The Influence of Regional Stress and Structural Control Over the Shape of Maar Craters

Abstract

This study aims to gain a better understanding of the factors leading to the expressed shape of individual maars in volcanic fields. Maars are volcanic features produced by phreatomagmatic eruptions which excavate a crater beneath the pre eruptive surface through the explosive interaction of magma and water. Maar craters take on a wide range of shapes including, circular, elliptical, and polylobate. Across all maar fields, the existence of one or more axis of elongation is commonplace. Identification of the factors which lead to, and control, elongation in individual maars will help improve the accuracy of current hazard mapping and safety protocols. In order to determine whether the orientations of elongation are controlled by existing structures and regional stress, the primary and secondary directions of elongation of maar craters from a range of tectonic settings were measured. Maars were found to exhibit similar primary elongation orientations within each field. Influence of regional stress was identified in the geographic placement of maars along lineaments in most fields. Although many maars were found in lineaments identified through nearest neighbor analyses, they rarely share primary elongation orientations with the lineaments they compose. Furthermore, maars which shared similar primary elongation orientations with one another were not found to be grouped close together geographically. Overall, the number of maars in each field which share primary elongation orientations with existing structures (faults) and nearest neighbor lineaments does not suggest regional structural control over primary elongation orientation in any field. The tendency of maars to exhibit similar primary elongation orientations within a field, coupled with the lack of correlation with structural controls shows that the elongation orientations of maars are likely governed by more local controls related to host rock material, explosion induced changes to the stress regime, or hydrology.