Waves of orange, glowing lava and smoky ash belched and sputtered Monday from the world’s largest active volcano in its first eruption in 38 years, and officials told people living on Hawaii’s Big Island to be ready in the event of a worst-case scenario.

The eruption of Mauna Loa wasn’t immediately endangering towns, but the U.S. Geological Survey warned the roughly 2,00,000 people on the Big Island that an eruption “can be very dynamic, and the location and advance of lava flows can change rapidly”.