Transitions between explosive and effusive phases of silicic volcanic eruptions have been related either to stratification of volatiles in the source magma body or to the loss of volatiles through the permeable host rock of the conduit. One way to distinguish between these two models is to map and analyze the vesicular and glassy textures found in silicic lava flows. In this paper we present textural observations and isotopic evidence from active and recent silicic lava flows which show that at least some vesiculation occurs during surface advance of extrusions, after magma has reached the earth's surface. This view is in contrast to the widely promoted ''permeable foam'' model, which states that all volatiles escape during ascent of the magma, and that all dense glassy material in lava flows forms from the collapse of pumiceous lava, i.e., that silicic lavas emerge as highly inflated foam flows. Such interpretations which claim that silicic lavas are completely degassed upon extrusion, and that all degassing must take place on the time scale of the eruption, neglect several important pieces of evidence, including the presence of obsidian in extremely small domes, and of vesicular zones in the interiors of silicic flows; the copious loss of volatiles through eruption plumes between eruptive phases; and direct observations of surface vesiculation during growth of the Mount St. Helens lava dome. The permeable foam model also implies the unlikely requirement that explosive-to-effusive transitions be associated with an increase in eruption rate. We present a more comprehensive model for the emplacement of silicic extrusions that allows for early gas loss during ascent, as well as late-stage vesiculation. We then discuss how the redistribution of volatiles during surface flow can increase explosive hazards from silicic lavas days, weeks, or months after the lava emerges from the vent. ¿ American Geophysical Union 1992