The history and dynamics of a welded pyroclastic dam and its failure

The 2,360 BP eruption of Mount Meager, British Columbia began as an explosive, dacitic sub-Plinian eruption that waned rapidly to a sustained period of Vulcanian, eruption-triggered dome collapse events producing voluminous block and ash flow (BAF) deposits. The earliest BAF deposits accumulated rapidly enough immediately downslope of the vent to retain heat and weld; using the deposit as a paleoviscometer determines an effective viscosity of 10(9)-10(10) Pa s during welding. This prolific production of hot lava and block and ash flows, in a steep mountainous terrain, created a similar to 110 m high, largely impermeable dam capped by permeable, non-welded BAF deposits and unconsolidated avalanche deposits that blocked the flow of the Lillooet River and created a temporary lake. The welded pyroclastic dam was compromised and overtopped at least once before the peak dam height was reached. Renewed eruption caused buildup of the dam to a maximum of 780 m above sea level (asl) and grew the temporary lake to an elevation of 740 m asl and a minimum volume of 0.55 km(3). The rise of lake level led to catastrophic failure of the top of the dam, generating an outburst flood that carved a canyon through most of the dam and resulted in a voluminous lahar that is traced at least 65 km downstream. Based on current flow rates of the Lillooet River, the lake would have overtopped the final dam at a minimum of 39-65 days after its formation. The peak deluge lasted approximately 8 h and eroded a 2.5-km long canyon into the still-hot dam core before returning to background flow rates.