1. Cascade Volcanism Is Back
The High Cascades
Cascade Volcanism Is Back

2. I. The New Cascade Arc The Shutdown ~16-18 Ma 
Western Cascade Group volcanism stopped
CRB’s start up
Why?

3. I. The New Cascade Arc B. Cascade subduction renewed
Juan de Fuca plate (old Farallon) subducts
~2-3 Ma
Juan de Fuca Ridge offshore

5. I. The New Cascade Arc C. Evidence for subduction?
Volcanoes & volcanic rocks
Yakima Fold & Thrust Belt

6. 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!

8. 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!

9. 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

10. I. The New Cascade Arc most flows & volcanoes <700,000 years old
deep erosion by glaciers 
most Cascade volcanoes 700,000-15,000 years

11. II. High Cascades Group Snow Cones
series of isolated composite cones  stratovolcanoes
defined alignment  N-S

13. The Snow Cones so prominent because they are built on the old WCG
also continued uplift of modern range

14. II. High Cascades Group volcanic histories of each 
low viscosity to high viscosity 
become more felsic with younger eruptions
quite variable eruptive materials 
tephra/ash – lava flows – lahars
olcanoes_cascade_range.html

15. II. High Cascades Group B. Present Day Tectonic Setting
continental volcanic arc
subduction of oceanic crust
Juan de Fuca & Gorda plates

18. II. High Cascades Group C. Hazards: historic & future
Volcanic eruptions
lava-steam-ash-pyroclastics
Nuee ardents (pyroclastic flows)
Example: Mount Mazama ~6950 BP

19. Mt. Mazama
~6950 years ago
Large eruption
Caldera collapse

22. Hazards Example: Mt. St. Helens youngest of High Cascades
most active last 4000 years

24. Hazards 2. Lahars  volcanically related mudflows
melting glacial ice or rainfall
Example: Mt. Rainier

25. Mt. Rainier Osceola flow ~5000 years ago Eruption – crater collapse
HUGE lahar flow 
all the way to Kent & south Puget Sound
~70 km

26. Mt. Rainier
Electron flow
~600 years ago
Puyallup - Summner
50-60 km