Volcanic unrest that began at Mayon during May 1999 led to growth of the lava dome beginning on 12 February 2000, and continued in the form of explosive eruptions from 23 February through 1 March (BGVN 25:02). Since the 1 March 2000 eruption, observations have indicated that activity is declining.

After 1 March activity was relatively quiet with the largest event being an ash puff that was produced on 12 March when hot lava at the summit came in contact with surface water. The ash puff rose to a height of ~1 km and drifted to the NW. After 1 March there was moderate seismicity, high volcanic gas outputs, no increase in ground deformation, continuing glow of the summit, and new lava effusion. These conditions were associated with very gradual return to repose. Therefore on 16 March PHIVOLCS reduced the alert status of Mayan volcano from Alert Level 4 (hazardous eruption imminent, possible within days) to alert Level 3 (less probability of a hazardous eruption).

On 17 March at 1254, 1350, 1609, and 1619 partial collapse of the new lava flow on the volcano's upper middle slopes produced voluminous secondary pyroclastic flows, with the associated ash clouds blown to the SW. On 19 March at 0138, 0203, and 0300 similar secondary pyroclastic flows occurred with ash clouds that blew NE. The billowing ash clouds did not originate from the crater, but from the side of the lava flow that failed on the volcano's upper slope, and from descending detached lava fragments along Bonga Gully. Scientists expected the series of secondary pyroclastic flows due to the instability of volcanic material deposited on steep ground.

After 25 March the number of low-frequency volcanic earthquakes was relatively high (up to 40 per day), presumably due to strong jetting of hot gases emanating from the crater. In addition, relatively high levels of SO₂ emissions (up to 9,000 metric tons per day) also occurred after 1 March that were associated with degassing of residual magma. Since the volcano showed no signs of an imminent eruption, on 1 April PHIVOLCS reduced the Alert Level to 2 (the probability of hazardous explosive eruption is minimal). The volcanic system is expected to continue producing earthquakes and to vent a large amount of gas because fresh magma still resides along the whole length of the volcanic conduit and near the summit. Due to the possibility of sudden explosions caused by the release of gas from localized pockets within the magmatic system, and the threat of secondary pyroclastic flows, rockfalls, and ash fallout, PHIVOLCS maintains off-limit zones around the volcano up to 7 km in radius.

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: Raymundo S. Punongbayan and Ernesto Corpuz, Philippine Institute of Volcanology and Seismology (PHIVOLCS), C.P. Garcia St. Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost. gov.ph/).