Magma supply, magma ascent and the style of volcanic eruptions

We propose a new way of looking at the sequence of events leading to different styles of silicic, volcanic eruptions. Small-to-medium sized eruptions, either explosive or effusive, are explained by the ascent of isolated magma batches from mid-crustal magma chambers. We separate magma ascent into four different zones: the Supply System, the Intermediate Storage System, the Transport System and the Eruptive System. Of primary importance is the concept that ascent from the Intermediate Storage System through the Transport System to the Eruptive System first requires the development of a fracture network. Initially, this fracture network allows the ascent of individual magma batches by opening and then closing after their passage. An increase in the complexity of the fracture network with time increases the connectivity of the fractures and hence the ease of upward magma movement. In this model, the dynamics of the ensuing eruptions are controlled entirely by the time spent in the Transport System. Large explosive eruptions require a full interconnectivity of the Transport System from the Intermediate Storage System to the Eruptive System. Moreover, we suggest that a fully connected conduit is rare, develops only under particular conditions, and typically generates catastrophic eruptions during formation. Here we examine two case histories that illustrate the interplay of these processes: Mt St. Helens, USA, between 1980 and 2004, and Mt. Pinatubo, Philippines, in 1991.