Approximately 2 cubic kilometres of tephra was erupted , more than Mount St. Helens ejected in 1980 . Many of the lakes surrounding the mountain had their shapes and areas dramatically altered , especially the eventual enlargement of Lake Rotomahana , the largest crater involved in the eruption , as it re-filled with water . The rift created during the eruption extends 17 km across the mountain , Lake Rotomahana and through the Waimangu Volcanic Rift Valley . The eruption was heard clearly as far away as Blenheim and the effects of the ash in the air were observed as far south as Christchurch , over 800 km away . In Auckland the sound of the eruption and the flashing sky was thought by some to be an attack by Russian warships
Mount St. Helens ejected in 1980
1980 eruption of Mount St. Helens
1886 eruption of Mount Tarawera