By Mark Niesse
The Associated Press

AP Photo/Marco Garcia As volcanic gas escapes from a vent, a young girl views the surrounding area of the Halemaumau Crater near the Jaggar Museum inside Hawaii Volcanoes National Park, Tuesday.

KILAUEA VOLCANO, 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 monitoring the summit say that there's a "remote possibility" of an eruption inside the half-mile-wide crater, but that it's unlikely because other indicators of an eruption aren't present.

"The recent explosive event represents a significant addition and change to Kilauea Volcano's ongoing activity, and Hawaiian Volcano Observatory is using every means available to study its causes and consequences," Kauahikaua said in a statement.

The explosion followed three months of increased activity in the crater, which has been releasing high levels of sulfur dioxide and carbon dioxide, said observatory geochemist Jeff Sutton.

The park had previously closed a part of the road near the gas emissions, and that area was expanded Wednesday following the explosion.

Officials shut off the road at the Kilauea Military Camp toward the Jaggar Museum and observatory. Only volcano employees were allowed inside that part of the park.

Rocks shot from the explosion damaged a wooden fence that visitors used to peer into the crater and created hazards across roads and paths.

Fire and police authorities are creating emergency plans to evacuate nearby villages if the winds blow toxic gases in their direction, said Duane Hosaka at Hawaii County Civil Defense. So far, the volcano's gas emissions continue to move toward the sea, rather than over populated areas.

"There's no evacuation or advisories. We're still in the planning stages in case something happens so we'll be prepared," Hosaka said.