Hummocky topography large volume valley filling

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Hummocky topography Large volume, valley-filling deposit Megablock and matrix faces Blocks are brecciated w/ jigsaw fractures Matrix is a mixture of disintegrated blocks Horseshoe-shaped crater in source region D. What happened in the 1980 Mt. St. Helens eruption? Magma was injected into the north flank of the volcano → bulge was created An earthquake triggered the movement of land in this area Resulted in a fast-moving debris avalanche E. Volcanic history of Mt. Rainier Lavas at the base 90,000-130,000 years old Most of current edifice built within past 40,000 years Current cone rebuilt within landslide scar Mid-19 th century → small tephra 19 th -20 th centuries → small lahars F. Mt. Rainier’s cold debris avalanche Debris flow 5,600 years ago Formed horseshoe-shaped crater G. What is Mt. Rainier’s collapse history? 500 years ago Electron mudflow 5,600 years ago → collapse event (Osceola mudflow) H. Rainier Lahars Age → 500 and 5,000 years ago Extent → Puget Sound lowland Origin → eruption and non-eruptive Hazard → lahar, little warning, little time I. What can we do? Find out whether we live in a high-hazard zone (and think about this when deciding where to live) Keep up with monitoring Plan an escape route (high ground) Find out about disaster assistance plans
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E. What are the hazards of Mt. Rainier? Mountain structure conducive to the landslide generation o Multiple thin flows near summit o Unstable rubble layers between flow o Abundant water and heat = hydrothermal alteration o Hydrothermal alteration focused along dike swarms Mountain conditions conducive to mudflow generation o Melt ice and mix w/ sediment o Pyroclastic flows can very effectively melt ice/snow o Glacial floods/outbursts also a source Hazards and risks o Recurrence interval for giant lahars 500-1000 years o 10% chance of lahar during lifetime o Could predict if triggered by eruption o Difficult to predict if not caused by eruption Interpretation of Osceola deposit as mudflow o Deposited as a flat plain o Valley filling, apparently flowed around hills o Deposit traced up to 10,00-feet level on Rainier Electron flow o 500 years old o Originated in Sunset Amphitheater o 2-hour flow time to Orting 10. Mt. St. Helens Pre-1980 History & Tectonic Setting A. What is MSH’s eruptive history? Intermittently active for at least a 26-year span from 1831-1857 Sporadically active during the 3 decades before 1831, including a major explosive eruption in 1800 Since 1857, viewed as a safe, beautiful mountain B. General information ~50,000-year history (possibly older) Youngest and most active Cascades volcano Current cone formed in the past ~2200 years Last major eruption → 1980-1986 Most recent eruption began October 1, 2004 C. Early activity March 15-21, 1980 o 1975-1980 → only 44 EQ’s o March 15-21 → over 100 EQ’s
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o A magnitude of 4.1 on March 20 indicated possible magma movement March 22-28 o Phreatic (stream-driven) explosion begins o
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