Lava extrusion ended on 6 July, more than a month after activity began to decline. Harmonic tremor had accompanied the extrusion, increasing in magnitude when the lava flow rate increased. Hollow rumbling sounds were occasionally heard. Some ash puffs continued to be ejected, the most recent at 1727 on 21 August, saturating a nearby seismograph. Chemical analysis of the ash and petrographic analysis of the lava both indicated a basaltic andesite composition. COMVOL recommended the return of the last of the evacuees on 7 July, after recommending a partial end to the evacuation several weeks earlier.

Further Reference. Peña, O., 1978, Notes on the Mayon eruption from May 3-July 4, 1978 and COMVOL's role: COMVOL Letter, v. 10, no. 3-4, p. 1-3.

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.

Information Contacts: G. Andal, COMVOL, Quezon City.