On the Fluctuations in Volcanic Plumes

Abstract

Abstract Some basaltic open vent volcanoes show that spectroscopic SO 2 measurements of a volcanic gas plume are characterized by a fluctuating signal. Understanding the origin of these fluctuations is of great interest for interpreting volcanic gas emission regimes. Although some fluctuations may be associated with intermittency in the source flux, some may be associated with the turbulence in the flow. A simple laboratory experiment, in which we release dye pulses into a turbulent wake in a small water‐filled channel, suggests that when the intermittency of the source has comparable or smaller timescale as the turbulence, the fluctuations are similar, whereas when the intermittency of the source has longer timescale than the turbulence, the fluctuations can be distinguished. We also present some small‐scale laboratory experiments of turbulent buoyant plumes produced from a steady source; these demonstrate a time‐fluctuating concentration of source fluid downstream and the fluctuation period tends to increase with distance. Key Points If the frequency of the source is comparable to the frequency of the turbulence associated with the plume, it is difficult to distinguish specific explosions If the frequency of the source is lower than the frequency of the turbulent fluctuations in the plume, discrete puffs form The frequency of turbulent fluctuations in a plume decrease with distance downstream