MAYON Volcano in Albay had a phreatic eruption on Sunday afternoon, its first visible sign of unrest since state volcanologists lowered the alert level last December. 

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) said the phreatic eruption happened at 4:37 p.m. and lasted four minutes and nine seconds. 

Along with the pyroclastic density currents, rockfall, and a 1.2-kilometer-tall plume that moved to the southwest, it also produced a booming sound. 

According to the United States Geological Survey, a phreatic eruption is ”made up of steam-driven explosions that occur when water beneath the ground or on the surface is heated by volcanic activity.”

”Phreatic eruptions can often precede, accompany, or follow a more traditional volcanic eruption. Groundwater can often be found near volcanic vents, and as the magma rises to the surface of the earth, it heats the groundwater and causes these phreatic eruptions,” it added.

Mayon Volcano remains at Alert Level 2, or moderate unrest, PHIVOLCS said.