Report on Mayon (Philippines) — February 2000
Bulletin of the Global Volcanism Network, vol. 25, no. 2 (February 2000)
Managing Editor: Richard Wunderman.
Mayon (Philippines) Strong explosions, lava flows, and pyroclastic flows following dome growth
Please cite this report as:
Global Volcanism Program, 2000. Report on Mayon (Philippines). In: Wunderman, R (ed.), Bulletin of the Global Volcanism Network, 25:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200002-273030.
13.257°N, 123.685°E; summit elev. 2462 m
All times are local (unless otherwise noted)
Volcanic unrest that began in May 1999, and intermittent explosive eruptions beginning in June 1999, eventually led to growth of a lava dome on 12 February 2000. By 23 February PHIVOLCS had recommended evacuation to 7 km from the summit in the SE and to 6 km for the rest of the volcano. The latter is a permanent danger zone.
At 2206 on 23 February the seismic network detected an explosion-type earthquake that coincided with rumbling and minor ejection of lava fragments from the summit. This earthquake was followed shortly by bright incandescence, indicating that lava emission and ejection had intensified. Low-frequency volcanic earthquakes then occurred beginning at 2217 and lasting until about 2326 when the seismographs began to record harmonic tremor. The tremor became pronounced at about 0034 on 24 February and was accompanied by minor lava fountaining to 50 m above the summit lava dome. The hazard status was raised to Alert Level 4 (hazardous eruption imminent, possible within days) at 0300 on 24 February. No additional evacuation was recommended, but residents within 8 km of the summit were advised to prepare for evacuation.
At 0826 on 24 February another explosion-type earthquake was recorded by the seismographs at Anoling, Sta. Misericordia, and Mayon Resthouse Observatory. The summit was obscured, but at 0829 a pyroclastic flow descended SE towards the Bonga Gully with a run-out distance of ~7.2 km, reaching the distal end of the Bonga fan. The hazard status was then raised to Alert Level 5, (hazardous eruption in progress). Because pyroclastic flows could continue to sweep down along well-incised gullies and channels, especially the Bonga Gully, PHIVOLCS recommended extension of the danger zone to 8 km along the SE sector of Mayon Volcano. Likewise, ashfall was expected mainly W, SW, and NW of the crater.
The SO2 emission rate increased on 24 February to 4,070-5,700 metric tons/day (t/d). Ground deformation measurements showed that the volcanic edifice swelled significantly in the previous two days, consistent with the growth of the lava dome.
By the morning of 25 February activity was mainly lava extrusion, with a flow channeled along the Bonga Gully. COSPEC readings conducted on 24 February reached 13,500 t/d. The abrupt increase in this value may be attributed to the series of highly gas-charged ash ejections comprising the volcanic plume.
Following a quiet interval that started at 1420 on 26 February, more vigorous activity resumed on the evening of the 27th. Seven ash-and-gas explosions occurred between 1950 and 2237, the most significant of which (at 2144 and 2237) were accompanied by lava fountaining with ejection of volcanic bombs. Large incandescent fragments were ejected to ~500 m above the crater rim. Ground deformation measurements showed that the edifice remained inflated. COSPEC readings of SO2 flux remained significantly above normal at 4,900 t/d. Explosion earthquakes and harmonic tremor accompanied the lava fountaining and persisted even when the activity had apparently subsided.
Explosive eruptions during 28 February-1 March. Mayon had another series of explosive eruptions during 0700 to 2100 on 28 February, with the most significant eruptions occurring at 1641, 1732, and 1940. The first explosion produced a 5-6-km-high eruption column and generated a large pyroclastic flow that descended the W portion of the Bonga Gully on the SE flank and entered the Mabinit channel to the S. This was followed by voluminous eruption clouds beginning at 1732 that rose to ~10 km above the summit and generated multiple pyroclastic flows to the SW, S, and SE. Vigorous explosions sustained the eruption column and discharged large volcanic fragments that splattered the upper portions of the cone. Thick ash clouds hovered around the volcano and created frequent lightning discharges.
Most of the ash clouds were eventually carried to the SW and W, affecting Ligao, Guinobatan, and Camalig. However, the pyroclastic flows did not travel beyond the present danger zones. The ash clouds contained high concentrations of sulfur dioxide, with COSPEC-recorded emission rates of 13,000 t/d, as expected for an eruption cloud. Aircraft were warned to avoid lingering ash clouds to the W of the volcano. The E side, towards the Legaspi airport, remained free from volcanic ash, debris, and SO2 emissions.
