Just after Thanksgiving, for the first time in almost 40 years, Hawaii's Mauna Loa volcano erupted. It's one of several ongoing eruptions – including Kilauea, also on Hawaii, and Indonesia's Mount Semeru. At just over half the size of the big island of Hawaii, Mauna Loa is the world's biggest active volcano. Today, volcanologist Alison Graettinger talks to Scientist in Residence Regina G. Barber about what makes Mauna Loa's eruption different than Indonesia's and others around the Pacific, and what it reveals about planet Earth.Watch the U.S. Geological Survey's live video of the eruption here.