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Download Lecture 15: The High Cascades
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The High Cascades Cascade Volcanism Is Back I. The New Cascade Arc A. The Shutdown • ~16-18 Ma • Western Cascade Group volcanism stopped • CRB’s start up • Why? I. The New Cascade Arc B. Cascade subduction renewed • Juan de Fuca plate (old Farallon) subducts • ~2-3 Ma • Juan de Fuca Ridge offshore I. The New Cascade Arc C. Evidence for subduction? 1. Volcanoes & volcanic rocks 2. Yakima Fold & Thrust Belt Yakima Fold & Thrust Belt • series of ridges in central & south central Washington • Rattlesnake Hills, Saddle Mtns., Horse Heaven Hills, Umtanum Ridge, Manastash Ridge • series of broad folds (anticlines & synclines) • and thrust faults • post basalts! Yakima Fold Belt • folding & thrusting after the CRB’s • uplift ~3000 ft. • So! What does that have to do with the price of rice? • Compressional stresses due to ? • plate convergence! I. The New Cascade Arc D. Quaternary Eruptions • began in Oregon & southern Washington • dominated by dark andesites & basalts • evolved into more typical andesites • oldest ? 2-3 Ma I. The New Cascade Arc • most flows & volcanoes <700,000 years old • deep erosion by glaciers • most Cascade volcanoes 700,000-15,000 years II. High Cascades Group A. Snow Cones • series of isolated composite cones stratovolcanoes • defined alignment N-S The Snow Cones • so prominent because they are built on the old WCG • also continued uplift of modern range II. High Cascades Group • volcanic histories of each • http://vulcan.wr.usgs.gov/home.html • low viscosity to high viscosity • become more felsic with younger eruptions • quite variable eruptive materials • tephra/ash – lava flows – lahars • http://vulcan.wr.usgs.gov/Volcanoes/Cascades/v olcanoes_cascade_range.html II. High Cascades Group B. Present Day Tectonic Setting • continental volcanic arc • subduction of oceanic crust • Juan de Fuca & Gorda plates II. High Cascades Group C. Hazards: historic & future 1. Volcanic eruptions • lava-steam-ash-pyroclastics • Nuee ardents (pyroclastic flows) Example: Mount Mazama ~6950 BP Mt. Mazama • ~6950 years ago • Large eruption • Caldera collapse Hazards Example: Mt. St. Helens • youngest of High Cascades • most active last 4000 years Hazards 2. Lahars • volcanically related mudflows • melting glacial ice or rainfall Example: Mt. Rainier Mt. Rainier Osceola flow • ~5000 years ago • Eruption – crater collapse • HUGE lahar flow • all the way to Kent & south Puget Sound • ~70 km Mt. Rainier Electron flow • ~600 years ago • Puyallup - Summner • 50-60 km
