Mechanical Interaction between volcanoes in the East Volcanic Zone of Iceland

When assessing volcanic hazards, it is important to know if volcanoes within a given area interact mechanically, that is, is unrest in one volcano likely to trigger unrest in one or more nearby volcanoes. Here we report a study of the mechanical interaction between central volcanoes within a part of the Neovolcanic Zone of Iceland. The Neovolcanic Zone is the area confining the majority of volcanism in Iceland today, containing rocks belonging to the Bruhnes magnetic epoch (<0.8Ma). The Neovolcanic Zone can be divided into 3 subzones: the North Volcanic Zone (NVZ), the West Volcanic Zone (NVZ) and the East Volcanic Zone (EVZ). All the volcanoes that we have studied belong to the EVZ. The prominent geological features of the Neovolcanic Zone are the volcanic systems, of which there are 30. 19 of these systems hold 23 central volcanoes, indicating that some systems hold more than one central volcano. A central volcano, by definition, has a shallow crustal magma chamber. The presence of a central volcano within a volcanic system indicates a level of maturity within it; for example, the Vestmannaeyjar Volcanic System, the southernmost system in Iceland, is thought to be developing a central volcano, and thus is a young volcanic system. The central volcanoes in the EVZ were modeled as two-dimensional structures, namely as circular holes or inclusions in an elastic crust, under a tensile stress of 5MPa in a direction parallel with the spreading vector. The modeling shows several results which allow an insight into the characteristically different volcanic activity within the EVZ. The first main result is that the modeling clearly indicates the presence of two clusters of central volcanoes: one at the northern end of the EVZ, under the Vatnajökull ice cap and over the mantle plume; the second at the southern end of the EVZ, and possibly influenced by the propagation of the rift-zone front of the EVZ to the southwest. The second main result is that the extent of mechanical interaction between the central volcanoes, as indicated by local stress fields, is governed by distance. There are no interactions that span the distance between the two clusters at the northern and southern end of the EVZ. Remarkably, this part of the EVZ, namely the part located between the central-volcano clusters, is the site of the largest Holocene fissure eruptions in Iceland. Whilst these models do not offer any answers to the question of why these large fissure eruptions take place in this part of the EVZ, the models suggest that the mechanical conditions for large fissure eruptions are most favorable away from clusters of central volcanoes.