VOLCANO, Hawaii — An explosion atop the erupting Kilauea volcano Wednesday rained gravel-sized rocks onto a lookout, road and trail, forcing parts of the park to close.

No one was injured by the early-morning blast hours before Hawaii Volcanoes National Park opened.

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

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

Scientists monitoring the summit say there’s a “remote possibility” of an eruption inside the half-mile-wide crater, but it’s unlikely because other indicators of an eruption aren’t present.

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

“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.

Fire and police authorities are creating emergency plans to evacuate nearby villages if the winds blow toxic gasses 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.

The explosion followed three months of increased activity in the crater, which has been releasing sulfur dioxide and carbon dioxide at levels higher than the dirtiest coal-fired power plant in the United States, said Hawaiian Volcano Observatory geochemist Jeff Sutton.

“That’s often caused by magma getting closer to the surface,” Sutton said, although he emphasized that scientists lack supporting evidence to show that magma is rising within the summit crater.

The sulfur dioxide gasses pose a health risk, especially to people who suffer from asthma and chronic coughing, Sutton said.

The American Lung Association of Hawaii advised people in the area to stay indoors, use air conditioners, avoid smoking, stock up on respiratory medications and quickly contact doctors if lung conditions deteriorate.

Hawaii Volcanoes National 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 nearby roads and paths.

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On the Net:

Hawaiian Volcano Observatory: http://hvo.wr.usgs.gov/