Volcanic activity remained relatively low at Mayon during the week. Few rockfalls were observed, seismicity was relatively low, incandescence was not visible in the crater, and no inflation was detected at the volcano's summit. SO2 emission rates reflected continuous degassing of residual magma. Because volcanic and seismic activity had been declining for the previous 2 weeks, on 21 August PHIVOLCS decreased the Alert Level at the volcano from 4 (hazardous eruption imminent) to 3 (increased tendency towards eruption). As a consequence, the Extended Danger Zone (EDZ) in the SE returned to the original 6-kilometer radius Permanent Danger Zone (PDZ).

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 term andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic flows 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.

Source: Philippine Institute of Volcanology and Seismology (PHIVOLCS)