Vent-derived and deposit-derived pyroclastic flows and ignimbrites: Examples at Pinatubo Volcano, Philippines

The climactic phase of the 1991 Mount Pinatubo eruption consisted of plinian activity and large-scale generation of pyroclastic flows, which were coeval for most of the 3.5 hours duration of the climax. The plinian column was maintained above 30 km high throughout this period, forming layer C1, spawning dilute pyroclastic flows fed by partial collapses from the lower buoyant column. About an hour after the onset of the plinian phase, fountaining at the base of the plinian column generated denser pyroclastic flows that flowed down at an average encroachment velocity of 5–6 m/s. Simultaneous plinian tephra and co-ignimbrite ash deposition formed the C2 fall layer.; Both the dilute column-generated and, the denser fountain-generated pyroclastic flows were the sources of the vent-derived ignimbrite. Fan-building, vent-derived pyroclastic flows created thick ignimbrite sequences in the upper valleys, pyroclastic fans, and in lower valley regions by vertical aggradation and lateral accretion of valley-ponded ignimbrites. The vent-derived ignimbrites lost as much as 1.6 km3 bulk volume of its vitric component into the co-ignimbrite ash that fed the umbrella region, fine fraction in layer C2 and ash fallout layers C3 and C4 on top of the plinian fallout and ignimbrite sequences.; Gravitational instability and loose, poorly consolidated nature of newly emplaced ignimbrite fan sequence caused dry-state remobilization that produced secondary or deposit-derived ignimbrites. Deposit-derived flows occurred at Pinatubo for more than 5 years after the June 15, 1991, eruption, leaving an avalanche scarp at their origin. Deposit-derived ignimbrites exhibit almost identical field characteristics to those emplaced by faster and more energetic flows originating from the vent, such as slight fines depletion and non-uniform TRM polarity of coarse clasts. Deposit-derived flows basically differ from other secondary movements in ignimbrite in that they occur long after the deposition of vent-derived ignimbrite.; This study has shown that the Pinatubo ignimbrite represents a continuum of pyroclastic sedimentation that began during the climactic eruption on June 15, 1991 and episodically continued with decreasing frequency for several years after the eruption. Moreover, deposit-derived flows are apparently common mechanism for redistributing ignimbrite after initial deposition and present post-eruption hazards that were previously unrecognized.