What mainly controls eruptive style? Viscosity in magma 2. Eruptive

Introduction to volcanoes, volcanic
eruptions, and volcanic landforms
Previous: Magmatic differentiation, Bowens Reaction series
(mineral melting points), mafic vs. felsic (fluid vs. sticky)
Video: “Lava Flows and Tubes” [basalt! A fluid mafic lava]
Today:
What are the eruptive processes?
What are the resulting volcanic landforms?
Why are there different kinds of volcanoes?
• Minerals have different melting points
• Minerals can vary in Fe, Mg, or SiO2 content
• Differentiation (magma evolves as it moves
through the crust via fractional xtl (partial melt)
• Result: Volcano types have tectonic context
• TODAY: basic volcano behavior and landforms
What mainly controls eruptive style?
• Gas content of magma
• Viscosity of magma
• http://dsc.discovery.com/convergence/pomp
eii/interactive/interactive.html
Viscosity in magma
2. Eruptive style:
Explosive or effusive?
• High viscosity magma = high silica content
= more explosive
EX: RHYOLITE
• Low viscosity magma = low silica content
=less explosive (effusive)
EX: BASALT
How would this affect volcanic hazards?
Felsic (high SiO2) = high viscosity
and more explosive (e.g. dacite)
Mafic (low SiO2) = low viscosity
and effusive (e.g. basalt)
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Explosive eruption:
pyroclastic flow
Unzen Volcano, Japan, 1991
Where do volcanoes erupt—and how does
this relate to plate tectonics?
types of volcanoes and volcanic landforms
Shield volcano ex: Mauna Loa; Larch Mtn, OR
Shield Volcano (typically
basaltic!)
Composite volcano (stratovolcano) ex: Vesuvius,
Mount Rainier, Popo, Mount Fuji
Dome
ex. Goat Mountain, Black Buttes & Mt. Shasta (CA)
Cinder cones and spatter cones
ex: Paracutin, Puu O
(Kilauea), Sunset Crater (AZ)
Maar volcano
Caldera
Below: aerial view of Hawaii
ex: Battleground Lake, Ubehebe (CA)
ex: Yellowstone, Toba, Crater Lake, Long Valley
Flood basalt
ex: CRBs; Deccan Traps, Siberian Traps
Kilauea’s summit caldera, southwest and east rift zones that
erupt most of the lava flows, and major fault zones
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Lower viscosity basaltic lava
(mafic) is ~45% to 54% silica
Shield
Volcanoes
Basaltic volcanism in the Cascade Range
Effusive eruptions - chiefly lava
flows.
<= Tumac Mtn shield,
NE of White Pass;
view to East
Clayton (1983) estimated
the age of Tumac Mtn at
30 to 20 ka.
Builds shield volcanoes --gentle
relief + cinder cones.
ka = kiloannum
Spray Park and Mount Rainier
view to the east; 1903 photo by
W.P. Romans
Kupaianaha lava pond, Kilauea: photo by Pat Pringle, July 1987
Mount Adams composite volcano is
constructed on top of a broad shield
Larch Mtn shield, OR
Underwood Mtn
shield, WA
Composite volcanoes
(aka stratovolcanoes)
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Related to subduction zones
~55–65 % silica (intermediate viscosity)
Moderately violent explosive behavior
~50/50 ratio of lavas/ fragmental deposits
Thick lavas (to 20 or even 100 m)
Steep cones; can have domes (implications?)
Lahars! (volcanic debris flows)
Composite volcano
Mount Hood
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Composite Volcano
Where are Composite
volcanoes?
aka stratovolcano
• Ring of Fire (Circumpacific belt)
• Mediterranean belt
Photo by Pat Pringle
May 18, 1980 Plinian
eruption column Looking
NNW
Photo by Bob Krimmel,
USGS
Mount St. Helens =>
changed the way we
think about composite
volcanoes!
Mount Rainier from
Glacier View Wilderness
Mount Baker
At composite volcanoes, glacial erosion can increase
relief and supply ground-water that contributes to
hydrothermal alteration and weakening of the cone.
Fig. 7-15, p.157
Fig. 7-16, p.158
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Martha Sabel, USGS,
1982
Incised canyon on the north flank of Mount
St. Helens near north end of 1980 Crater
floor; note great percentage of fragmental
debris! View looks NNE; photo by Pat
Pringle, September, 1982
Fig. 7-18, p.160
Lava domes
Summit dome dacite of Kalama age
(post AD 1479)
Photo by Pat Pringle
Basalt of Castle
Creek age
(~1900 yr B.P.)
basaltic dike
Crater walls of Mount St.
Helens; photos by Pat Pringle; above
Note person for
scale
from south crater rim; right, from top of
Lava Dome.)
Dacite dome rock of Pine
Creek age (~2500 yr B.P.)
Mount St. Helens hot dome avalanche of May 9, 1986
Mount Shasta, a composite volcano, and Shastina
Mount Shasta’s summit
consists of 4 domes
Domes
Lidar images of
crater floor, Sept.
2003 and Sept.
2004
Black Buttes
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MSH04_dome_from_sugarbowl_10-10_to_11-21-04
Aerial view of
Spiral Butte
dacite dome at
White Pass
Clayton ~ 30 – 20 ka;
Tom Sisson of USGS
estimates its age at
~100 ka
Cinder cones
Typically basaltic
Paricutin,
Mexico,
1941
Calderas
Mount Katmai caldera
lake, below
Aniakchak caldera, AK
1.2 Ma basalt lava flows from the Uinkaret Plateau in Grand Canyon
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Rhyolitic volcanism and calderas
• ~65 – 76 % Silica (viscous!!!)
Can be very violent and highly explosive
Produce 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
Fig. 7-23, p.163
Fig. 7-32, p.168
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Fig. 7-33, p.168
Fig. 7-36, p.169
Lava Creek ash
from Yellowstone
Caldera, 1,000 km3
Fig. 7-37, p.170
Flood Basalts
Flood Basalts (aka Plateau Basalts)
Fissure eruptions of basalt—Earth’s largest lava flows
CRB surface exposures ~175,000 km2 & extend 400-600 km!
Exs: Columbia River Basalt Gp, Deccan Traps, Parana Brazil
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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
Volcanic necks—Beacon Rock in
Columbia River Gorge
Maar volcanoes-created by
phreatic eruptions
Wind Mountain
shallow intrusive body
in the Columbia Gorge
(~6 Ma).
Volcano Hazards
•Proximal areas (up to 15 mi)
subject to multiple hazards
•Lahars and floods affect
valleys for many tens of miles
•Tephra fall affects areas tens
to hundreds of miles, or
more, downwind
•Excess sediment in
watersheds makes channels
unstable for years to decades
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Learning objectives
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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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