CHAOTIC COLLAPSE OF CALDERAS

A study of the formation of Krakatoan-type calderas indicates that they probably form by chaotic collapse. The energy required to form these calderas by explosive decapitation of the volcano is seldom available during caldera-forming eruptions. Gravity anomalies resulting from different mechanisms of collapse show that chaotic-collapse calderas are characterized by a negative residual Bouguer anomaly with a gentle gradient toward the center of the caldera. Calderas of the Valles type have steeper gradients near of the caldera edges. The residual anomaly derives from the low-density shattered rocks that make up the chimney above the collapsed magma-chamber. The chimney flares toward the top giving the substructure of the caldera a funnel-shaped form. During eruptions forming calderas of the chaotic-collapse type, the eruption and emplacement of coignimbrite breccia is indicative of explosions due to a pressure decrease within the magma chamber. The pressure decrease is due both to drainage of magma and to collapse of the roof of the chamber both of which cause a decrease of the lithostatic load on the magma chamber. In some cases water can gain access to the magma chamber through collapse of entire sections of the hydrothermal system of the volcano with consequent explosive vaporization within the chamber itself. © 1990.