How We Tell if a Volcano Is Active, Dormant, or Extinct

Tongariro (left) and Ngauruhoe (right) in New Zealand, seen from the Tongariro Alpine Crossing, November 2010. Jeff P. / Flickr | CC by 2.0 Yesterday, GNS Science in New Zealand declared that the volcanic unrest at Tongariro that started in 2012 is over. They lowered the volcanic alert status to Level o (background) and declared that Tongariro was back asleep (even if some steam might persist from the Te Maari vent). So, we can probably consider Tongariro "dormant" again after 3+ years of being an "active" volcano. However, it is surprising for most people to hear that there are no hard-and-fast definitions for what a volcano is considered active, dormant or extinct. Many times, what one person might think is an extinct volcano might actually merely be dormant and that transition is shades of grey, anyway. So, how do I differentiate between active, dormant and extinct volcanoes. This is my personal take (with input from the volcanological community) and to me, it's all about potential. If you have a different definition, feel free to leave that in the comments.