Permeability controls on expansion and size distributions of pyroclasts

We synthesize field and experimental data to evaluate relationships between the textures and sizes of pyroclasts and the processes that shape them, with emphasis on the role of permeability. In (mafic) scoria, the important competition that determines preserved vesicularity is between gas escape and magma expansion by bubble growth; post-fragmentation expansion occurs rapidly until the permeability increases substantially. In (silicic) pumice, the competition is between gas escape and fragmentation by bubble overpressure; sufficiently permeable regions allow gas escape although high viscosity hinders further expansion. Thus the preserved vesicularity of both pumice and scoria is controlled by the permeability threshold, the vesicularity at which there is an abrupt increase in permeability over a small increase in vesicularity, which appears to be similar to 70-80%. High permeability thresholds may also explain the high fine ash content of silicic Plinian eruptions. At the local scale, magma ruptures because of stresses in viscous melt around expanding isolated bubbles. Local control is illustrated by the correspondence between the size distributions of the ash and of bubbles in individual pumice clasts of the 1980 Mount St. Helens Plinian eruption: the medians and modes for grain and bubble sizes are of order tens of microns, and fractal dimensions are similar (3.1-3.2 for grains; 3.4 for bubbles). Fractal dimensions for total grain size distributions and bubble size distributions in pumice from other silicic eruptions are similar (3.0-3.2, 2.9-3.9, respectively) and larger than generated by crushing and grinding rocks. This suggests that fragmentation efficiency depends on the balance between rates of magma decompression (overpressurization) and gas escape, which explains relationships between ash content and eruption rate. In summary, the vesicularity distribution of pyroclasts places important constraints on the permeability threshold of expanding magma, whereas the whole deposit grain size distribution of pyroclastic deposits places critical limits on the permeability structure of the magma at the point of fragmentation.