Page 275 - Caldera Volcanism Analysis, Modelling and Response

250 J. Martı ´ et al.

Figure 6 Schematic representation of an explosive caldera-collapse process.The central vent erup-
tion progressively decompresses the overpressurised (P L +DP START ) magma chamber.The chamber
pressure decreases well below the lithostatic pressure. Caldera collapse starts when chamber decom-
pression reaches a critical value (P L DP COLL ) at which the chamber roof fails under shear. Caldera
collapse brings the magma pressure at the chamber roof to lithostatic pressure. Subsidence of the
chamber roof will continue either until the pressure at the base of the magma column occupying
the ring fault is equal to the lithostatic pressure of the chamber (P L +DP L ), or until all magma has
been erupted. Solid dots indicate points inside the magma chamber for which P M is indicated.
P M , magmatic pressure; P L , lithostatic pressure; DP START , overpressure required to fracture the
country rock and to form a conduit to the surface; DP COLL , shear strength of the rock, i.e. the
underpressure necessary to initiate caldera collapse (modi¢ed after Mart|¤ et al., 2000).