The first explosive eruption at Kilauea volcano in almost a century scattered boulders and smaller rocks over 75 acres of Hawai'i Volcanoes National Park in the middle of the night when no one was around.

Falling rocks damaged the popular Halema'uma'u lookout and its parking lot, and were scattered along Crater Rim Drive, the road visitors drive throughout the park. The largest boulder was about 3 1/2 feet wide.

The rocks flew through a vent about 230 feet below the Halema'uma'u crater rim and left scientists at the Hawaiian Volcano Observatory wondering if, and when, it will happen again.

"Now I know how Capt. Kirk felt — we're exploring new worlds," said Jim Kauahikaua, scientist-in-charge at the observatory. "The recent explosive event represents a significant addition and change to Kilauea volcano's ongoing activity, and the Hawaiian Volcano Observatory is using every means available to study its causes and consequences."

The explosion left a crater along the east wall of Halema'uma'u about 65 to 100 feet wide.

NO LAVA FOUND

Scientists originally said the eruption at 2:58 a.m. yesterday was a 3.7-magnitude earthquake, followed by about 10 smaller quakes in the Halema'uma'u area. But when dawn arrived, scientists discovered volcanic rocks strewn about and knew something unusual had happened.

By noon, they knew that the recent increase in hydrothermal or gas sources had forced the explosive eruption, the first since May 1924. No lava was found in the area.

"It's very exciting for all of us," said Jim Gale, chief of interpretation at the observatory. "It's such a change from the overall activity I've seen in my seven years here.

"We have passive lava fields here in Hawai'i where people usually get up within six feet and experience them. They get so close they can feel the intense heat. Volcanoes are a fantastic experience no matter what, but this is a new experience for us," Gale said.

He said most of the park remains safe and open, but one mile of Crater Rim Drive was closed yesterday from Kilauea Military Camp to Jagger Museum as a precaution. Four miles of the road have been closed since last month because of increased sulfur dioxide gas emissions, which are still increasing.

"Our No. 1 priority is to make sure the visitors and area residents are safe," Gale said. "We're monitoring the events.

"Sulfur dioxide levels have been on the rise since December. There's enough sulfur dioxide being put out at the Kilauea summit right now to fill 150 Goodyear blimps a day," he said. "It is a concern."

SLEPT THROUGH IT

Debbie Smith, a teacher at Le Jardin Academy in Kailua on O'ahu, is helping lead about 60 fourth-graders who are visiting the Big Island this week and staying at the Kilauea Military Camp. She said they have experienced no breathing problems.

"They closed off another part of the road, but that was it," she said. The group slept through the explosion and didn't know it happened until much later, she said.

Sulfur dioxide levels within the park are already monitored, and a system is being developed to monitor the levels outside the park. That effort is being coordinated by the observatory, Hawai'i County Fire Department and state Department of Health.

Sulfur dioxide is an invisible gas that can aggravate pre-existing heart and breathing problems such as asthma and can be dangerous to anyone venturing too close to its source at the high levels the park is now experiencing.

Reach Dave Dondoneau at ddondoneau@honoluluadvertiser.com.