SPRING 2015

GEOLOGY 285:INTRO. PETROLOGY

Dr. Helen LangDept. of Geology & GeographyWest Virginia University

The Cascade Volcanoes

are a good example of the Circum-Pacific “ring of fire” of subduction-

related andesitic volcanoes

Calc-alkaline Magmas• Subalkaline• 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 suites• Much more likely to have explosive eruptions

than Hawaiian volcanoes

AFM Galapagos (tholeiitic)vs. Cascades (calc-alkaline)

Cascades and Galapagos Molar AFM

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Complex Tectonics of Western North America

Cascades Tectonic Setting

Young Juan de Fuca Plate is being subducted beneath North America

Called Strato-volcanoes or Composite Volcanoes

• Steep-sided (slopes up to 36o)• Typically explosive, violent eruptions• Composed of lava flows interlayered with

pyroclastic material (“composite”)• Pyroclastic material - any volcanic material

that is ejected from volcanic vents as loose or fragmental material

• Only 1/100th of the volume of a large shield!!

Mount Rainier

Prehistoric Osceola mudflow (5,600 years b.p.) from Rainier reached Tacoma WA (45 miles away)

Little Tahoma Peak

1963 debris avalanche

Mount Hood

USGS - 1985

Eroded Cascade Volcanoes - Mt. Washington and Three-fingered Jack

Eroded Volcanoes like Three-fingered Jack show the Composite, Stratiform nature of Cascade volcanoes

“Sherpa Jeffey” - 2003

South Sister (of the Three Sisters) with Obsidian Flow

Once upon a time there was a big volcano called Mount Mazama in southern Oregon . . .

~7700 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 ~700 yrs ago

Climactic Ash Flows 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; darker, more mafic material from the bottom of the chamber is on top.

Back to the Cascades Map

Mount Shasta

Crater Glass Flow -rhyolite obsidian dome

Black Butte -basaltic cinder cone

Note great variety of volcanic rock types in a single Cascade volcano

Mount St. Helens 1980 Eruption• Small earthquakes• Small steam and phreatic ash eruptions in

March and April 1980• USGS monitoring station set up north of the

mountain on Coldwater Ridge• May 1980 - North side of mountain began

to bulge, many micro-earthquakes, eruption was imminent; “red-zone” closed

• See USGS Professional Paper 1250 (1981) for much more information and pictures

8:32 AM 5/18/80 Magnitude 5.1 earthquake triggered the Big Eruption

• The unstable bulge on the North side collapsed and exposed magma in a shallow chamber to air

• Gas bubbles formed instantaneously in the magma causing expansion, a shock wave and a big ash eruption

• Most of the force of the eruption was directed horizontally to the North

• Only 57 people died in the eruption

“Vancouver! Vancouver! This is it . . .” David Johnston

Photo of Dave at Coldwater II, May 17

Gary Rosenquist photos made reconstruction of the eruption sequence possible

Photos taken from a ridge about 10 miles NE of the summit

Rosenquist 4 and 5

Rosenquist 6 & 8

Rosenquist 10

Reconstructed Eruption Sequence

After the first few minutes, the ash erupted upward and drifted ENE with the prevailing winds (Plinian eruption)

Effects of May 18, 1980 eruption

National Geographic,January 1981, specialMt. St. Helens issue

Ash flows

Aug. 7, 1980 ash flow(USGS)

Aug. 7, 1980 deposit on Aug. 8 (HL)

Dome growth may eventually fill the crater

1986

The dome - 1983

2005October 2004-2008 eruptions, first inmore than a decade, expanded the dome

2004

Dome growth may eventually fill the crater

The dome - 1983

The most recent activity was 2004-2008

Before and After May 18, 1980

National Geographic, 1981

We think Mount St. Helens 1980 was a big deal

By comparison to other eruptions of subduction-related volcanoes, it was tiny!

1816 “the year without a summer” Pinatubo 1991

~5 cubic km

National Geographic>1000 km3 = Supervolcano (Elements, 2008)

Volcanic Hazards

• Directed blast• Hot ash flows, lava flows• Airfall ash (threatens airplanes)• Mudflows = lahars

– Mixture of melted ice, debris, water, and ash– Threaten people, cities and towns far away– Lahars from Mt. Rainier could threaten Seattle

and/or Tacoma

USGS-USAID - Volcano Disaster Assistance Program (VDAP)

• http://vulcan.wr.usgs.gov/Vdap/description_vdap.html• Established after volcanic disaster in Columbia

(Armero) in 1985 (23,000 killed in mudflow)• VDAP is a mobile, well equipped team of

experienced volcanologists who can respond whenever a volcano crisis threatens anywhere in the world

• VDAP monitoring and successful prediction of 1991 eruption at Pinatubo saved thousands of lives

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