HAWAII VOLCANOES NATIONAL PARK » It has been a quarter-century since Mauna Loa last erupted and lava rushed down the volcano to within four miles of Hilo.

The U.S. Geological Survey and the National Park Service are preparing to mark the anniversary of the event with events to keep residents informed about the world's largest mountain mass.

“;Mauna Loa is an active volcano. Because it hasn't erupted for 25 years, people tend to forget that,”; said Frank Trusdell, a geologist at Hawaiian Volcano Observatory.

Mauna Loa's last eruption started on March 25, 1984, and lasted for three weeks.

The anniversary comes as the Big Island's youngest volcano, Kilauea, continues to erupt in two places. Since 1983, Kilauea has been releasing lava and sulfur dioxide from Puu Oo along the mountain's East Rift. It has also been spewing sulfur dioxide and small lava fragments from Halemaumau Crater at the summit since last March.

Mauna Loa erupted about every five years between 1880 and 1950 but was quiet from 1950 to 1975, when there was a brief eruption. It was quiet again until 1984.

Kilauea's eruptions tend to last longer. But Mauna Loa's can be more dangerous for people because the volcano produces greater amounts of lava and because the slopes are steeper, allowing the molten rock to reach populated areas more quickly.

Scientists estimate it could take just hours for Mauna Loa's lava to travel to the ocean through South Kona or Kau. The trip to Hilo would take days or weeks. But even that is fast compared with Mauna Loa's neighbor, Kilauea.

In 1984, for example, Mauna Loa paved 16 square miles of land in just three weeks. It took Kilauea three years to cover a similar area during the Puu Oo eruption.

Twenty-five years ago, Mauna Loa produced a mile-long line of lava fountains that sent molten rock firing high into the air. Four parallel lava flows moving as fast as 700 feet per hour poured down Mauna Loa's northeast flank.

Lava came near the back wall of Kulani prison camp, forcing an evacuation of the inmates.

The eruption was preceded by a swarm of small earthquakes and volcanic tremors. A few hours before the eruption began at 1:30 a.m., two to three earthquakes hit every minute.

To mark the anniversary, Trusdell and former observatory volcanologist Jack Lockwood will speak at the national park on March 17 and 24.

The observatory will also host “;talk story”; sessions

in Hilo, Ocean View and Kealakekua in late March and

early April.

Scientists say Mauna Loa is not showing signs of erupting soon, but they want to keep residents informed.

“;If you're middle-aged or younger, the chances are high that you'll witness a Mauna Loa eruption,”; Trusdell said.

“;I encourage residents to educate themselves - to learn about volcanic hazards, including both earthquakes and lava flows - so that they are prepared for the next eruption,”; he said.

Mauna Loa rises 29,400 feet from the floor of the Pacific Ocean to its summit, 13,677 feet above sea level. Measured from top to bottom, it is the largest single mountain mass on earth. The elevation of Mount Everest, by contrast, is 29,035 feet.