• Eruptions of Mt. Ranier in the holocene have procduecd “lahars” that reach Seattle. • Volcano erupts, melts these glaciers, water flows down the steep sides and carries debris with it and form what are called lahars o Lahars: Flows of mud etc. Anything that can be swept away with water o Holocene: the period since the last glaciation • Volcanic debris flow—wetted debris that moves downhill o Occur where volcanoes are covered with ice and snow or drenched in abundant rain o Volcanic debris flows move downslope like wet concrete. o Lahar—water-rich debris flow of ash and blocks Can move very fast (~50 km/hour) and very far (~tens of km). • Threat of falling ash and lapili (size of tephra) • Lapili: pea sized fragments • Tephra: fragments thrown into the air o Floodwaters easily move tephra and other debris as deadly lahars Lahars—mudflows result when water moves ash. o Like concrete, this material is more dense than water. o It can carry away everything (people, houses, bridges).
o Nevado del Ruiz, Colombia, buried the town of Armero in 5 meters of ash and mud, killing over 25,000 people. __________________________________________________________________________________ 13. Major explosive super volcanoes of the past million years?
•Toba (Indonesia), 73,000 B.C.E. (2,800km3)Not supervolcano: Mount St. Helens – erupted May 18, 1980, at 8:32 amHigh-viscosity felsic magma erupts explosively.Yield huge volumes of ash that can cover large regionsPyroclastic flow—volcanic ash and debris avalancheoRaces down the volcanic slope as a density currentoOften deadlyAn initial vertical blast led to a much stronger lateral blastLateral blast tore off entire north side of the mountainAbout 440m blasted awayAn earthquake-triggered landslide released pressureThe eruption immediately followed a 3-km3slope failure.Slide material moved more than 20 km from the peak.Devastated 600km2 of land and killed 61 peopleLahars plugged the Toutle River; closed the Columbia.Ash fell in North Dakota; highways and rail lines stopped.Destroyed timber was valued at several hundred million dollarsPyroclastic flows are often augmented by glacial melt water debris flows that can travel many kilometres from the volcano.
- Fall '09