Subduction zones are the most geologically active areas on Earth and they present some of the most complex questions faced by Earth scientists. After many decades of study, consensus has been reached on a model for magma generation in volcanic arcs, the chains of volcanoes above subduction zones. Main stage Cascade volcanism (North America) is a popular achetype for the classical conceptual view of a continental arc. However, north of the Nootka fault, which defines the terminus of the Juan de Fuca plate, is problematic for the classic model, which predicts that arc volcanism will only occur where a subduction zone is active. Indeed, Cascade arc volcanism persists well north of the end of the subduction zone, which is called Nothern Cascade arc or the Garibaldi Volcanic Belt.
Taken together, the Main Stage and the Northern Cascade Arcs record both the life and death of an arc. Thus in this region lies a profound opportunity to characterise for the first time the evolution of a continental volcanic arc in space and time and develop understanding of this evolution in terms of links among key mantle, magmatic, tectonic and surface processes.