Transcript Chapter 8: Major Elements

Volcanic Hazards
50-60 volcanoes erupt each year!
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2-3 in USA
Usually in unpopulated areas, but not always:
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Japan
Philippines
Indonesia
Italy
NW USA
Volcanic Hazards
Plate tectonics and volcanism
Hot spots can be either oceanic (Hawaii) or continental (CRB, Yellowstone)
Volcanism
Mid-ocean ridges
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Basalts from partial melting of the mantle
Volcanism
Mid-ocean ridges
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Iceland is the only major threatened area (hot spot?)
75 o
Reykjanes
Ridge
60 o
Eurasian
Plate
Juan de Fuca
and Gorda Ridges
Eurasian
Plate
Walla Walla
t
30 o
Pacific
Plate
African
Plate
Galapagos Spreading Center
0o
Indian
Plate
Nazca
Plate
American
Plate
30 o
60 o
Central Indian
Ocean Ridge
South East Indian
Ocean Ridge
S
Antarctic Plate
75
o
Volcanism
Hot Spots
Ocean islands: Hawaii, Azores, Fiji, Canaries...
 Continental hot spots: CRB, Yellowstone...
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75
Iceland
60
Cobb
Azores
Yellowstone
Madeira
Bermuda
30
Baja
Pacific
Plate
Hawaii
Caroline
0
Samoa
Cocos
30
Eurasian
Plate
Bowie
Eurasian
Plate
Indian
Plate
Marquesas
Society
SE
Australia
Galapagos
Easter
Pitcairn
Tubai
G. Meteor
Cape Verde
Canary Tibesti
Darfur
Hoggar
Fernando
Cameroon
American
Plate
Ascension
Trindade St Helena
San Felix
Juan Fernandez
African
Plate
Tristan
Ethiopia
Comoro
E. Africa
Vema
Gough
Cape
Reunion
Crozet
Kerguelen
Bouvet
60
Antarctic Plate
75
Volcanism
Hot Spots
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Plates move as hot spots remain ~ stationary
Hawaii:
Oceanic Hot spot
plumes  basalt
Low viscosity
Shield Volcano
Volcanism
Hot Spots
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Plates move as hot spots remain ~ stationary
CRB - SRP - Yellowstone:
WA
MT
Walla Walla
CRBG
ID
Present
OR
Yellowstone
WY
SRP
CA
NV
UT
Volcanism
Ridge and Hot Spot volcanism is
predominantly basaltic
Basalts are low viscosity melts
 Gases escape easily and eruptions are not explosive
 Most common hazards are flows and ash deposits
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Subduction Zone Volcanism
Melting of slab and/or dehydration  melting of
mantle wedge above the slab
 Island Arc: Japan, Aleutians, Indonesia, Caribbean...
 Continental Arc: Cascades, Andes
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Subduction Zone Volcanism
Basalt - andesite - dacite - rhyolite
 Each is progressively more viscous and explosive
 Strato-volcanoes (composite volcanoes)
 Flows and pyroclastics
 Far more dangerous due to explosive nature
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Subduction Zone Volcanism
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Volcanism in the NW USA
Continental Subduction Zone (and some hotspot) Volcanism
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Real biggies: rhyolite caldera eruptions
 Long Valley, Yellowstone, Vallez in New Mexico
 Melt base of silica-rich continental crust
 Subduction related or hot spot? Behind SZ proper
 No historic eruptions (thank goodness!)
Lassen Peak is a rhyolitic dome
Hydrothermal activity: hot springs, geysers 
geothermal energy
Volcanic Hazards
Flows
Mostly basaltic
 Hawaii: ~ 1 m/hr
 CRBs faster
 Run through property, but easy to evacuate in
advance, so little threat to life
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Basalt flows in
Kalapana area,
Hawaii
The Columbia River Basalts and Oregon High Lava Plateau are
dominated by lava flows. During the peak of CRB activity (1618 million years ago) some massive individual flows may have
exceeded 2000 km3 or even 3000 km3, which would qualify
them as the largest known terrestrial lava flows
Volcanic Hazards
Flows
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Control?
 Spray water and chill (Heimaey & Mt. Helgafell:
modest success... fortuitous?)
 Construct walls and diversions
 Bombing? Block established channelway
 Nothing will stop a large flow
Volcanic Hazards
Pyroclastics
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Basalts in ridge-hot spot
 Ash of minor importance
 Can be locally destructive, however
Wind-blown ash deposits
Heimay, Iceland 1973
Pyroclastics
Subduction-related eruptions
Pyroclastics much more important and damaging
1. Vertical ash columns (“Plinian” eruptions)
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Vertical plinian ash
column from Mt. St.
