Photoballistics of volcanic jet activity at Stromboli, Italy

Volcano Stromboli, Italy, has been in a nearly continuous state of activity since the earliest records were made by Aristotle. Well-collimated jets of incandescent gases loaded with basaltic vitric lapilli characterize these eruptions, for which the term strombolian activity has been coined. We report results of a photographic study of two eruptions, one relatively large and one small, using principally high-speed movies taken with a Hulcher 102 (70-mm) camera at a rate of 10 frames/s and exposure time of 1/72 s. Ninety-six frames, containing about 13,000 streaks, were analyzed with a computer interfaced digital film reader yielding detailed data on the bulk and time-dependent properties of these two eruptions. We believe that these two eruptions bracket typical conditions at Stromboli; hence the results can be taken as representative of strombolian activity at the type locality. Geometric ambiguities, data acquisition, and reduction problems are discussed. Results for the two eruptions were as follows: duration of eruptions, 8 and 5 s; number of particles ejected, 2594 and 152; average (and maximum) particle size, 2.2 (31.8) and 2.5 (13.5) cm; average (maximum and minimum) particle velocity, 26.2 (2.5–72) and 15.3 (4.5–38) m/s; median angular dispersion from the jet axis, 8° and variable (10°–45°); mass of particles ejected, 100 and 7.6 kg; total kinetic energy of particles, 16.2 and 2.5×1010 ergs; calculated possible range of gas density, 0.05–0.30 mg/cm3 for both eruptions; maximum gas velocity, 112 and 94 m/s; calculated mass of gas ejected, 240 and 120 kg; and gas to lava mass ratio, 2.4 and 15.8. Particle velocity and calculated gas velocity decayed from their initial maximum values, but well-marked oscillations with a period of about 1.6 s were superimposed. One model explaining these fluctuations is organ pipe resonance implying a tube length of 300–400 m. We estimate the flow to be subsonic, with a Mach number of 0.2–0.3. The range of estimated gas density is consistent with mixtures of CO2 and H2O.