After a week of puffing out ash and sending bursts of lava trickling down its steep slopes, the 8,070-foot (2,460-meter) mountain overlooking the Gulf of Albay and Legazpi city shook with nearly 2,000 volcanic earthquakes and tremors between Sunday and Monday, state volcanologists said.

The emission of sulfur dioxide — an indication of magma rising inside the volcano — jumped to 6,000 tons per day from the normal 500, said the Philippine Institute of Volcanology and Seismology. It also reported "audible booming and rumbling sounds" in the eastern flank of the volcano, accompanied by intensified crater glow at night.

Lava fountains bursting from the cone-shaped volcano overnight rose 650 feet (200 meters) in the air, the institute said.

Scientists raised the alert level Sunday to one step below a hazardous eruption