The volcano of Whakaari or White Island in New Zealand erupted just over a week ago. More than a dozen people were killed, including tourists to the popular attraction. Volcanologist Alison Graettinger explains the science behind this particular eruption, a hydrothermal eruption and why they can be especially difficult to predict. Reach out to the show at shortwave@npr.org. Plus, keep the conversation going with host Maddie Sofia on Twitter — she's @maddie_sofia.