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Geol 285 - Petrology, Dr. Helen M. Lang, West Virginia University, Fall 2005
The Cascade Volcanoes
A good example of the Circum-Pacific "ring of fire" of subduction-related
andesitic volcanoes
Calc-alkaline Volcanism occurs inboard of Subduction Zones at Convergent
Plate Boundaries
Calc-alkaline Magmas
Flat trend on AFM diagram (no Fe-enrichment)
Strongly plagioclase porphyritic
Andesite-dominated strato-volcanoes
Wider variety of rock types (basalt-andesite-dacite-rhyolite suite) than in tholeiitic
Two-thirds of Earth’s Active Volcanoes are along the Pacific "Ring of Fire"
Complex Tectonics of Western North America
Cascades Tectonic Setting
Young Juan de Fuca Plate is being subducted beneath North America – see handout
Called Strato-volcanoes or Composite Volcanoes
Steep-sided (slopes up to 36o)
Typically explosive, violent eruptions
Composed of layers of lava flows interlayered with pyroclastic material
Pyroclastic material - any volcanic material that is ejected from volcanic vents as
loose or fragmental material; includes many specific terms that refer to shapes or
sizes of particles (ash, bombs, pumice, cinders, etc.)
Only 1/100th of the volume of large shield!!
Glacier Peak
Mount Rainier
Mount St. Helens erupted May 18, 1980 - more later
Mount Jefferson
Eroded Cascade Volcanoes - Mt. Washington and Three-fingered Jack
Eroded Volcanoes like Three-fingered Jack show the Composite, Stratiform
nature of Cascade volcanoes
Interlayered Pyroclastics & Lavas
Typical Platy Andesite
South Sister (of the Three Sisters) with Obsidian Flow
Once upon a time there was a big volcano called Mount Mazama in southern
Oregon . . .
6600 years ago it had a HUGE ash eruption, its magma chamber emptied and its top fell
in, leaving the caldera called Crater Lake
Wizard Island Cinder Cone erupted inside the caldera ~700 yrs ago
Ash Flows from the Climactic Eruption filled valleys surrounding the Volcano
Chamber emptied from the top down producing foam-like ash flows, "like a ricepot
boiling over." Notice lighter, more felsic material from the top of the chamber is on the
bottom of the flow; darker, more mafic material from the bottom of the chamber is on
As the ash flows cooled, gases escaped through pipe-like conduits, chemicals
precipitated to harden the rock surrounding the pipes leaving The Pinnacles
Mount Shasta
Obsidian Flows near Mt. Shasta
Note the great variety of volcanic rock types in a single Cascade volcano, very different
from Hawaii