Abstract
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. Buoyancy promotes coalescence of cracks. When differential stress in the vertical plane is small, parallel, vertical, buoyancy-driven cracks can coalesce. An increase of differential stress prevents buoyancy-driven cracks from interacting. Crack propagation system including the above theory is proposed for understanding variation of basaltic volcanoes. Magmatic input rate, and stress (stress gradient with depth including buoyancy, and differential stress) are important boundary conditions. The output-stress diagram relating magmatic output rate and crustal deformation rate is introduced. It is possible to convert the diagram into the input-stress diagram. Applications of crack propagation system are discussed, compared with the field data. (1) Crack interaction is available for magma accumulation during its ascent in the lithosphere. (2) The crack interaction theory is able to explain the difference between a monogenetic volcano field and a polygenetic volcano. At smaller input rate and larger differential stress, monogenetic volcanism occurs. Under a monogenetic volcano field, stress and magmatic input are balanced. At larger input rate and smaller differential stress, a polygenetic volcano grows. (3) According to crack interaction theory, a magma chamber under a basaltic volcano, as a local stress source, can be a dike-sheet complex. At small differential stress and large input rate, radial dikes develop under a polygenetic volcano ; the increase in differential stress causes intrusion of parallel dikes. At large input rate, the local stress under a polygenetic volcano increases so that stress relaxation process plays an important role in development of a volcano. Variation in stress relaxation depends on the crustal structure as well as the stress balance under a volcano. The present local stress field generated by the arrangement of previous dikes controls where the next fissure eruption will occur. (4) Some volcanoes interact with each other mechanically. The output rates of these volcanoes oscillate compensationally. Crack interaction theory can explain this interaction.
