Development of a subvolcanic structure by the interaction of liquid-filled cracks

Abstract

The interaction of liquid-filled cracks may be an important process for magma accumulation and the development of subvolcanic structures. The horizontal interaction was studied through the stress analyses around pressured cracks in various regional stress fields. The vertical interaction was studied using gelatin experiments. Buoyaney-driven cracks coalesce more easily than pressure cracks. The theory of the three-dimensional crack interaction was discussed. There are two types of crack interaction. The interaction between liquid-filled cracks may cause coalescence of cracks. The interaction between a liquid-filled crack and a solid-filled crack never causes coalescence. The stress relaxation by ductile or brittle behavior, or by the driving-out of magma, decreases the local stress field generated by the accumulation of magma-filled cracks. As the regional differential stress increases and the minimum compressive principal stress becomes tensile, magma-filled cracks become parallel to each other. As the supply rate of magma-filled cracks becomes relatively large, or as the regional differential stress decreases and the minimum compressive stress increases, the arrangement of magma-filled cracks varies from an offset pattern to a radial one, because the local compressive stress field cannot be decreased. This process leads to the development of subparallel or radial dikes in two dimensions, or a dike-sheet complex in three dimensions. A large magma chamber, such as a sphere or a cylinder, is not always necessary for the development of a basaltic volcano. In case of the interaction between liquid-filled cracks, magma-filled cracks coalesce easily into one large crack and diminish the local compressive stress field. Last, the theory of the crack interaction is applied to developments of various volcanoes. The fissure pattern of Hekla volcano, Iceland is explained in terms of the variation of the stress relaxation. The difference in fissure pattern between Izu-Oshima and Miyakejima volcanoes, Japan is interpreted by the variation of the differential stress. The local stress balance including the stress relaxation is kept in the developments of Etna volcano, Italy and Kilauea volcano, Hawaii.