As the last of the Kula Plate decayed and the Farallon Plate advanced back into this area from the south, it once again started to subduct under the continental margin of Western Canada 37 million years ago, supporting a chain of volcanoes called the Cascade Volcanic Arc. At least four volcanic formations along the British Columbia Coast are associated with Cascadia subduction zone volcanism. The oldest is the eroded 18-million-year-old Pemberton Volcanic Belt which extends west-northwest from south-central British Columbia to the Queen Charlotte Islands in the northeast where it lies west of mainland British Columbia. In the south it is defined by a group of epizonal intrusions and a few erosional remnants of eruptive rock. Farther north in the large Ha-Iltzuk and Waddington icefields, it includes two large dissected calderas called Silverthrone Caldera and Franklin Glacier Complex while the Queen Charlotte Islands to the northeast contain a volcanic formation ranging in age from Miocene to Pliocene called the Masset Formation. Although widely separated from each other, all Pemberton Belt rocks are of similar age and have similar magma compositions. 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.