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Mount St. Helens hot dome avalanche of May 9, 1986

Lava domes Photo by Pat Pringle

Note person for scale

Nov. 4, 2004 photos by Jim Vallance, USGS; looking east

FLIR –forward-looking infrared radar

Photos by Jim Vallance: right, oblique view looks SSE

above: aerial oblique looks WSW

Lidar images of crater floor, Sept. 2003 and Sept. 2004

MSH04_dome_from_sugarbowl_10-10_to_11-21-04

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Domes

Black Buttes

Mount Shasta, a composite volcano, and ShastinaMount Shasta’s summit consists of 4 domes

Aerial view of Spiral Butte dacite dome at White Pass

Photo from Geoff Clayton’s thesis; Tom Sisson of USGS estimates its age at ~100 ka

Cinder conesTypically basaltic

Paricutin, Mexico

1.2 Ma basalt lava flows from the Uinkaret Plateau in Grand Canyon

~1900 yr old scoria cone (reddish rocks) exposed in

Mount St. Helens crater wall; geologist Janet Babb in

foreground

Caldera

Aniakchak caldera, AKMount Katmai caldera lake, below

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Calderas – Rhyolitic volcanism

• ~65 – 76 % Silica (rhyolite) viscous!!!)

Violent and highly explosive

Mostly pyroclasts/ extensive ash flows

Significant % of magma erupts producing calderas

Rhyolitic volcanism and calderas• 100 to 1000+ km3 deposit volumes!!!• >138 active in historic times• Examples: Toba, Yellowstone, Campi Flegrei,

Long Valley (CA), Crater Lake, Aniakchak• Largest known (Miocene) La Garita in the San

Juans (CO) => >3000 km3 !!!!!

Volcanoes and human history: Santorini volcano on the island of Thera

Santorini

Santorini

Toba ~74 ka eruption possibly linked to human DNA diversity bottleneck? Humans may have been reduced to <10,000 individuals; tephra volume = 2,800 km3

http://www.andaman.org/BOOK/originals/Weber-Toba/ch4 climate/fig4-4.gif

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Yellowstone Track of the Yellowstone hot spot to the NE

Flood BasaltsAka “plateau basalts”

Flood Basalts (aka Plateau Basalts)Fissure eruptions — Earth’s largest lava flowsTremendous volumes and aerial extent—CRB surface exposure~175,000 km2; flows extend 400-600 km!Ex: Columbia River Basalt Gp, Deccan Traps, Parana Brazil, Siberian Traps

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Features of flows (simplified): pillows and pillow palagonite breccia, colonade, entablature, vesicular top.

Grande Ronde Basalt of the Columbia River Basalt Group as exposed at Table Mountain, Columbia River Gorge; photo by Pat Pringle

contact with Miocene Eagle Creek Formation—Table Mtn

Entablature

Colonade

Grand Ronde flow GSOC, 1997, SR 410

Palagonite pillow delta of Pliocene age, Columbia River Gorge

Longview quarry, 2006

Columbia River Basalt flows

Columbia River Basalt flows separated by sedimentary interbeds

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Sill of Columbia River Basalt on

north coast of Oregon; basalt

invaded marine sediments of the Miocene coastal

plain

Maar volcanoes-created by phreatic eruptions

Volcanic necks—Beacon Rock in Columbia River Gorge

Wind Mountain shallow intrusive body in the Columbia Gorge (~6 Ma).

Learning objectives

• What is a volcano?• Where do we find volcanoes? (Review)• What is the origin of volcanoes?• Why do volcanoes erupt? • Why do some volcanoes erupt explosively and

some non-explosively? • How does eruptive style affect creation of

volcanic landforms and volcanic hazards?