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Material toughness, internal structure, and caldera-collapse frequencies in basaltic and composite edifices
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2008 IOP Conf. Ser.: Earth Environ. Sci. 3 012023 (http://iopscience.iop.org/1755-1315/3/1/012023) View the table of contents for this issue, or go to the journal homepage for more
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Collapse Calderas Workshop IOP Conf. Series: Earth and Environmental Science 3 (2008) 012023
IOP Publishing doi:10.1088/1755-1307/3/1/012023
Material toughness, internal structure, and caldera-collapse frequencies in basaltic and composite edifices Agust Gudmundsson Department of Earth Sciences, Queen’s Building, Royal Holloway University of London, Egham TW20 0EX, UK (e-mail:
[email protected])
Keywords: material toughness, fracture, structure, rock mechanics, caldera
Abstract Why is caldera collapse, including slip on existing calderas, so much more common in basaltic edifices (shield volcanoes) than in composite edifices (stratovolcanoes and evolved calderas)? Since large landslides and dyke-fed eruptions are also apparently more common in basaltic edifices than in composite edifices, the question can be generalised as follows: Why is it easier to propagate a large-scale fracture in a basaltic edifice than in a composite edifice? To answer this question, I focus on three related topics: (1) fracture initiation and propagation, (2) material toughness, and (3) the internal structure of the edifices. An active volcano undergoing deformation generates and propagates numerous fractures. However, most of these soon become arrested and thus remain short. Short fracture cannot generate collapse calderas, large landslides, or dykefed eruptions; only large fractures can, namely fractures that propagate through many layers and reach the surface of the volcano. Most fractures become arrested and remain short because after a short propagation they meet with unfavourable mechanical conditions, particularly at contacts between layers. The mechanical conditions at contacts are primarily controlled by three parameters, namely: (a) local stresses; (b) tensile (or shear) strength of the contact versus that of adjacent layers; and (c) material toughness of the contact versus that of the adjacent rock layers. Parameters a-c all contribute to the edifice material toughness, which is also referred to as critical strain energy release rate. It is a measure of resistance to fracture; it is the energy absorbed in a rock per unit area of fracture and has the units J m-2. Thus, in a tough edifice, large amounts of energy are needed to cause failure through fracture propagation. Using field observations as well as conceptual and numerical models, I show that it is c 2008 IOP Publishing Ltd
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Collapse Calderas Workshop IOP Conf. Series: Earth and Environmental Science 3 (2008) 012023
IOP Publishing doi:10.1088/1755-1307/3/1/012023
normally much more difficult for a large fracture such as a caldera fault, a ring dyke, or a feeder-dyke to propagate (through many layers) to the surface in a composite edifice than in a basaltic edifice. This difficulty is related to difference in internal structure between these edifice types. A composite edifice is a structure made of layers, rock units, and contacts whose mechanical properties vary widely. By contrast, a basaltic edifice is made of rock units, layers, and contacts whose properties vary much less. A large elastic mismatch (difference between Young’s moduli) normally encourages fracture arrest or deflection at contacts, and thus increases edifice toughness. Also, unfavourable local stresses commonly develop at contacts between mechanically dissimilar layers. Thus, a composite edifice tends to arrest its fractures while they are short; it is therefore tougher and more resistant to any type of large-scale fracture propagation than a basaltic edifice. I suggest this difference in material toughness between these types of edifices is one principal reason for their difference in caldera (and landslide and feeder-dyke) failure frequencies.
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