One model for formation of obsidian pyroclasts suggests that they form through sintering of ash particles on volcanic conduit walls, which are subsequently torn out and entrained in the gas-particle dispersion out of the erupting vent. Here, we investigate microlite abundances and textures in obsidian pyroclasts in order to determine the time required to produce adequate numbers of microlites, and hence the pyroclasts themselves. We measured microlite number densities (MNDs) and microlite and vesicle orientations in obsidian pyroclasts in tephra deposits from the 1340 A.D. North Mono eruption. MNDs increase with decreasing dissolved H2O concentrations. Also, microlite spatial orientations become less aligned and differ more from vesicle orientations with decreasing dissolved H2O concentrations. MNDs increase from the second layer (P2) through the final layer (P10). To investigate timescales required to replicate MNDs in the North Mono obsidian, we performed time, temperature and pressure-controlled experiments with rhyolitic glass from the same eruption. MNDs in our experiments initially increase with decreasing pressure (50-35 MPa), then decrease as pressure decreases further(35-10 MPa). MNDs in obsidian from layers P2-P10 were replicated in ~7 hours or less. Based on these observations we propose a model where during the initial phase of the North Mono eruption most obsidian formed close to the magmatic fragmentation depth, equilibrated for short time periods (< 7 hours) and were then erupted out of the volcanic vent. These obsidian clasts have lower MNDs than subsequent phases, and microlites are well aligned with each other and with vesicles, reflecting their short residence time in the conduit, higher dissolved H2O contents, and lower viscosities. During later phases of the North Mono eruption obsidian formed at various depths in the conduit, equilibrating for longer periods of time (≤ ~7 hours) before being erupted out of the vent or sintering together with other clasts and equilibrating at shallower depths before being erupted. These obsidian clasts have higher MNDs than earlier phases of the eruption, and microlites are not well aligned with each other or with vesicles, reflecting their variable residence times in the volcanic vent, lower dissolved H2O contents, and higher viscosities.

Energetic ash-producing volcanic eruptions are driven by the diffusive and decompressive growth of bubbles (mostly water) during ascent in a magma conduit. The spatial distribution of bubble nucleation sites is one of the primary controls on ash-forming fragmentation. However, the initial formation of bubbles in a supersaturated magma is problematical, especially for homogeneous nucleation. Excessive surface tension pressure should preclude the existence of small bubbles, because exsolved water is driven back into the melt. This is the “tiny bubble paradox.” We suggest that—under special circumstances—the tiny bubble paradox may be circumvented by spinodal decomposition, a process in which uphill diffusion enables spontaneous unmixing of phases to reduce the free energy of the system. As spinodal decomposition progresses, three dimensional, quasi-spherical, zones of water-rich magma develop. These zones are characterized by an increasingly high concentration of dissolved water at the centers and reducing concentration at the margins. Bubbles are born when the concentration of water in the interior of the water-rich zones goes to 100% and the concentration of melt goes to zero. The small, nascent, bubbles that emerge will be buffered from melt by water-rich shells with increasing melt concentration away from newly formed bubbles. This diffuse concentration gradient of water means that there is no surface, per se, for surface tension to arise. This is the crux of the solution of the tiny bubble paradox. Particle morphology may be used to distinguish ash with spinodal origins from ash associated with typical (metastable) bubble nucleation. Spinodal decomposition occurs at a wavelength determined by the pressure, temperature, and viscosity of the magmatic system. This wavelength should create bubbles of uniform size and bubble walls of equal strength in a fragmenting magmatic foam, leading to sharply mono-modal vesicle and ash particle size distributions. Classical bubble nucleation should create more-variable bubble sizes and bubble wall strengths, leading to a broader particle size distribution. Better understanding the mechanism of bubble formation in magmatic systems will, in turn, enable better understanding of hazardous, explosive, eruptions.

Tree damage can provide insights into internal dynamic pressure changes of pyroclastic density currents (PDC). On 18 May 1980, Mount St. Helens erupted a laterally directed PDC that decimated ~600km2 of forest, referred to as the blowdown zone. The head of the current contained the peak dynamic pressure, which uprooted or broke off most trees and stripped them of vegetation; however, some partially stripped tree trunks were left standing. Tree damage was assessed using aerial photography taken one month after the eruption. The flow direction of the PDC was mapped from shadows of root balls of toppled trees and directions of fallen trees. Along given flow paths, the density of standing trees was measured by the number of shadows within 200m2 areas. Towards the northwest, the average tree density increased from 0.01 to 0.58 (± 0.19) trees/m2 with distance. Additionally, analysis identified 95 clusters of trees still standing in the blowdown zone, situated on the lee sides of hills or plateaus. Blurry, cylindrical shadows versus well-defined, cylindrical shadows distinguished standing trees with foliage in clusters from those without. Five variables were used to determine the heights of trees: ground slope and aspect, bearing and length of shadows, and the sun angle above the horizon. Trees stripped of foliage in patches have average heights of 16 ± 7m and occur where the PDC reached 66 ± 24% of its runout. Foliage patches have average heights of 12 ± 7m and occur where the PDC reached 91 ± 9% of its runout. Tree heights in the patches indicate a localized height the peak dynamic pressure must jump as it travels over hills and away from its source. Patches with foliage imply that the peak dynamic pressure has risen above the tops of the trees, whereas patches without foliage suggest that the peak dynamic pressure was still low enough to damage trees even though the current had jumped over topography. Outside of the patches, increasing tree density suggests that dynamic pressure waned with distance.