Page 365 - Caldera Volcanism Analysis, Modelling and Response

340 Agust Gudmundsson

Figure 19 Sill-like magma chamber, 8 km wide and 2 km thick.The chamber is located at
3 km depth in a 20 km thick and 40 km wide layered crustal segment subject to doming stress
(pressure) of 10 MPa at the segment bottom as well as horizontal stress of 5 MPa.The lower part
of the segment (hosting the chamber) has a sti¡ness of 40 GPa, whereas the uppermost 3 km
above the chamber consists of 30 layers alternating in sti¡ness between 1and 100 GPa. (A)
Con¢guration of the model; (B) maximum principal tensile stress s 3 and von Mises shear stress
s at the free surface; (C) tensile stress s 3 and the shear stress s at the upper boundary of the
magma chamber; (D) contours of tensile stress s 3 around the magma chamber and (E)
contours of shear stress s around the magma chamber.

shear t stresses at the free surface peak above, and the upper margin stress of the
chamber at, the lateral ends. Since the theoretical peak stresses are close to 57 MPa
at the chamber margin and 33 MPa at the free surface, they are likely to initiate a
ring fault and a ring dyke.

7. Discussion

Collapse calderas are common on the Earth and on several of the rocky planets
and their satellites (Figure 3). While many extraterrestrial calderas as well as calderas in
basaltic edifices on Earth may form and slip without significant eruptions, most large
calderas on Earth are related to large explosive eruptions (Newhall and Dzurisin,
1988). In fact, the largest and most devastating explosive eruptions on Earth are