Vesicle zonation and vertical structure of basalt flows

Observation and measurement of vertical sections of thin (< 10 m) basaltic lava flows show that the vertical structure of basalt flows, regardless of variation in chemical composition or thickness, can be divided into three, previously unrecognized, zones consisting of a fundamental and regular pattern in vesicle size and abundance. These zones can be characterized as follows: (1) an upper vesicular zone, (2) a middle nonvesicular or dense zone, and (3) a lower vesicular zone. The thickness of the upper vesicular zone is generally about one-half of the total vertical section, and the thickness of the lower vesicular zone is generally 30–40 cm regardless of the total flow thickness. In the upper vesicular zone, vesicles increase in diameter and decrease in number per unit cross-sectional area downward attaining a maximum diameter near the base of the upper vesicular zone. In the lower vesicular zone, vesicles increase in diameter and decrease in number per unit cross-sectional area upward attaining a maximum diameter at the top of the lower vesicular zone.Numerical simulations, performed for this study, suggest that these characteristic patterns of vesicle zonation are the result of the growth and rise of gas bubbles in cooling lavas rather than the result of dynamic conditions such as flow movement or convection. As a bubble grows, it begins to ascend, and continues to ascend until it is overtaken by solidification progressing inward from either the upper or lower cooling surfaces of the flow. Bubbles that start out high in the flow will ascend ahead of the lower solidification front and cease rising only after encountering the downward-advancing upper solidification front, and bubbles near the base of a flow will be entrapped by the upward-advancing lower solidification front. Bubbles that start and rise just above the lower solidification front form the lower part of the upper vesicular zone. Such bubbles will also have longer times in which to grow than bubbles that are either higher or lower and are therefore among the largest in the flow. A zone free of vesicles will remain between the last bubbles to ascend to the upper solidification front and the last bubbles to be overtaken by the lower solidification front.