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.