What was first believed to be an earthquake this morning turned out to be the first explosive eruption at Kilauea Volcano since 1924, U.S. Geological Survey scientists announced this afternoon.

The last eruption of any kind in the Kilaeau caldera occured in September 1982.

Today's small explosion occurred at the Halemaumau Crater at 2:58 a.m. It was originally reported as a 3.7 magnitude earthquake by the U.S. Geological Survey's Hawaian Volcano Observatory, but after discovering volcanic rocks up to about three-fourths of an inch in diameter in the Halemaumau parking lot early this morning, officials at Hawaii Volcanoes National Park closed one mile of Crater Rim Drive from Kilauea Military Camp to Jagger Museum as a safety precaution. Four miles of the road were closed last month because of increased sulfur dioxide gas emissions, which continue to increase.

This morning's explosion scattered debris over about 75 acres, covering a portion of Crater Rim Drive and damaging the Halemaumau overlook. No lava was ejected as part of the explosion, and scientists say that suggests the activity was driven by hydrothermal or gas sources.

Explosive debris also covered the overlook, the Haleamaumau parking area and the portion of Crater Rim Drive next to the parking area. The size of the explosive debris increased toward the overlook, with the largest observed block ejected being about 35 cubic feet. Scientists believe it was propelled from the vent located more than 230 feet below the crater rim. Small impact craters are abundant in the overlook area. Rock debris extends halfway across the Halemaumau Crater. The debris is composed of rock fragments derived from the walls of the crater, but no fresh lava was observed on the debris or the crater floor.

According to the USGS report, a series of seismic events started at 2:55 a.m. that may have been shallow or high-frequency earthquakes or minor explosions. The main explosion started at 2:58 a.m. was associated with long-period seismicity. Low-frequency sound waves were also detected by the University of Hawaii infrasound laboratory. The signals persisted through the morning, indicating continuing energetic release of gas from the vent in the Halemaumau Crater.

The USGS said the explosion produced a crater along on the east wall of Halemaumau about 65 to 100 feet in diameter.

Future explosive activity is possible, the USGS said, and the area will continue to be monitored.

Jim Gale, the park's chief of interpretation. said the sulfur dioxide being produced at the Kilauea summit has increased dramatically over the past month.

"There's enough sulfur dioxide being put out at the Kilauea summit right now to fill 150 Goodyear blimps a day," Gale said. "It's a concern. At Pu'u O'o crater, it is further away from people and there is also increased sulfur dioxide being put out, but at Kilauea you're near the observatory and closer to the visitors center."

Sulfur dioxide, or SO2, is an invisible gas that can aggravate pre-existing heart and breathing problems such as asthma, and can be dangerous to anyone at the high levels the park is now seeing.

Just after 7 this morning, scientists at the Hawaiian Volcano Observatory posted a status report on the volcano that listed four hazardous conditions on Kilauea volcano:

1. The current increase in sulfur dioxide emission rates at the summit means that SO2 concentrations in the air are likely to be at hazardous levels for areas downwind of emission sources in Halemaumau crater. Most people are sensitive to sulfur dioxide at these levels. Children and individuals with asthma, chronic obstructive pulmonary disease, or other breathing problems especially should avoid downwind areas. During trade-wind conditions (brisk winds from the northeast), concentrations will be highest and most hazardous in the south caldera: southwest rift zone, south caldera pullouts, and the Halemaumau overlook parking lot - The National Park Service has closed this section of Crater Rim Drive. During weak or southerly winds, concentrations may be high and hazardous throughout the summit area of Hawai`i Volcanoes National Park and nearby communities.

2. Vent areas and lava channels are hazardous and conditions can change rapidly. Sulfur dioxide emissions from Pu`u `O`o are high and result in hazardous concentrations downwind.

3. There continues to be a threat from lava flows to anyone within Royal Gardens subdivision and the coastal plain. As long as lava flows are active south of fissure D, the subdivision is within likely flow paths of future flows. The rootless shields, which are now less than two miles from the subdivision, can collapse and release lava flows that advance that distance within a few days. Tube- or channel-fed lava flows established within the subdivision and on the coastal plain remain a threat due to tube ruptures which can produce new lava flows. Lava flows advancing through vegetation are hazardous and can produce fire and methane explosions that propel chunks of lava and rock several feet into the air.

4. Lava entering the ocean poses two additional hazards - potential collapse and laze. Lava entering the ocean builds a delta over its own rubble that is extremely unstable. That delta can collapse without warning and expose very hot surfaces to waves which can explode and throw rock debris up to one-quarter mile inland. For these reason, spectators should avoid the delta and the area one-quarter mile inland. The interaction between seawater and lava produces a steam plume laced with acids and fine particles of volcanic glass or 'laze' that is unhealthy if inhaled and can produce skin or eye irritation if contacted.

Reach Dave Dondoneau at ddondoneau@honoluluadvertiser.com.