Philippines

The February - March 2000 Mayon Eruption

Format
Situation Report
Source
Posted
Originally published
by Ernesto G. Corpuz, Raymundo S. Punongbayan

Mayon Volcano in southeast Luzon erupted from 23 February to 07 March 2000. The initial eruption was characterized by relatively quiet dome extrusion at the summit crater and emission of lava flow to the southeast flanks. The main explosive phase commenced on the morning of 24 February, with the occurrence of pyroclastic flows which also descended the Bonga Gully southeast of the crater. Vigorous pyroclastic ejection with tall ash columns and voluminous and multiple pyroclastic flows generally characterized the peak of eruptions from 28 February until 01 March 2000. Gentle emission of lava flows with sporadic outbursts of ash plumes from the summit and from still-hot deposits dominated post-explosion activity. All hazardous eruption phenomena and products were contained within the previously delineated danger zones.

The Mayon 2000 eruption was actually preceded by a series of explosions as early as eight months before the onset of lava extrusion. During the pre-eruption period in 1999 and in early January 2000, five discrete phreatic or steam-driven explosions occurred. The first of these explosions on 22 June 1999 heralded the beginning of unrest, hurling ash and steam up to six kilometers in the sky. Subsequent explosions on 22 September and 08 December 1999 and 05 January 2000 were of similar character, each producing a single outburst that quickly subsided to unremarkable state of unrest and apparently lacking clear-cut indicators for further explosions. For example, the usual glow of the crater, a precursor to the arrival of magma beneath the surface, did not precede the explosions and therefore suggested that magma was not present in any sufficient amount to drive a full-scale eruption. It was on this basis that PHIVOLCS did not issue further alerts except for declaring Alert Level 1 on 22 June and then Alert Level 2 (02 July 1999) and extending the six-kilometer radius danger zone to seven kilometers in the southeast portion of the volcano.

It was not until 07 January 2000 that a precursor to a probable magmatic eruption was manifested. A brief period of intense glow of the crater suggested that magma ascended to very shallow levels in the volcanic cone. This situation was confirmed by appearance of new lava on the crater bottom during an aerial survey on 09 January. However, the lack of other usual precursors such as high seismicity and accelerating ground movements or ground tilt indicated that an eruption was not imminent. The appearance of a lava pile or lava dome on the summit crater by 13 February 2000, however, gave strong evidence that magma was close to the crater. Thus, Alert level 3 was issued and meant that an eruption could be possible within weeks should trends continue. By 19 February, it became apparent that some swelling of the volcano's edifice was occurring, based on precise geodetic measurements. Other evidence for progressive volcanic activity became noticeable at this time. Volcanic gas output became voluminous and the Sulfur Dioxide (SO2) component of the gas, one clear sign of magma involvement, increased to very high levels. Later, when the summit became clear of clouds, persisting crater glow further suggested that the summit lava dome was degassing profusely and that intrusion of magma into the lava pile had caused cracks in the lava dome, exposing the hot and incandescent interior. Further growth of the summit lava pile from 19 - 23 caused part of the said lava dome to collapse from time to time so that incandescent fragments were seen to fall into the Bonga Gully.

On 23 February 2000, the unconfined side of the lava dome facing the southeast overlapped the summit crater rim, causing an avalanche of incandescent rockfalls into the Bonga Gully. With imminent explosive eruption, PHIVOLCS announced an Alert Level 4 over Mayon at 0300 H of 24 February 2000. The onset of explosions began at 0826 H of the same day, and when small pyroclastic flows descended the Bonga Gully at this time, the status of the volcano was raised to the full Alert Level 5 to indicate that hazardous eruptions were in progress. In addition to raising the alert condition, PHIVOLCS also recommended an extension of the southeast danger zone from 7 kilometers to 8 kilometers. The 6-kilometer radius Permanent Danger Zone, however, was retained for other areas around the volcano.

Eruptions from 24 February to 01 March generally alternated between highly explosive discharge of pyroclastic material and gentle emission of lava flows from the summit crater. The strongest eruptions were evident on 28 and 29 February and on 01 March 2000, based on the apparent volume of material ejected, the height of lava fountaining, height of eruption columns and generation of multiple pyroclastic flows around the cone. Thick ash-laden eruption clouds generally drifted to the western sections of Mayon, enveloping the Municipalities of Guinobatan and Ligao and partial fallout at Camalig that were within 10 kilometers of the crater. From time to time, pyroclastic flows that descended the Bonga Gully also produced ashfall to the southeast, but the fallout never reached beyond the danger zones and were in fact delivered by surface winds to the west. During violent discharge of pyroclastics on 28 February, some pebble-sized scoria rained on the western slopes of Mayon, pelting the Barangays of Bubulusan in Guinobatan and other places some 10 kilometers due west of the crater.

After explosive activity ceased, eruptions continued in the form of gentle lava emission. It also became evident that with persisting rains since 01 March to 05 March, pyroclastic deposits perched high on the mountain slopes would become susceptible to remobilization. PHIVOLCS therefore initiated a mapping effort to delineate river channels and the localities which may be affected by lahar flows. The areas identified include the Mabinit and Buyuan-Padang river channels in Legaspi City; Miisi and Anuling channels in Camalig; Tumpa channel in Camalig; Maninila and Upper Nabonton channels in Guinobatan; Basud-Lidong channel in Sto. Domingo, Bulawan channel in Malilipot, San Vicente and Buang channels in Tabaco and Upper Nasisi in Ligao.

All in all, some 24,000 people were evacuated out of the danger zones with no deaths attributed to direct impact by the eruptions. The number of people temporarily transferred ballooned to about 66,000 persons after local governments evacuated residents out of the ashfall-hit areas beyond and outside the previously delineated danger zones. Although a slow return by the volcano to quiet conditions has become apparent, about 30 million cubic meters of pyroclastic deposits remain on the slopes of the volcano. Such deposits remain unstable and are likely to experience periodic remobilization and subsequent deposition onto the lower slopes. While the alert levels will likely be brought down progressively to reflect the waning of olcanic activity, danger areas associated with potentially life-threatening lahar flows will remain for some time.