MAGMA CHAMBERS MODELED AS CAVITIES EXPLAIN THE FORMATION OF RIFT-ZONECENTRAL VOLCANOS AND THEIR ERUPTION AND INTRUSION STATISTICS

The volcanic rift zone in Iceland is characterized by elongate volcani c systems, consisting of tension fractures, normal faults and volcanic fissures, where most of the volcanotectonic activity takes place. Mos t volcanic systems develop central volcanoes, the formation of which i s still poorly understood. There is great difference in the eruption a nd intrusion statistics of the volcanic systems inside and outside the central volcanoes. Ln the central volcanoes, eruptions are frequent ( typically one every several hundred years), of small volume (normally less than 0.1 km(3)) and fed mainly by thin (average thickness of 0.5 m), inclined sheets. Outside central volcanoes the eruptions are rare (typically one every several thousand years) but of large volume (typi cally more than 1 km(3)) and fed mainly by thick (average thickness of 4-5 m), subvertical dikes. Using the results of a boundary element st udy of magma chambers modeled as a cavities or holes in a semiinfinite plate, these empirical relations, as well as the formation of specifi c central volcanoes in the volcanic systems, can be explained. Once a cavity-like magma chamber has formed, its existence in the rift zone c oncentrates tensile stress. This stress concentration causes the segme nt containing the magma chamber to rupture much more frequently than t he other parts of the volcanic system, which partly explains the forma tion of central volcanoes and their high eruption frequencies. This co ncentration also gives rise to a local stress field that encourages in jection of small-volume sheets in all directions and at various dips f rom their source magma chamber. The magma chamber acts as a trap for u pward propagating dikes from mantle reservoirs and channels magma, thr ough inclined sheets, toward a limited area at the surface where the c entral volcano gradually forms. The inverse relationship between erupt ion frequency and eruption volume, when the central volcano is compare d with other parts of the volcanic system, is also partly due to the t rap-like nature of the magma chamber. The chamber is normally much sma ller than its source mantle reservoir, so that a single magma flow (th rough a dike) from the reservoir (lasting perhaps many years) may trig ger tens of magma flows (through sheets) from the chamber, many of whi ch would reach the surface in the central volcano.