PHIVOLCS reported that the eruption at Mayon continued during 19-25 July, with slow lava effusion from the summit crater feeding lava flows on the S, SE, and E flanks. The length of the lava flow in the Mi-Isi (S) drainage remained at 2.8 km and the flow in the Bonga (SE) drainage advanced to 2.8 km by 23 July. The lava flow in the Basud drainage on the E flank did not advance, remaining 600 m long. Collapses at the lava dome and from the lava flows produced incandescent rockfalls and pyroclastic density currents (PDCs, or pyroclastic flows) that descended the Mi-Isi, Bonga, and Basud drainages as far as 4 km. Seismicity was dominated by weak low-frequency volcanic earthquakes (LFVQs) originating from a shallow source and were associated with a rapid release of volcanic gases from the summit crater. Some of the events produced audible thunder-like sounds and short dark ash plumes that drifted SW. Between 1733 on 18 July and 0434 on 19 July there were 30 of these events (called “ashing” by PHIVOLCS) recorded by seismic, infrasound, and visual and thermal monitors; each lasted 20-40 seconds long and generated ash plumes that rose 150-300 m above the summit. Sulfur dioxide emissions were reported on most days; variable amounts averaged between 1,581 and 3,135 tonnes per day, with the highest value recorded on 24 July. Each day seismic stations recorded 137-175 rockfall events, 3-4 PDC events, and 5-304 LFVQs. During 19-20 the network recorded three ashing events. At 1956 on 21 July a short-lived (28 seconds) ejection of lava was accompanied by seismic and infrasound signals. The Disaster Response Operations Monitoring and Information Center (DROMIC) reported that as of 1800 on 24 July there were 5,372 families, or 18,782 individuals, that were either in evacuation shelters or staying in other types of alternative housing. The Alert Level remained at 3 (on a 0-5 scale) and residents were reminded to stay away from the 6-km-radius Permanent Danger Zone (PDZ). PHIVOLCS recommended that civil aviation authorities advise pilots to avoid flying close to the summit.

Geological Summary. Symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the most active volcano of the Philippines. The steep upper slopes are capped by a small summit crater. Recorded eruptions since 1616 CE range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer periods of andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic density currents and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often damaged populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Sources: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Disaster Response Operations Monitoring and Information Center (DROMIC)