Magmatic Gas Composition Reveals the Source Depth of Slug-Driven Strombolian Explosive Activity

Strombolian-type eruptive activity, common at many volcanoes, consists of regular explosions driven by
the bursting of gas slugs that rise faster than surrounding magma. Explosion quakes associated with
this activity are usually localized at shallow depth; however, where and how slugs actually form
remain poorly constrained. We used spectroscopic measurements performed during both quiescent
degassing and explosions on Stromboli volcano (Italy) to demonstrate that gas slugs originate from as
deep as the volcano-crust interface (~3 kilometers), where both structural discontinuities and
differential bubble-rise speed can promote slug coalescence. The observed decoupling between deep
slug genesis and shallow (~250-meter) explosion quakes may be a common feature of strombolian
activity, determined by the geometry of plumbing systems.

1. R. S. J. Sparks, J. Volcanol. Geotherm. Res. 3, 1 (1978).
2. L. Wilson, J. W. Head III, J. Geophys. Res. 86, 2971 (1981).
3. E. A. Parfitt, J. Volcanol. Geotherm. Res. 134, 77 (2004).
4. C. Jaupart, S. Vergniolle, Nature 331, 58 (1988).
5. C. Jaupart, S. Vergniolle, J. Fluid Mech. 203, 347 (1989).
6. B. Chouet et al., J. Geophys. Res. 102, 15129 (1997).
7. B. Chouet et al., J. Geophys. Res. 108, 2019 (2003).
8. M. Ripepe, S. Diliberto, M. D. Schiava, J. Geophys. Res.
106, 8713 (2001).
9. C. A. Rowe, R. C. Aster, P. R. Kyle, R. R. Dibble,
J. W. Schlue, J. Volcanol. Geotherm. Res. 101, 105 (2000).
10. M. T. Hagerty, M. Protti, S. Y. Schwartz, M. A. Garces,
J. Volcanol. Geotherm. Res. 101, 27 (2000).
11. S. Vergniolle, G. Brandeis, J.-C. Marechal, J. Geophys.
Res. 101, 20449 (1996).
12. M. Ripepe, A. J. L. Harris, R. Carniel, J. Volcanol.
Geotherm. Res. 118, 285 (2002).
13. N. Métrich, A. Bertagnini, P. Landi, M. Rosi, J. Petrol. 42,
1471 (2001).
14. A. Bertagnini, N. Métrich, P. Landi, M. Rosi, J. Geophys.
Res. 108, 2336 (2003).
15. P. Landi, N. Métrich, A. Bertagnini, M. Rosi, Contrib.
Mineral. Petrol. 147, 213 (2004).
16. P. Allard, J. Carbonnelle, N. Métrich, H. Loyer,
P. Zettwoog, Nature 368, 326 (1994).
17. P. Allard et al., Geophys. Res. Lett. 27, 1207 (2000).
18. F. Barberi, M. Rosi, A. Sodi, Acta Vulcanol. 3, 173 (1993).
19. A. Aiuppa, C. Federico, Geophys. Res. Lett. 31, L14607
(2004).
20. T. Mori et al., Earth Plan. Sci. Lett. 134, 219 (1995).
21. P. W. Francis, M. Burton, C. Oppenheimer, Nature 396,
567 (1998).
22. P. Allard, M. Burton, F. Muré, Nature 433, 407 (2005).
23. C. Oppenheimer, P. Bani, J. Calkins, M. Burton,
G. M. Sawyer, Appl. Phys. B 85, 453 (2006).
24. Data were collected with a Bruker OPAG-22 FTIR
spectrometer, working at 0.5 cm−1 resolution.
Single-scan, double-sided interferograms were collected
every ~4 s and were Fourier transformed offline with the
use of Norton-Beer medium apodization. Spectral
analysis was performed with a nonlinear least-squares
fitting program and an adapted forward model based
around the Reference Forward Model (33). The different
physical conditions of atmospheric and volcanic gases
were taken in account using a two-layer atmospheric
model. Volcanic gas temperature was retrieved by fitting
this parameter during analysis of the SO2 n1 + n3
combination band at 2500 cm−1, whose rotational line
envelope is highly temperature dependent. Source
temperatures were determined from the ratio of the
observed signal at 4400 and 4460 cm−1 and fitting to a
Planck curve. Quiescent degassing compositions between
the explosions were determined with the use of linear fits
to correlation plots of volcanic gas amounts.
25. P. Allard, abstract GMPV7-8044, presented at the
European Geophysical Union General Assembly, Vienna,
Austria, 16 to 20 April 2007.
26. S. Newman, J.B. Lowenstern, Comput. Geosci. 28, 597 (2002).
27. M. Chaigneau, C. R. Acad. Sci. Paris 261, 2241 (1965).
28. M. L. Carapezza, C. Federico, J. Volcanol. Geotherm. Res.
95, 227 (2000).
29. M. Burton, H. Mader, M. Polacci, P. Allard, Eos 86 (Fall
Meeting Suppl.), 52 (abstr. V13G-01) (2005).
30. H. Langer, S. Falsaperla, Pure Appl. Geophys. 147, 57 (1996).
31. M. Ripepe et al., Geology 33, 273 (2005).
32. M. R. James, S. J. Lane, B. A. Chouet, J. Geophys. Res.
111, B05201 (2006).
33. A. Dudhia, University of Oxford, www.atm.ox.ac.uk/RFM.
34. N. Métrich, R. Clocchiatti, Geochim. Cosmochim. Acta 60,
4151 (1996).