Why Stratovolcanoes Are Mechanically Stronger than Shield Volcanoes

In comparison with stratovolcanoes, shield volcanoes tend to have more frequent dike-fed eruptions and large lateral and vertical collapses, as well as more gently dipping flanks. In many stratovolcanoes, dike-fed eruptions occur once every several hundred or thousand years but once every few years in many shield volcanoes. Using Hamilton’s principle of least action as a basis for determining potential dike/sheet propagation paths, it is shown that the probability of arrest of an injected dike/sheet is normally much greater in a stratovolcano than in a shield volcano. This is primarily because in stratovolcanoes rock layers and units are of contrasting mechanical properties, so that many dikes become arrested and thus do not feed eruptions. Similarly, many faults in stratovolcanoes become confined to one or several layers/units and do not reach the surface to generate landslides or ring faults. Consequently, the formation of large landslides is generally more difficult—requires more energy—in composite volcanoes than in shield volcanoes. For the same reason, formation of calderas in stratovolcanoes is normally more difficult than in shield volcanoes. More energy is needed to propagate fractures through many layers/units in stratovolcanoes than in shield volcanoes. It follows that stratovolcanoes tend to be tougher, more resistant to tectonic fracture propagation, and thus mechanically stronger than shield volcanoes. This may partly explain differences in the frequencies of dike-fed eruptions, large landslides, and caldera collapses between shield volcanoes and stratovolcanoes.