The second phase of activity began three million years ago, emplacing rhyolitic magma thick and long during a single event of activity. A broad circular lava dome was eventually created called the Spectrum Range. This is the southernmost of the four central volcanoes and is over wide and up to thick on the southwestern flank of Armadillo Peak and north of the Arctic Lake Plateau. Named for its extensive colourful alteration, it overlies a basal shield volcano and contains deeply carved circular valleys displaying portions of massive silica-rich comendite and trachyte lava flows which comprise the lava dome. The deeply carved circular valleys also display the bounding faults of a buried, cogenetic caldera approximately across. More than of rhyolite and trachyte was erupted during the Spectrum Range dome eruptive period, with its activity ending 2,500,000 years ago.