The history and dynamics of a welded pyroclastic dam and its failure
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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 109–1010 Pa s during welding. This prolific production of hot lava and block and ash flows, in a steep mountainous terrain, created a ∼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 km3. 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.
KeywordsBlock and ash flows Volcanic dams Welding Dam failure Lahar
Research costs were met through support from NSF CREST award 1137774 and a CSU Bakersfield RCU award to GDMA, an NSERC Discovery Grant to JKR, and a Postgraduate (PGS-A) scholarship to MS. Thoughtful reviews and comments from Vernon Manville, Nancy Riggs, Brittany Brand, Adrian Pittari, and an anonymous reviewer greatly improved the clarity of our arguments and the manuscript in general.
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