Abstract

Activity at Stromboli is exceptionally well characterized on time scales from milliseconds to millennia but it remains a challenge to identify what portion of that activity should be defined as “Strombolian’ in the strict sense of naming an eruption style, in a fashion that is useful at other volcanoes. Here I look at delineating the limits of ‘Strombolian style’, particularly in the context of demarcation from Hawaiian fountaining eruptions. That demarcation has generally been articulated in terms of transient versus sustained activity linked to mechanical decoupling versus partial coupling of exsolved volatiles. Plotting basaltic activity as an erupted mass-duration plot reveals some of the issues. Individual ‘normal’ Strombolian explosions occupy the short duration, small volume rate corner of such a plot, very clearly distinguished from high Hawaiian fountains that have durations that are 3 to 5 orders of magnitude longer and mass discharge rates that are 10 to 100 times larger. However the spectrum of activity called names such as ‘low fountains’, ‘gas pistoning’ and ‘paroxysms’ fills much of the area between these extremes. Finally, while Hawaiian fountains are sustained for hours to days, they are seldom steady, and new high-speed imagery shows that even normal Strombolian explosions are pulsatory in nature. The implication is that typical Strombolian explosions are not compatible with simple slugs (Taylor bubbles) and that typical Hawaiian fountains are not simple examples of either purely coupled bubbly flow or annular flow.