The influence of magma supply rate on eruption style and textures of erupted ash particles

Persistent low- to moderate-level eruptive activity of basaltic to andesitic volcanoes is difficult to monitor because small changes in magma supply rates can cause abrupt transitions in eruption style. Direct measurement of magma ascent rate is not possible; therefore, we must develop indirect measures of ascent rate. Here, we use a petrologic approach to this problem. We quantify textural variations in Vulcanian and Strombolian eruptive products (erupted between 1999-2006) from Tungurahua volcano, Ecuador. We compare average crystallinities of pyroclasts from a succession of ash eruptions with magma supply rates determined using satellite observations of plume heights and seismic recordings of explosion frequency. We show that tephra crystallinity correlates inversely with magma supply rate, similar to correlations described from decompression experiments of intermediate composition magma. We further suggest that the range of textures present in any individual sample indicates recycling from one eruption to the next and/or simultaneous eruption of particles with different crystallization histories. The correlation between crystallinity and magma supply rate is most easily explained by efficient degassing at very low pressures and degassing-driven crystallization between eruptions. A similar correlation has been shown at Crater Peak, USA (Gardner et al. 1998); Merapi, Indonesia (Hammer et al. 2000); Unzen, Japan (Nakada and Motomura, 1999); and Sakurajima, Japan (Miwa et al., 2009). Futhermore, systematic changes in crystallinity have been shown to extend across a range of eruption styles (including Vulcanian and Strombolian), as at Etna, Italy (Taddeucci et al. 2002) and Croscat, Spain (Cimarelli et al., 2010). This variation with eruption style suggests that the observed transition from intermittent Vulcanian explosions to more continuous periods of Strombolian eruptions and lava fountains can be explained simply by changes in the average crystal content of the shallow magma and its effect on the ability of bubbles to rise through (Strombolian) or become trapped beneath (Vulcanian) vent-capping magma. Cimarelli C, Di Traglia F, Taddeucci J, 2010, Basaltic scoria textures from a zoned conduit as precursors to violent Strombolian activity: Geology, 38: 439-442 Gardner CA, Cashman KV, Neal CA, 1998, Tephra-fall deposits from the 1992 eruption of Crater Peak, Alaska: implications of clast textures for eruptive processes: Bulletin of Volcanology, 59: 537-555 Hammer JE, Cashman KV, Voight B, 2000 Magmatic processes revealed by textural and compositional trends in Merapi dome lavas: Journal of Volcanology and Geothermal Research, 100: 165-192 Miwa T, Toramaru A, and Iguchi M, 2009, Correlations of volcanic ash texture with explosion earthquakes from vulcanian eruptions at Sakurajima volcano, Japan: Journal of Volcanology and Geothermal Research, 184: 473-486 Nakada S, and Motomura Y, 1999, Petrology of the 1991-1995 eruption at Unzen: effusion pulsation and groundmass crystallization: Journal of Volcanology and Geothermal Research, 89: 173-196 Taddeucci J, Pompilio M, Scarlato P, 2002, Monitoring the explosive activity of the July-August 2001 eruption of Mt. Etna (Italy) by ash characterization: Geophysical Research Letters, 29