There are three main types of volcanoes: active volcanoes that have erupted recently and could erupt again like Kilauea in Hawaii and Arenal in Costa Rica, dormant volcanoes that are currently inactive like Mount Fuji in Japan, and extinct volcanoes that cannot erupt again like Vesuvius in Italy. Underwater volcanoes can form volcanic islands like the Galapagos Islands near Ecuador. Active volcanoes present hazards from ash and lava flows while eruptions in the past have buried cities like Pompeii in ash.