THE CONTROL OF CHAMBER GEOMETRY ON TRIGGERING VOLCANIC-ERUPTIONS

The pressure evolution of a cooling and crystallising body of magma st ored in the crust is sensitive to the depth and vertical extent of the chamber and also to the volatile content of the magma, In shallow sil l-like chambers or in chambers containing magma of high volatile conte nt, the magma becomes vapour-saturated on emplacement. The chamber pre ssure then increases as the melt cools and crystallises owing to the c oncomitant exsolution of volatile species. This can lead to the erupti on of relatively unevolved, crystal-poor magma. In deep sill-like cham bers or chambers with low volatile (H2O) contents, the magma remains u ndersaturated until a significant fraction of the melt has crystallise d. While unsaturated, crystallisation leads to a decrease in chamber p ressure since the crystals are typically denser than the melt. However , once the magma becomes saturated, vapour bubbles are exsolved, leadi ng to an increase in pressure and possible eruption of a crystal-rich magma, although, at this stage the magma may be so crystalline that it is essentially immobile. In chambers of significant vertical extent, the upper part of the chamber may be saturated and exsolving volatiles while the lower part remains unsaturated. This may cause a decline in pressure during the early stages of crystallisation when the contract ion of the undersaturated deeper magma dominates. However, as the satu ration surface migrates downwards through the magma, the pressure may build up again, eventually leading to eruption of relatively crystal-r ich magma. These results also suggest that inter-eruption time-scales are greater in deeper chambers owing to the greater amount of crystall isation required before eruption. We show that the model is broadly co nsistent with data from several historical eruptions. (C) 1997 Elsevie r Science B.V.