The High Cascades Cascade Volcanism Is Back I
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
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