Electronic distance measurements revealed that the volcano's edifice remained inflated. Such inflation was thought to be caused by the ascent of magma as indicated by the near-continuous seismic tremor associated with active magma transport.
The series of major ash ejections and subsequent pyroclastic flows that occurred along Bonga Gully, Mabinit, and Miisi Channels started at 1641 on 28 February 2000. The maximum height estimated for the vertical ash plume was 12 km during the 1732 event. The approximate runout of the pyroclastic flows reached to ~5-6 km downslope. Severe ashfall occurred in the SW sector of the volcano, especially at Barangay Tumpa in Camalig and Barangays Maninila and Masarawag in Guinobatan. Lava fountaining with ballistic bombs was also frequently observed starting at 1732 with maximum heights estimated at 1 km.
After the vigorous activities late in the afternoon to early evening on 28 February, only quiet effusion of lava was noted during times when the summit was not obscured through the morning of 29 February.
Another series of ash ejections began at 1211 on 29 February. The largest event occurred at 1501 and produced a 14-km-high eruption column. This event also generated several pyroclastic flows that descended all sides of the volcano. Pyroclastic flows that were channeled by gullies in the SW, S, and SE reached up to 5-6 km from the summit. Smaller pyroclastic flows that followed gullies in other sectors stopped ~2-3 km from the crater. Ash from the tall eruption column and from pyroclastic flows drifted to the W and SW. The ash ejections were generally accompanied by rumbling sounds. Vigorous lava fountaining began at 1531 and ballistic projectiles fell within 1.5 km of the crater. Lava flows were observed on 1 March to have reached the 1,000 m elevation, or about 2.3 km from the summit.
COSPEC measurements on 29 February were hampered by thick ash cover. Ground deformation measurements made the morning of 29 February along the Buang and Masarawag EDM lines showed that the volcano edifice remain inflated. Significant potential was noted for lahars along major tributaries draining from the NW due to the presence of ash and pyroclastic-flow deposits, which could be eroded and remobilized during heavy rainfall.
Mayon exhibited another series of eruptions that began on 1 March and produced dense and highly convective ash columns that rose up to 7 km above the summit. Part of the eruption column would occasionally collapse to produce pyroclastic flows that traveled along major gullies around the volcano. Pyroclastic flows were observed along the main gullies facing Anoling. The largest of these pyroclastic flows occurred along Bonga Gully and traveled ~6 km from the crater, while smaller flows at other gullies descended some 4 km downslope. Explosive eruptions produced lava fountaining with discrete ballistic volcanic fragments hurled out to ~500 m above the crater rim. Frequent rumbling accompanied the explosions, which lasted until 1609. By the end of this episode of explosive activity, quiet lava extrusion followed and continued to be observed up to the present. Areas SW and W of the volcano were severely affected by ashfall with the most significant deposition in Camalig, Guinobatan, and Ligao. Minor ash and steam were continuously being generated by lava deposits from the summit crater and Bonga Gully and drifted to the SW and W areas by prevailing winds.
Lava emission phase. Mayon was relatively quiet during 2 March as the seismic network recorded short-duration harmonic tremors and some discrete low-frequency volcanic earthquakes. This departs from the continuous tremor recorded in the past days during periods of relative quiet. The volcano has apparently entered a phase of lava emission with sporadic episodes of minor ash puffs. Ash and steam emission from both the summit crater and new lava flow deposits produced a haze over the SW sector, particularly in the municipalities of Camalig, Guinobatan, and Ligao. SO2-flux measurements on 2 March yielded a value of 14,500 t/d. Ash clouds derived from the new lava flow deposits apparently produced a significant portion of this emission rate. Ground deformation measurements indicated that the volcano deflated slightly following the 1 March ash ejections.
Lava emission with sporadic episodes of minor ash puffs dominated the eruptive activity on 3 March. This relatively quiet state was reflected in the low-level but significant seismicity comprised by short-duration harmonic tremors and some discrete low-frequency volcanic earthquakes. Thick clouds covered the summit area, but below the cloud line and on the middle and lower slopes of the volcano ash clouds and steam emanated from the new lava flows and pyroclastic-flow deposits. A high emission rate of 8,900 t/d SO2 was measured by COSPEC. Much of the ash and steam clouds resulting from this degassing drifted to the W and SW sections of the volcano due to prevailing winds. The haze produced by fine ash suspended in the air temporarily precluded ground deformation measurements.
