Lava fountaining began on 21 March after almost 2 months of intermittent precursors, including a small, but deadly, phreatomagmatic eruption on 2 February. Following the 2 February eruption, sluggish and intermittent production of lava continued until 19 March when its extrusion rate increased. Several tens of small collapse-type pyroclastic flows on both 19 and 20 March resulted from portions of the flow front detaching and rolling down the steep upper slopes of the volcano. On the night of 21 March, lava fountaining to heights of several hundred meters began, forming a small cone at the head of Bonga Gully where it descends from the summit crater. Most of the lava fell back into the crater and around the vent, but eventually flowed SSE down Bonga Gully. By 26 March the flow front was 4.5 km from the summit, and the estimated volume of lava extruded was 60 x 10⁶ m³, more than half the volume of the 1984 flow. Ash-bearing steam clouds from the fountains rose 2-3.5 km above the crater and deposited a few millimeters of ash on nearby towns. This was less ash than resulted from the pyroclastic flows, which stopped when fountaining began.

The fountaining episodes typically lasted 20 minutes; the longest lasted 50 minutes. They were separated by repose periods lasting 30 minutes to 1 hour. Some episodes were followed by 10-20 minutes of intermittent 2-Hz tremor, the amplitude of which varied greatly suggesting that each tremor episode consisted of a series of tremor events. The tremor did not correlate with any visible steaming. Continuous, strong gas jets, glowing "like a blowtorch" and emitting a continuous "jet plane sound," were visible from Legazpi city, 14 km SE of the summit. They appeared to be in the summit crater, 100-200 m upslope from the vent.

COSPEC measurements of SO₂ flux increased from 4,000 metric tons/day (t/d) on 1 March, to 5000 t/d on 24 March. On 26 March, the SO₂ flux measured in the morning was 3,920 t/d rising to 7,600 and 8,800 t/d in the afternoon (two sets of measurements).

By 2 April, lava fountaining had ceased, and little or no new material was feeding the flow. Seismicity was low to moderate and dominated by small explosion-type earthquakes. Ash puffs were rare and weak. A single small pyroclastic flow occurred on 1 April, originating in the crater. The glow from the crater persisted, but was considerably dimmer and the gas jets burning in the crater had disappeared. However, SO₂ emission remained high and variable, 3,000-8,000 t/d, and the volcano was not deflating.

The lava flow front, still about 5.4 km from the vent, was not expected to advance much farther, having moved only a few meters on 1 April. The flow was confined to Matanag Gully except for a small finger that reached the lower Bonga Channel. Lateral levees and pressure ridges were well defined.

On 9 April a dirty-white steam plume rose only 50 m above the crater rim. At night, a faint glow from the crater was visible. Small "explosion-type" earthquakes, continued; 57 were detected in the 24-hour period beginning at 1700 on 8 April. Most, however, were associated with incandescent materials detaching from the lava deposits in the Bonga Gully. The rate of SO₂ emission was 2,272 t/d.

More than 45,000 people fled their homes during the early stages of the eruption, from 2 February to 19 March, filling 43 evacuation centers. An additional 12,000 evacuated their homes as the eruption entered its Strombolian phase on 19-21 March. Since the 2 February event, which killed 75 people, no deaths directly attributable to the eruption have been reported.

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: PHIVOLCS; Chris Newhall, USGS; Reuters.