The new blast from Mount Merapi came as Indonesia also struggles to respond to an earthquake-generated tsunami that devastated remote islands. The, unfolding simultaneously on opposite ends of the seismically volatile country, have killed nearly 500 people and severely tested the government's emergency response network. In both events, the military has been called in to help.
One of 129 active volcanoes in Indonesia, Merapi has killed 38 people since it started erupting a week ago.
Even in the crowded government camps miles away, people still instinctively ran for shelter at the power of Monday's eruption, which was accompanied by several deafening explosions, said Subrandrio, an official in charge of monitoring Merapi's activity. About 69,000 people villagers have been evacuated from the area around its once-fertile slopes - now blanketed by gray ash - in central Java, 250 miles east of Jakarta, the capital.
As massive clouds spilled from the glowing cauldron and billowed into the air, sending debris and ash cascaded nearly four miles down the southeastern slopes, Subrandrio said. Local officials and witnesses initially described it as the biggest since the initial blast a week ago, but Surono, chief of the Center for Volcanology and Geological Hazard Mitigation, said an explosion on Saturday was actually more powerful.
Merapi has erupted many times in the last two centuries, often with deadly results. In 1994, 60 people were killed, while in 1930, more than a dozen villages were incinerated, leaving up to 1,300 dead.
More than 800 miles to the west, meanwhile, a C-130 transport plane, six helicopters and four motorized boats were ferrying aid to the most distant corners of the Mentawai Islands, where last week's tsunami destroyed hundreds of homes, schools, churches and mosques.
President Susilo Bambang Yudhoyono said relief efforts must be sped up, expressing dismay it took days for aid to reach the isolated islands, though he acknowledged that violent storms have previously prevented most planes, helicopters and boats from operating.
The tsunami death toll stood at 431 Monday after initially being raised to 450. The National Disaster Management Agency said on its website that the number dropped as officials double-checked reports to verify them. The number of missing was 88.
Indonesia, a vast archipelago of 235 million people, straddles a series of fault lines and volcanoes
The fault line that caused last week's 7.7-magnitude earthquake and killer wave that followed - and also the 2004 tsunami that killed 230,000 people in a dozen countries - is the meeting point of the Eurasian and Pacific tectonic plates that have been pushing against and under each other for millions of years, causing huge stresses to build up. It runs the length of the west coast of Sumatra island.
Both earthquakes and volcanoes can be related to movements in the overlapping plates that form the earth's crust. As plates slide against or under each other, molten rock from the layer of mantle can break the surface via a volcano, or create energy released in an earthquake.
The government has raised alert levels of 21 other volcanoes to the second- and third- highest levels in the last two months because they have shown an increase in activity, said Syamsul Rizal, a state volcanologist, said Monday. Many of those are already rumbling and belching out heavy black ash.
Indonesia has several volcanoes smoldering at any given time, but another government volcanologist Gede Swantika said there are normally only five to 10 on the third-highest alert level, indicating an increase in seismic activity and visible changes in the crater, and none at the second-highest, signifying an eruption is possible within two weeks. He said monitors noticed more volcanoes were exhibiting seismic activity starting Sept. 2.
"We can say this is quite extraordinary, about 20 at the same time," Swantika said. "We have to keep an eye on those mountains. ... But I cannot say or predict which will erupt. What we can do is monitor patterns."
Geologist Brent McInnes said as he hadn't seen the raw data but would find such a rash of volcanic activity significant.
"If it's true that there are over 20 volcanoes demonstrating increased levels of seismic activity, then that is something we should pay attention to," said McInnes, a professor at Australia's Curtin University who has done extensive volcanic research in Indonesia.
He said such an increase could indicate "maybe there is a major plate restructuring going on, and that would be significant."
Two of the closely watched volcanoes - Karangetang and Ibu - are at the second-highest alert level. Karangetang erupted in August, killing four people, and both mountains shoot out ash daily, local monitors said. The two mountains lie within a few hundred miles of each other more than 1,400 miles northeast of Jakarta.
Anak Krakatua, a volcano known as the "Child of Krakatoa" also started shooting lava last week. Although the firebursts look spectacular, there were no immediate signs of major eruption, said Anton Tripambudi, a government seismologist.
The mountain, about 80 miles west of Jakarta, was formed after the Krakatoa eruption of 1883, the largest volcanic eruption in recorded history that, along with a tsunami, killed at least 36,417 people.