Helens, 1980
Mount Pinatubo,
Philippines, on
June 12, 1991
Pyroclastics
Subduction-related eruptions
Pyroclastics much more important and damaging
1. Vertical ash columns (Plinian eruptions)
 Ash fall blanket
 Huge area, but more of a nuisance at a distance
 Destroys vegetation
 Contaminate surface water
 Collapse roofs
 Can be thick and noxious close by (Pompeii)
 Also hot
 Pompeiian body postures suggest heat
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Pyroclastics
Martinique after Nuée Ardente from Mont Pelée, 1902
Subduction-related eruptions
2. Pyroclastic flows
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Mt. St.
Helens
Aug. 7,
1980
Travel at several hundred km/hr and  800oC or
higher
A. Gravitational collapse of ash column 
B. Lateral blast (Mt St Helens & others)
C. Caldera overflow
Any may  nuée ardente (glowing cloud) 
ignimbrite
 Hug ground as base surge, fluidized by gas
content
Huge Rhyolitic Eruptions
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Long Valley
Mono Craters
Mammoth Mtn
Long Valley caldera along the east side of the Sierra Nevada in east-central California
Many eruptions in last 3 Ma incl. massive, caldera-forming eruption 760,000 years ago
Most recent eruptions occurred just 500 to 600 years ago from vents along the Mono
Crater-Inyo crater volcanic chain, Mono Lake to western part of Long Valley caldera
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Long Valley
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Earthquakes began to increase in 1978 & 1st swarms in 80's
Related to magma chamber 8 km below Long Valley
32 x 15 km caldera from eruption only 760,000 yrs ago
4 large earthquakes in May 1980
Resurvey  "resurgent dome" had risen 25 cm since 1975
By 1982 had risen another 7 cm & caldera up 18-20 cm
Swarm = 1) magma motion? (most were harmonic)
2) stress from deeper magma source pushing all
Swarms  shallower by mid May, 1982
 steam vents
 formal notice of volcanic hazard
 later found not a strict progression of shallowing e'quakes
& latest May ones, although shallowest, were over a
broad area
Huge Rhyolitic Eruptions
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Long Valley
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Why concern??
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760,000 yr old eruption was HUGE
Bishop ash fall was hundreds of meters thick locally &
welded
Ignimbrite = Bishop Tuff
1 meter thick over 75 km radius & 1 cm at Kansas
City!!
Huge Rhyolitic Eruptions
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Yellowstone
Huge Rhyolitic Eruptions
Poisonous Gases
Mostly steam, but also
CO2, CO, SO2, H2S
 Rarely a problem since
diffuses
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Sulfur dioxide from Mt. Pinatubo, 1991
Japan has gas monitors and sirens for some
volcanoes
 Lake Nios, Cameroon, 1986
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Lake Nios, Cameroon, 1986
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Slow release by
dormant volcano 
bottom of lake in
crater
Suddenly overturned
and released CO2
which is heavy and
hugged the ground
suffocating ~2000
Mudflows and debris (coarser) flows: lahars (Java)
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Ash & other tephra become water saturated
The premier threat of NW volcanoes: so much snow
and ice to melt
 They can be huge (Redoubt 1990  flow with
discharge ~ Mississippi at flood)
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Mudflows and debris (coarser) flows: lahars (Java)
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Osceola (5000 yrs) and Electron mudflows from Mt
Rainier
Hundreds of
thousands of people
live in threatened
valley bottoms
Little warning
(unlike floods)
Predicting Volcanic Activity
Some general patterns, but usually pattern for one
volcano not so good for another
 Seismic activity
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earthquakes are earliest warning
harmonic tremors = magma motion
may die to quiescent or may be calm before storm
Geophysical Monitoring
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shallow chamber usually fills prior to eruption  surface
swelling and tilt
heat: IR images
magnetic fields hot springs and hydrologic activity
Predicting Volcanic Activity
Tilt Record of Kilauea, Hawaii
Predicting Volcanic Activity
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Gases
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Changes in gas geochemistry (relative amounts of steam,
CO2, SO2, etc)
Worked at Mt St. Helens and Pinatubo (use at MSH
predicted 13 eruptions)
Geologic History
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Mapping of volcanic rocks and deposits characterize
common eruption types
Example: > 90% of Kilauea surface covered by lava in
last 1500 yrs.
 If had known, may never have built Kalapana
Best for long-range (of course)
Volcanic Hazard Warnings
Restricted-Access, Mt. St.
Helens, 1980
Hazard Zones on Montserrat