Chapter 8 Magma-Chamber Geometry, Fluid Transport, Local Stresses and Rock Behaviour During Collapse Caldera Formation

Abstract

Abstract Collapse calderas are common on Earth and some other solid planetary bodies, particularly on Io (a satellite of Jupiter), Mars and Venus. Caldera structures are generally similar on all these bodies but the sizes vary considerably. Here I present numerical models of caldera-fault formation in volcanoes with shallow, spherical or sill-like magma chambers. In all the anisotropic models, the crustal segment (including the volcano) above the shallow chamber is composed of 30 comparatively thin layers with stiffnesses (Young's moduli) alternating between 1 and 100 GPa. The chamber itself is located in a single, thick layer with stiffness different from that of the other layers. The crustal segment hosting the chamber is either 20 or 40 km wide but has a constant thickness of 20 km. The loading conditions considered are crustal segments subject to: (a) underpressure (lack of magmatic support) of 5 MPa; (b) tensile stress of 5 MPa; (c) excess magmatic pressure of 10 MPa at the bottom of the crustal segment (doming of the volcanic field containing the chamber) and (d) combination of tension and doming. In all the models, the magma-chamber top is at 3 km depth; the diameter of the sill-like chamber is 8 km (its thickness is 2 km) while that of the spherical chamber is 4 km. The main results are as follows: (1) Underpressure and excess pressure in a shallow, crustal chamber normally result in dyke injection rather than caldera formation. (2) For doming or tension, a spherical magma chamber normally favours dyke injection rather than ring-fault initiation. However, when the spherical chamber is located in a very soft (10 GPa) layer, the local stress field may be suitable for caldera-fault formation. (3) For a sill-like chamber in a 20-km-wide volcanic field, a ring fault may be initiated either during horizontal tension or a combination of tension and doming. (4) For a sill-like chamber in a 40-km-wide volcanic field, doming alone is sufficient to initiate a caldera fault. The results indicate that the local stresses in composite volcanoes most likely to initiate caldera faults are associated with sill-like chambers subject to tension, doming or both.