hotspot, a plate-edge effect or slab window leading to magma ascent along the northern edge of the Juan de Fuca Plate, or a propagating crack controlled by stress fields related to large-scale plate tectonics of western North America. The generally preferred tectonic model is a mantle hotspot as the AVB decreases in age from west to east, a trend that is consistent with the westward motion of North America at the rate of about per year. There are also observed chemical differences between the oldest (western) and youngest (eastern) basalts. The youngest basalts are undersaturated alkaline lavas and are classified as basanite, suggesting a deeper source zone for magma in the Nazko area compared to the rest of the AVB. Western segment The Bella Bella and Gale Passage dike swarms on the Central Coast of British Columbia form the westernmost extent of the AVB. They are both over wide, with individual dikes measuring up to thick. Both swarms attain K–Ar ages of 12.5 to 14.5 million years and consist of a bimodal suite of basalt, trachyte and comendite. Basaltic and comenditic dikes are the most abundant, distributed in about equal proportions. Near the centre of each dike swarm are erosional remnants of rhyolite breccia with locally occurring rhyolite flows that comprise the Bella Bella Formation. The breccias contain a high content of coarse basement clasts, indicating that they are the product of explosive volcanism. Rhyolites associated with the Bella Bella dike swarm are distributed on Denny Island whereas rhyolites belonging to the Gale Passage dike swarm are found on the northern ends of Athlone Island and Dufferin Island. Exposed in high cliffs along the Dean and Burke channels is the King Island Pluton. This easterly trending intrusion has a length of over and a width of , cutting across the predominantly northwest structural trend of the Coast Plutonic Complex. Two distinct phases of magmatism created the King Island Pluton 10.3 to 13 million years ago. The first and principal phase formed the core of the pluton which comprises coarse-grained syenite. These rocks form most of the pluton exposures at sea level. The second phase resulted in the creation of a marginal zone of alkali granite at the extreme western end of the King Island Pluton, as well as satellite stocks west of the main pluton. Central segment Three large shield volcanoes and two locally extensive volcanic fields form the central