The theoretical basis for textural indices of eruption dynamics: review and new conceptual models

The textural indices of eruption products (pumice, scoria and lavas), number density of crystals or bubbles, or characteristics of their size distributions, have been proposed to estimate dynamic properties such as magma ascent rates, on the basis of theoretical and experimental studies. To clarify the applicability and limitation of these indices, I review the fundamental mechanism of nucleation and growth of crystals and bubbles during change of temperature and pressure, together with discussion of applications of these methods to case studies. Discrimination between homogeneous nucleation (HON) and heterogeneous nucleation (HEN) are critically important in interpreting rock texture such as microlite number density (MND) and bubble number density (BND) in matrix-textures. In HON cases, traditional rate-meters for matrix-texture are applicable, whereas in HEN cases, these meters should be used carefully, consulting natural observation such as compositional zoning of crystals, laboratory experiments, and geological occurrence. Pheno-textures such as phenocrysts and pheno-vesicles (-bubbles) which can be expected to carry information about the state of deeper processes in magma chamber and magma supply to magma chamber from the mantle source regions are still limited to constrain magma dynamics. As an example of study on phenocrysts, the slope of crystal size distribution as an index of magma supply rate to a shallow magma chamber from deeper region is discussed with a simple model and existing experimental data, which allow us to estimate deeper conduit cross-sectional area and ascent velocity from deeper magma source region in mantle beneath Sakurajima volcano. Pheno-vesicle content as an index for eruption volume is proposed on the basis of recent numerical experiments reproducing cumulative volume evolution (Step-diagram) of the last 500 years at Sakurajima volcano and textural observation. Pheno-bubble number density can also be leveraged to evaluate the decompression rate during caldera-forming eruptions as the index of decompression rate during magma evacuation when caldera-forming eruption occurs.