1.The life and death of a volcanic dam: the 2360 BP eruption of Mt. Meager, BC Graham Andrews Franklin & Marshall College UBC Collaborators: Kelly Russell, Krista Michol, Martin Stewart

2. Why study volcanic dams? natural dams 101 how do dams fail? when dams fail Mount Meager and the 2360 BP eruptionOutline non-volcanic debris flows Volcaniclastic stratigraphy and dam architecture Welded vs . non-welded porosity compaction Paleo-Salal Lake Lessons for the future? 3. The 2360 BP eruption of Mt. Meager gives us a unique opportunity to explore:This study 1.the volcanic damming (and failure) process, 2.the timescales of damming and failure, 3.the influence of changing properties in volcanic dams, and 4.ways to prepare for and mitigate against similar events in the future. 4. beaver damNatural Dams Natural dams form in 3 different ways: Wikipedia biologicalESA geological glacial Lake Sarez,Tajikistan 5. USGSNatural Dams - IceHubbard glacier, AK In 1986 and 2002 the surging Hubbard Glacier temporarily created a 5 km3 Russell Lake in 24 hours before it burst. The outburst flood generated was 105 m3/s. Lake Missoula floods 107 m3/s 6. Dams typically fail in 1 of 2 ways: overtopping, undermining by seepage.How dams fail1. - Overtoppinggradually but rapidlyerodes the top and thedownstream side of thedam like a knick-pointmigration until thedam fails. e.g., Johnstown Flood, PA,1889 2,200+ fatalitiesWikipedia 7. Dams typically fail in 1 of 2 ways: overtopping, undermining by seepage.How dams fail2. - Undermining removes a large section of the damin one go. It usually follows seepage of water into theporous dam structure or bedrock.e.g., Teton Dam, Fremont and Madison Counties, ID, 1976St. Francis Dam, CA, 1928 450+ fatalities 8. How dams fail 9. dam failureUSGS test excavationsHow dams faildamremnantflooddebrisTeton Dam, ID USGSThick dams are rarely completely destroyed usually most ofthe dam is left and the water escapes out the side of the dam. 10. USGS 11. Dam failure - effects Catastrophic dam failures are devastating to theenvironment downstream.Outburst floods form debris flow depositscommonly contain house-sized clasts of rock / dammaterial weighing 10s of metric tons (10,000s oflbs).13t piece of St. Francis damVolcanic outburst mile from sourcefloods and mudflowsare called lahars. USGS 12. Andrews et al., in prep.Quaternary Garibaldi volcanic belt (GVB)Mount MeagerNorthernmost of 3deeply-eroded felsicstratovolcanoes:-Garibaldi, Cayley,and Meager. 13. Andrews et al., in prep.Quaternary Garibaldi volcanic belt (GVB)Mount MeagerNorthernmost of 3deeply-eroded felsicstratovolcanoes:-Garibaldi, Cayley,and Meager.Part of the Cascadearc and Pacific Ringof Fire. 14. Andrews et al., in prep.Quaternary Garibaldi volcanic belt (GVB)Mount MeagerNorthernmost of 3deeply-eroded felsicstratovolcanoes:-Garibaldi, Cayley,and Meager.Part of the Cascadearc and Pacific Ringof Fire.Prospective forgeothermal andhydroelectricpower. 15. The last eruption formed the extensive Bridge River tephra (14C - 2360Mount Meager BP) the last explosive eruption in Canada. Geological Surveyof Canada 16. The Meager volcaniccomplex is a series ofMount Meager4 edifices built one ontop of the other.Each new edifice isfurther north.The volcano hasexperienced >5 periodsof glaciation.The volcano is highlyunstable. Hickson et al., 1999 17. Meager Peak With over 1800 CapricornPlinth Peak (P) m of relief Mt Peak2677 m Meager is veryMount Meager rugged and steep. The 2360 BPlooking NW vent (V) is in a glacier-filled col ~800 m above the adjacent Lillooet valley floor (1 in 3 gradient).looking SW 18. Mount Meager 19. Andrews et al., in prep. 2360 BP Pebble Creek Formation Pebble Creek Formation 20. Andrews et al., in prep. Pebble Creek Formation misfit stream and canyon 21. Keyhole Canyon & misfit Lillooet Riverslot-canyon =2300 years ofnormal erosionlooking NW - upstream looking SE - downstream 90 m2000 m300 m 22. The 2360 BP eruption went through 3 majorphases:2360 BP eruption 1. sub-Plinian explosive eruption depositeddacite ash across much of British Columbia.Local pumice fall deposits and thin ignimbrites(pyroclastic flows). 23. The 2360 BP eruption went through 3 major phases:2360 BP eruption 1. sub-Plinian explosive eruption deposited dacite ash across muchof British Columbia. Local pumice fall deposits and thinignimbrites (pyroclastic flows). 2. Vulcanian explosions of a hot lava domegenerated welded and non-welded block-and-ash flow deposits. 24. The 2360 BP eruption went through 3 major phases:2360 BP eruption 1. sub-Plinian explosive eruption deposited dacite ash across muchof British Columbia. Local pumice fall deposits and thinignimbrites (pyroclastic flows). 2. Vulcanian explosions of a hot lava dome generated welded andnon-welded block-and-ash flow deposits. 3. Collapse of an extrusive dacite lava flowgenerated more non-welded block-and-ashflow depositsexplosive effusive 25. Montserrat Feb 5th 2010Vulcanian eruptions block &ash flows MVO 26. Vulcanian eruptions 27. 2360 BP eruption Andrews et al., in prep. 28. lake sedimentsPebble Creek Formationblock & ash deposits lahar deposit Andrews et al., in prep. 29. 1 Pebble Creek Formation 2Event stratigraphy downstream upstream Andrews et al., in prep. 3 30. block & ash deposit damVolcanic dam Michol, Russell, Andrews, JVGR 2008 780 masl680 masl 31. Volcanic dam non-welded B&Awelded B&A Michol, Russell, Andrews, JVGR 2008 32. Volcanic damwelded B&A strongly-welded glassy matrix, blocks of dense dacitic obsidian, ~31% compacted, deposited hot (>600 C), HARD like lavaMichol, Russell, Andrews, JVGR 2008 33. Volcanic dam non-welded B&A unconsolidated to weakly-indurated ashy matrix; blocks of dense dacite; deposited cold (