Inclined pipe vesicles as indicators of flow direction in basalts: A critical appraisal

Inclined pipe vesicles and stretched vesicles near the base of basalt flows have long been regarded as reliable flow-direction indicators. However, attempts to use such structures in the Santa Rosa Basalt of Southern California to determine regional flow patterns, paleoslope, and source of eruption yielded inconclusive and contradictory results. Orientations of 1070 vesicles at 37 localities were obtained directions of inclination from vertical were plotted on circular histograms. At any specific locality vesicle orientations typically are normally distributed through a 40–60 degree sector. Commonly a pronounced maximum lies within a 20–40 degree sector. Even at localities having bimodal or trimodal distribution patterns, most vesicles plot within a 90 degree sector. The frequency distribution of oriented vesicles at individual localities strongly suggests a limited direction of flow and implies a source in the opposite direction. Comparison of such «flow directions» from locality to locality within the same flow, however, yielded highly divergent results over short distances. Likewise, comparison of directions from different flows yielded results ranging from parallel to diametrically opposed. Composite circular histograms from three small mesas censisting of thin, flat-lying flows showed little apparent preferred direction of vesicle inclination. Possible reasons for the highly divergent readings include sub-flow surface irregularities, turbulent rather than laminar flow, and/or convection in the lava during cooling. Although inclined vesicles may well indicate motion in a flow, their use for determination of flow directions and for regional paleogeographic interpretations is questionable.