Toughness and failure of volcanic edifices

Major volcanic edifices are of two main types: stratovolcanoes (composite edifices) and basaltic edifices (shield volcanoes). These types of edifices show several remarkable differences. For example, while some basaltic edifices erupt very frequently, many stratovolcanoes erupt very infrequently. Also, most basaltic edifices have gentle slopes, commonly 2–12°, whereas stratovolcanoes have steeper slopes, particularly in their uppermost parts where the slopes may reach 35–42°. There have been various attempts to explain these differences, but here the focus is on edifice strength or rather toughness. Toughness of a material is a measure of its resistance to fracture; alternatively, for a tough material, large amounts of energy are needed to cause failure. Most quoted material toughness (critical energy release rate) values for rocks are 20–400 J m− 2, for extension fractures such as dykes, and 0.01–0.03 MJ m− 2 for shear fractures such as dip–slip faults. The related property of critical stress intensity or fracture toughness is, for most rocks, 0.5–3 MPa m1/2. These values for material and fracture toughness refer to small, essentially homogeneous laboratory rock samples. Here I present equations for calculating the material toughness in volcanic edifices based on the measured or estimated dimensions of associated large structures such as dykes and faults. For typical dykes, the calculated material toughness is 1.3–47 MJ m− 2 , with a corresponding stress intensity of 114–690 MPa m1/2. Similarly, for a 3.8-km-long dip–slip fault in an edifice the calculated material toughness is 2.3 MJ m− 2  and the corresponding stress intensity 150 MPa m1/2. These results compare well with previous material toughness estimates for large strike-slip faults and stress intensity estimates for large dykes. Generalising these results, and using field data as well as materials science results, I conclude that stratovolcanoes normally have a much higher material toughness than basaltic edifices. This is primarily because of different internal structures: a basaltic edifice is made of layers and contacts all of which have essentially similar mechanical properties whereas a stratovolcano is made of layers and contacts many of which have widely different properties. Accordingly, it requires much more energy (is more “difficult”) for a fracture to propagate through a stratovolcano than through a basaltic edifice. Thus, contrasting toughnesses may be one principal reason why these edifices differ in slope and in failure frequencies, including the number of dyke-fed eruptions, caldera collapses, and landslides.