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CHARACTERISTICS AND ORIGIN OF THE
PYROCLASTIC FLOWS AND SURGES OF
THE 1993 MAYON VOLCANO ERUPTION

Sandra G. Catane
and Ma. Hannah T. Mirabueno
Philippine Institute of Volcanology and Seismology, Philippines

 

 

 

 

ABSTRACT


A small, vent-clearing phreatic explosion at the summit of Mayon Volcano on 02 February 1993 sent a pyroclastic flow down a 250-m deep gully on the southeast side of the volcano and killed 77 persons. Slow but sustained intrusion of viscous lava dome at the southeastern edge of the crater floor occurred two days after. lntermittent collapse due to oversteepening of the growing lava dome resulted in the generation of pyroclastic flows for more than a month. Pyroclastic-flow events of similar volume to the 02 February occurred on 12-13 February, between 14 February-20 March, and another on 21 March of the same year. These pyroclastic flows left behind various types of deposits that vary in texture and composition that include blocks and ash, lapilli and ash and fines-depleted breccia.

Pyroclastic flows were observed as ground hugging, outwardly expanding dense clouds with cauliflower shaped vortices. Associated ash-laden vertical column was about 300 m to 1 km high. Flows traveled 4-6 km from the summit at an initial speed of roughly 120-180 km/hr. Pyroclastic flows of 02 February and 12-13 February consisted of two parts, (1) a ground hugging basal avalanche, and (2) an overlying ash-cloud surge. Basal avalanches left behind deposits that were channel confined and formed lobes with well-defined marginal levees. Individual basal avalanche and ash cloud surge deposit are about 5 m and less than 50 cm thick, respectively. Basal avalanche deposits are thick, structureless, non-welded and coarse-grained, consisting of large lithic blocks and ash. The basal avalanches probably flowed in roughly laminar fashion and as poorly expanded high concentration dispersions. Deposits from the ash cloud surges were spread over the ridges and other elevated areas. They are thin, massive beds of ash and lapilli with abundant plant debris. Field evidence and interpretation based on the descriptions by survivors indicate that surges were expanded, low-density ash laden, laterally moving air currents that occur as pulses of flows.

The casualties and widespread devastation occurred along the paths and margins of the ash cloud surges. Laterally directed flows are evident from blown down and splintered trees, ramping of deposits, impact marks, and surmounted ridges. When caught in a surge, one can possibly survive by taking refuge on the lee side of a hill, behind a broad tree trunk, or other stable natural or man-made structures. An emplacement temperature range of 100-300 C was inferred for the 02 February pyroclastic flow, although temperatures of succeeding pyroclastic flows may have been higher.
 

 

 

 

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