomb mass emission rates were therefore 2 × 104 kg s–1. We find a unimodal weight distribution with a mode at φ = –10 (diameter 102.4 cm; Fig. 3B). The median diameter (Mdφ) is φ = –8.2, with a standard deviation (σφ) of 1. Thus, in Walker’s (1971) diagram a third bomb field needs to be added to take into account such a deposit (Fig. 3C). We note that, although bombs with φ
