VARIATIONS IN INITIAL VESICULARITY OF LATE CENOZOIC BASALTS

Measurements of vesicularity may provide insight into variations in eruptive volatile content and vent conditions during eruptions.Initial studies of vertical vesicle distributions in lava flows that were erupted from a variety of vent types throughout New Mexico imply that initial vesicularity is tightly controlled.The variables governing vesicle formation, redistribution, and growth within lavas are restricted and quantifiable. Initial vesicularity (the ratio of the volume of gas bubbles to the volume of dense lava equivalent) at a given level within a lava flow is a function of the gas law. Variations in vesicle dimensions within a lava flow are a function of initial lava flow volatile retention, pressure at the crust-melt interface, and rate of solidification (time available for bubble coalescence). It is suggested that de-volatilization at fire fountains is the principal control on lava initial vesicularity. Because the pressure that satisfies the ideal gas law has both atmospheric and hydrostatic terms, differences in vesicularity within lava sections are expected to be relatively sensitive to variations in volatile abundance in the upper lava section and to the over pressures associated with surges of lava supply inside crusted flows. Variations in the volatile content within individual batches of lava, as it exits the vent, will contribute to variations in vesicularity profiles of lava sections and should be observable in the measured sections.