MANILA, Philippines — A pyroclastic density current (PDC), locally known as “uson,” was observed at Mayon Volcano on Saturday, according to the Philippine Institute of Volcanology and Seismology (Phivolcs). Phivolcs, in an initial advisory, said an ongoing PDC was seen moving along the Mi-isi gully on the volcano’s south-southwestern slopes. Pyroclastic density currents are fast-moving
