REEVALUATION OF VESICLE DISTRIBUTIONS IN BASALTIC LAVA FLOWS

A fundamental dichotomy in the study of basaltic lava flows is that ob servations of active flows are restricted to flow surfaces, yet older flows are often exposed only in vertical cross section. Cross-sectiona l exposures of an inflated basaltic sheet flow emplaced in Kalapana, H awaii, from 1990 to 1991 provide an unusual opportunity to merge these two viewpoints, permitting the development of the internal structure of the flow to be viewed in the context of its known emplacement histo ry. We demonstrate that fundamental features of the flow structure-a t hick upper vesicular crust that diminishes downward in overall vesicul arity, a dense flow interior, and a thin lower vesicular zone-are gene rated through syn-emplacement cooling of upper and lower flow crusts. Both the inverse correlation of overall vesicularity and vesicle size and the constant relative thickness of the upper vesicular zone are un ique to inflated flows and permit a reinterpretation of flows previous ly interpreted to be ponded (rapidly emplaced). Identification of infl ation, in turn, implies near-horizontal paleoslopes and permits estima tes of flow duration based on upper flow crust thickness.