Injection and arrest of dykes: implications for volcanic hazards

Dykes are the principal channels through which magma reaches the surface in volcanic eruptions. For this reason dykes observed in the field are common ly assumed to be feeders to lava flows. The actual proportion of dykes reac hing the surface is, however, poorly known. In order to develop models for the purpose of estimating volcanic hazard, this proportion must be known. T his follows because such models should not only consider the probability of dykes being injected from magma chambers during periods of unrest in the a ssociated volcanoes, but also the probability of the injected dykes being a rrested. This paper presents field data on several thousand dykes from Icel and and Tenerife (Canary Islands) indicating that many, and probably most, dykes become arrested at various crustal levels and never reach the surface to feed eruptions. Using the results of analytical and numerical models, i t is shown that, for common loading conditions, the stress field in the vic inity of a magma chamber may favour the injection and propagation of dykes while the stress field at a certain distance from the chamber favours dyke arrest. This means that many dykes that are injected from the chamber propa gate only for a very limited distance from the chamber to the point where t hey become arrested. The implication is that during periods of unrest in vo lcanoes, the probability of volcanic eruption is only a small fraction of t he probability of dyke injection from the source magma chamber. (C) 1999 El sevier Science B.V. All rights reserved.