Formation and development of normal-fault calderas and the initiation of large explosive eruptions

Abstract

The ring fractures that form most collapse calderas are steeply inward-dipping shear fractures, i.e., normal faults. At the surface of the volcano within which the caldera fault forms, the tensile and shear stresses that generate the normal-fault caldera must peak at a certain radial distance from the surface point above the center of the source magma chamber of the volcano. Numerical results indicate that normal-fault calderas may initiate as a result of doming of an area containing a shallow sill-like magma chamber, provided that the area of doming is much larger than the cross-sectional area of the chamber and that the internal excess pressure in the chamber is smaller than that responsible for doming. This model is supported by the observation that many caldera collapses are preceded by a long period of doming over an area much larger than that of the subsequently formed caldera. When the caldera fault does not slip, eruptions from calderas are normally small. Nearly all large explosive eruptions, however, are associated with slip on caldera faults. During dip slip on, and doming of, a normal-fault caldera, the vertical stress on part of the underlying chamber suddenly decreases. This may lead to explosive bubble growth in this part of the magma chamber, provided its magma is gas rich. This bubble growth can generate an excess fluid pressure that is sufficiently high to drive a large fraction of the magma out of the chamber during an explosive eruption.

How caldera collapse shapes the shallow emplacement and transfer of magma in active volcanoes

“…Topographic effects have been previously considered as a notch in the morphology. Such notches concentrate stresses around them and are found to influence dike propagation only at very shallow levels (few tens of m to few hundreds m; e.g., Gudmundsson, 1998;Gudmundsson, 2011), while gravitational unloading due to the removal of mass from the surface may lead to significant rotation of the principal stresses and affect the dynamics of magma propagation also at deeper levels (Hooper et al, 2011;Maccaferri et al, 2014). We test this possibility using numerical Finite Element (FE) models to calculate the stress field within a volcanic edifice decompressed during the formation of a caldera and investigate the expected orientation of the magma intrusions.…”

Numerical modeling of magma withdrawal during explosive caldera‐forming eruptions

“…The occurrence of explosive calderaforming events depends on the strength of the chamber walls and the depth, water content, and aspect ratio of the magma chamber [Marti et al, 2000]. Gudmundsson [1998] and Gudmundsson et al [1997] have proposed that regional loading favors the formation of ring faults. Recently, McLeod [1999] has proposed that magmatic buoyancy can also play a major However, the thermodynamic evolution of the magma chamber during a caldera-forming process is still unresolved.…”

Great challenges in volcanology: how does the volcano factory work?

“…Caldera collapse occurs under specific eruptive conditions and, in the last decades, it has occurred a few times only (Geyer and Martì, 2008;Stix and Kobayashi, 2008;Michon et al, 2011). While caldera collapse is relatively infrequent, it is still difficult to forecast it, as there is uncertainty on its triggering causes and the proper detection of these causes at the surface: in fact, caldera collapse may be triggered by both overpressure or underpressure conditions within a magma chamber, including lateral intrusion of magma, giving the system a wide spectrum of dynamic variability (e.g., Gudmundsson, 1998;Martì et al, 2009). To better constrain this variability, more effort should be given at investigating the magmatic and dynamic conditions within the magma chamber at the onset of caldera collapse.…”