Transcript Volcanic Activity

Mt. Unzen, Japan, 1991
• Dormant for 200 years
• Woke up in 1990; erupted into
1992
• Lots of pyroclastics
• Killed the Krafts and Harry
Glicken, a survivor of Mt. St.
Helens
• Site of volcano warning system
1991 - 43 scientists and journalists
were killed by a three-mile-long
pyroclastic flow, a fast-moving
river of hot gas and rock that can
speed along at speeds up to 450
miles per hour.
Introduction to Volcanic Hazards
• 50-60 erupt each year
– 3-4 in the US (mostly in Alaska)
– Many potentially active in
northwestern US and Alaska
• Often at remote locations
• Sometimes near population
centers
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Japan
Philippines,
Mexico
Indonesia
Introduction to Volcanic Hazards
Introduction to Volcanic Hazards
Locations of Volcanism 1: Plate Boundaries
• Mostly along plate boundaries;
80% in the "ring of fire“.
• Molten rock, including small
components of dissolved
gases, produced where
lithospheric plates interact with
other earth materials is called
MAGMA
• Lava- magma from a volcano
Typically produce composite
volcanoes, whose magma is high in
silica content.
Locations of Volcanism 2: Hot spots
Typically produce shield volcanoes,
whose magma is low in silica content.
Example: Hawaiian Islands
Volcano Types
• Shield volcanoes
– Largest type
• Hawaii
• Iceland
• Indian Ocean Islands
– Gentle slopes (about 10° )
– Among the tallest mountains
– Generally non-explosive eruptions:
low silica basaltic flows
– Some occurrences
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Pyroclastic materials (tephra)
Lava tubes
Calderas
Rift zones (normal faults)
Volcano Types
• Cinder cones
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Much smaller (few km2)
Steeper (>30-35°)
Mostly pyroclastic materials
Easily eroded-poorly
preserved
– Often initial phase
Volcano Types
• Composite/Stratovolcanoes
– Many in NWUS (Rainier, St.
Helens, Hood)
– Andesitic (intermediate)
– Layers of andesite/ pyroclastics
– Slopes ± 30-35°
– May erupt explosively –
considered to be “most
destructive”, due to eruptive
style
Three Types of Volcanoes
Q: What sets the shape of these volcanoes?
A: Magma viscosity, for the most part, which is determined
by silica content and termperature
Mt. St.Helens – a typical composite volcano
http://www.youtube.com/watch?v=gewmUaR5sQo
After
Before
Composite Volcanoes commonly
produce Andesites – a silica-rich
igneous rock
Volcano Types
• Volcanic Domes
– Siliceous, viscous magmas
(rhyolite)
– Mt. Lassen, CA
– Mt. St. Helens
Volcanic Origins
• Occurrences
– Mid-oceanic ridges (basalts)
• Shield volcanoes in Iceland
– Shield volcanoes over hot spots (Hawaii)
– Composite volcanoes (subduction zones)
• Andesitic
• Common around Pacific Rim
– Fissure Flows
• Columbia River
• Deccan Basalts, India
• South Africa
Volcanic Origins
– Caldera eruptions
• Extremely explosive and violent
• Rhyolitic magmas
• Volcanic domes
• Craters, Calderas, and Vents
– Craters
• Depressions around the tops of
volcanoes
• Form by explosion or collapse
• May be flat bottomed or funnel shaped
• Much smaller than calderas
Long Valley, Ca
Volcanic Features
– Hot Springs and Geysers
Old Faithful
Crater Lake, Oregon a good
example of a caldera
Volcanic Features
– Calderas
• Large diameter (20+ km)
circular depressions
• Explosive ejection-large scale
collapse
• May contain multiple vents
• None in Recent times
– 10 in the last 1My
– 3 in North America
(Yellowstone, WY. and Long
Valley, CA.)
» Classified as resurgent
• Produce large amounts of
pyroclastic debris (1,000 km3)
Caldera-Forming Eruptuion @
Yellowstones ~600 ka
October 7, 2003
Caldera
Migration
Volcanic Hazards
• Effects
– Primary effects
• Flows
• Pyroclastics
• Release of gases
– Secondary effects
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Debris flows
Mudflows
Floods
Fires
Lava Flows
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Pahoehoe
– Fast moving (m/hr)
– Low viscosity, smooth textured
• Aa
– Slow moving (m/day)
– Blocky, sintered appearance
• Control methods
– Bombing (most successful in Italy)
– Chilling (most successful in Iceland
– Deflection walls (being tried in
Iceland)
– Results have been mixed
Pyroclastic Hazards
• Blowing of tephra into the
atmosphere
• Pyroclastic activity
• Volcanic ash eruptions or ash falls
– Rock fragments
– Volcanic glass
– Gases
• Lateral blasts (Mt. St. Helens)
• