NW Silicic Volcanism
Cascade volcanoes
Cascades are sitting on a base of Basalt
Cascades are sitting in graben
3 episodes of eruption
Western Cascades (35-20 mybp)
Ancestral Cascades (7 mybp)
High Cascades (1 mybp)
Activity
South Sister
Since 1997 uplift of the west flank of S. Sister by as much as 10 inches. (2 in/year)
Probably caused by magma
Depth ~4 miles
Volume ~ 50 million cubic yards
Located by Harmonic tremors
Most recent report (March 2007): slowed growth 1 in/year
Radar interferogram. Measures changes in position. Each full color band (from blue to red) represents uplift of about 2.8 cm. Total uplift has been about 10 cm.
Mt St Helens
Young volcano
Formed about 40,000 years ago
Previous eruption before 1980: 123 yrs ago
Eruption began with a series of earthquake
March 20th
Fractures develop due to quakes
March 24th and 25th
Phreatic eruptions began
March 27th and 28th
Harmonic tremors started in April
Small eruptions through May 14th
North side had been developing a bulge the whole time
May 18th 8:32am eruption began
Started as mag. 5.1 earthquake triggered the slide on North side
Lateral blast followed by upward blast
Debris avalanche from landslide covered 23 square miles with a depth of 150-600 ft. Moved at 70-150 mph
Lateral blast reached 17 miles from crater. Moved at 300 mph with a temp of 600oF
4 bil board feet of timber blown down
Summit was lowered by 1300 ft
Eruption column reached 80,000 ft in less than 15 minutes. Circled earth in 15 days
Pyroclastic flows coved 6 miles2 and reached 5 miles from crater
Multiple flows 3-30 ft thick reaching 1300oF and moving 50-80 mph
Lahars: Flowed down river valleys and damaged 27 bridges, ~200 homes and ~185 miles of hwys. Traveled at 10-25 mph and reduced channel depth from 40ft to 14ft
John Day area
~55-30 mybp
Created by old subduction zone
Primarily ash and lahar deposits
GREAT FOSSIL RECORD!
Newberry
Stratified Magma Chamber
2 possible scenarios
Start erupting felsic and change to mafic
Felsic material is more buoyant so at top of chamber
Mafic material is erupted at end as deep parts of chamber is tapped
Possibly occurred during eruption at Mt St. Helens about 2000 yrs ago
Start erupting mafic and change to felsic
Early eruption controlled by rifting
When conduits are established and subduction zone is near(i.e. Cascadia subduction) begins erupting felsic material
This is what happened at Newberry
Last eruption was Rhyolite and Obsidian 1300 years ago
Other:
Steens Mountain: ~16mybp
Bimodal eruption
Silica rich then basalt
Harney Basin: 9mybp; collapsed caldera
Crater lake: 7000 years; collapsed caldera with rhyolite flows
Volcanic Hazards
Basaltic Volcanism?
Silicic Volcanism?
How can Volcanic eruptions help change the climate?
1815 Tambora Indonesia
year without a summer
NW Volcanic Hazards
Pyroclastic flows
Ash
Lahars (biggest Hazard why?)
Tacoma area
Other Notable Volcanoes
Yellowstone
Krakatoa
Mt Pinatubo
Tambora
http://volcano.und.edu/vwdocs/current_volcs/current.html