Potential exists for hot lahar flows due to the presence of highly erodible pyroclastic deposits, which may be remobilized during heavy rainfall. Gullies with confirmed pyroclastic-flow deposits in their headwaters, which may therefore be sites for future lahars, are the Mabinit and Matanag river channels in Legaspi City, Miisi channel in Daraga, Basud-Lidong channel in Sto. Domingo, San Vicente and Buang channels in Tabaco, and the Bulawan channel in Malilipot.
Short-duration harmonic tremors and low-frequency volcanic earthquakes continued on 4 March. This type of seismicity indicated that eruptive activity was limited to quiet lava emission. Ground deformation measurements showed that the volcano was still inflated in its lower portion, while the SO2 emission rate was determined to be at a minimum of 12,100 t/d. Preliminary estimates of the volume of deposits emplaced by the eruptions yielded at least 40 million cubic meters of lava flow and pyroclastic flow deposits. Lava flow deposits account for the major proportion of this estimate.
Activity for the next day was mainly characterized by gentle outpouring of lava. During cloudbreaks the night of 5-6 March, intense glow from the crater and from some portions of the advancing lava flow along the upper and middle Bonga gully were evident. Rockfalls and minor collapses along the length of the flow contributed to some localized ash and steam emission. However, the majority of the thick volcanic plume came from the summit crater which emitted about 8,300 t/d of SO2. The PHIVOLCS seismic network continued to record short-duration harmonic tremors and low-frequency volcanic earthquakes. Ground deformation measurements showed some slight inflation of the volcano on the lower NW flank. The very high sulfur dioxide emission rate, occurrence of tremor and volcanic earthquakes associated with magma ascent, and slight swelling of the Mayon edifice indicate that some ascent of magma is still ongoing. Due to cessation of explosive eruptions, the sky W and SW of the volcano was generally clear of ash.
During 6 March the volcano exhibited quiet lava effusion accompanied by intense crater glow and rolling incandescent materials along the upper and middle reaches of the Bonga Gully. Moderate to strong emission of steam drifted generally to the N from the summit crater. The high steam output also yielded an elevated SO2 emission rate of at least 8,800 t/d. Seismic activity consisted of 11 low-frequency volcanic earthquakes and 25 episodes of short-duration tremors. Slight inflation of the lower NW flank of the volcano continued.
At 0746 on 7 March, a parallel collapse of the new lava flow deposit in the upper middle slopes produced a voluminous secondary pyroclastic flow. The billowing ash cloud descended the Bonga Gully to the SE.
The seismic network recorded low-frequency volcanic earthquakes and short-duration harmonic tremors on 7 March. The measured SO2 gas emission rate of 3,900 t/d, although low compared to recent measurements, was still well above the volcano's baseline level. Likewise, ground deformation surveys showed that the edifice was slightly inflated. At night on 7-8 March, when the volcano's summit area was visible, intense crater glow continued.
A PHIVOLCS report on the morning of 9 March noted that since the last eruption of 1 March, a waning trend in Mayon's overall activity has been evident. The number of volcanic earthquakes decreased and remained at unremarkable levels. In addition, tremor associated with emission of lava from the crater ceased. Seismic activity only reflected sporadic surface disturbances such as occasional rockfalls caused by oversteepened slopes. The Electronic Distance Meter (EDM) and precise leveling surveys also showed a return to the baseline levels, indicating a probable deflation of the edifice. Mayon continued to vent a large amount of steam, but the SO2 component measured by COSPEC had decreased. Although the summit and isolated spots on the new lava flow deposits continued to glow at night, this incandescence was attributed to residual heat.
In view of these recent developments at Mayon, PHIVOLCS lowered the volcano status to Alert Level 4. On 9 March the 8-km-radius extended danger zone in the SE quadrant was reduced to 7 km. PHIVOLCS emphasized that the 6-km radius Permanent Danger Zones should remain evacuated at all times because of instability of new pyroclastic and lava deposits that may be dislodged towards the lower slopes with resultant secondary explosions and life-threatening secondary pyroclastic flows.
Geologic Background. Beautifully symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the Philippines' most active volcano. The structurally simple edifice has steep upper slopes averaging 35-40 degrees that are capped by a small summit crater. Historical eruptions date back to 1616 and 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 devastated 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/).