Volcanic rift zones and their intrusion swarms

Abstract

Most volcanoes have rift zones, underlain by swarms of dykes or other minor intrusions. This paper reviews the subject and presents some new data and ideas. It plots rift zone width against length for different volcano types, and finds that the zones on strato- and central volcanoes are on the whole narrower and shorter than on other types. Among the longest and narrowest zones are those on Hawaiian shield volcanoes; there are several reasons for the focussing. Hawaiian rift zones however become diffuse when volcanic activity declines. Monogenetic volcano fields include some that have clearly identifiable rift zones, and others that have vent-fields lacking fissures or dykes. Here the vent-field justifiably can be taken to proxy for a rift zone. The zones visited in several volcanic areas, (including the Azores and Samoa), are localised by deep crustal structures or tectonic activity, and often involve strike-slip faults. This paper then suggests how insertion of dykes could cause structural changes such as bending or initiation of a rift zone, and how departures from the “normal” balance between magma flux and extensional strain rate could determine whether rift zones are vertical or horizontal. This leads to a possible mechanism for the circumferential (annular) rift zones of some Galapagos volcanoes.