Hawaii ? An explosion atop the long-erupting Kilauea volcano rained gravel-size rocks onto a tourist lookout, road and trail before dawn Wednesday, injuring no one but forcing parts of a national park to close.

It was the first explosion in Kilauea’s main Halemaumau Crater since 1924, scattering debris over about 75 acres, said Jim Kauahikaua, scientist-in-charge at Hawaiian Volcano Observatory on the Big Island.

The 4,190-foot volcano has been erupting from fissures along its side steadily for more than a quarter-century. Hawaii Volcanoes National Park draws thousands of people daily, with a visitors center and lodge near the crater rim.

No lava erupted as part of the 3 a.m. explosion. That suggests it was caused by hydrothermal or gas buildup, Kauahikaua said.

Scientists said there was only a “remote possibility” of an eruption inside the half-mile-wide crater.