The world's largest active volcano, Mauna Loa, erupted for the first time in nearly 40 years Sunday night, joining its neighboring volcano, Kilauea, which has been erupting for more than a year. The rare dual-eruption event is expected to attract an influx of visitors eager to see their molten flows, the national park said in a Facebook post.

Just 21 miles apart, the two volcanoes have not erupted together since 1984, when Mauna Loa last erupted. Neither are threatening homes or nearby infrastructure at this time, the park said, though state health officials warned of potential impacts on air quality.

Fountains of lava as tall as 200 feet were flowing from Mauna Loa, the US Geological Survey said Monday evening, though most remained a few yards tall.

The eruption is currently concentrated in Mauna Loa's Northeast Rift Zone, where lava is flowing from at least one split in the volcano, according to the US Geological Survey. Volcanic gas, fine ash and Pele's Hair (strands of volcanic glass) could be carried downwind, the agency said.