The internal structure of lava flows - Insights from AMS measurements .1. Near-vent a'a

Two small near-vent a'a flows (one from Xitle, Mexico and the other from Mauna Kea, Hawaii) showing a conspicuous pattern of concentric vesicle foliation were used to investigate how anisotropy of magnetic susceptibility (AMS) measurements relates to the internal structure of lava flows. The results show an almost perfect match between the vesicle foliation and the plane containing the maximum and intermediate principal susceptibilities, or plane of magnetic foliation. Both types of foliation can be explained by variations in the relative magnitudes of local shear stresses, assuming a unidirectional velocity within the lava flows. The attitude of the plane of magnetic foliation provides a three-dimensional picture of the dynamic behavior of flowing lava, and might be applicable also in instances when no other foliation is observable. A strong imbrication occurs, marked by the distribution of magnetic foliation parallel to a family of nested cones the axis of which coincides with the flow axis, and the apices of which point down-flow. Flow of lava having, at least in part, a stationary roof and possessing a small yield strength is consistent with the velocity profiles inferred from the orientation of the foliations. A model based on its fluid behavior is proposed to explain the origin of the AMS of lava. At variance with existing qualitative models, our model allows a semi-quantitative systematic explanation of the different circumstances under which any principal susceptibility is parallel to the flow direction.