 Therefore, these magmatic rocks are believed to be products of arc volcanism related to subduction of the Farallon Plate.By late Pliocene time the Farallon Plate had been greatly reduced in size and its northern portion ultimately broke off between five and seven million years ago to form a new plate boundary called the Nootka Fault.This rupture created the two small Juan de Fuca and Explorer plates that lie off the west coast of Vancouver Island.