THE INITIATION OF VOLCANIC ERUPTIONS IN THE CASCADE RANGE

A key source of information regarding volcanic eruptions are the set of processes that lead from storage of magma in a stable or quasi stable mode in the shallow crust, to movement of magma upward, eventually to breach the surface and cause an eruption. We describe this as eruption initiation. An understanding of the key physical and chemical processes that lead to eruption initiation are important for considering hazards and responses associated with future eruptions, for resurrecting the past behavior of volcanic systems from the geological record, and for correctly interpreting monitoring and other data which allow us to anticipate future and ongoing volcanic behavior.

Although there is surprisingly little published literature dedicated to understanding eruption initiation, petrological approaches are particularly amenable to studying initiation in both modern and prehistoric eruptions. Initiation typically involves some of the last high temperature processes that affect erupted magmas, and these often leave clear signals preserved in the petrographic record. Petrological techniques are also increasingly providing access to the timescales of eruption initiation, which are important for evaluating future volcanic hazards.

We present a survey of eruption initiation mechanisms deduced from petrology and other observations for selected volcanoes of the Cascade arc. A number of different mechanisms are evident, related to magma recharge, magma mixing and vapor accumulation or second boiling. Timescales associated with initiation, including the time between recognition of volcanic unrest via seismic, gas or other remote means, and the time at which magma breaches the surface, appear to range between weeks and months.