 This resulted from the changing geometry of the Kula Plate, which progressively developed a more northerly movement along the mainland of Western Canada.Instead of subducting beneath Western Canada, the Kula Plate began subducting underneath southwestern Yukon and Alaska during the early Eocene period.Volcanism along the entire length of the Coast Range Arc shut down about 50 million years ago and many of the volcanoes have disappeared from erosion.