Lava domes
Photo by Pat Pringle
Nov. 4, 2004 photos by
Jim Vallance, USGS;
looking east
FLIR –forward-looking
infrared radar
Note person for
scale
Mount St. Helens hot dome avalanche of May 9, 1986
Photos by Jim Vallance: right,
oblique view looks SSE
Lidar images of
crater floor, Sept.
2003 and Sept.
2004
above: aerial oblique looks
WSW
MSH04_dome_from_sugarbowl_10-10_to_11-21-04
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Mount Shasta, a composite volcano, and Shastina
Mount Shasta’s summit
consists of 4 domes
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
Black Buttes
Cinder cones
Typically 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
Mount Katmai caldera
lake, below
Aniakchak caldera, AK
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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 !!!!!
Santorini
Volcanoes and
human history:
Santorini volcano on
the island of Thera
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/ori
ginals/WeberToba/ch4 climate/fig4-4.gif
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Yellowstone
Track of the
Yellowstone hot
spot to the NE
Flood Basalts
Aka “plateau
basalts”
Flood Basalts (aka Plateau Basalts)
Fissure eruptions — Earth’s largest lava flows
Tremendous 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.
Entablature
Grande Ronde Basalt of the
Columbia River Basalt Group as
exposed at Table Mountain,
Columbia River Gorge; photo by
Grand Ronde flow GSOC,
1997, SR 410
Pat Pringle
Colonade
contact with Miocene Eagle
Creek Formation—Table Mtn
Columbia River Basalt flows
separated by sedimentary interbeds
Palagonite pillow delta of Pliocene age,
Columbia River Gorge
Longview quarry, 2006
Columbia River Basalt flows
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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?
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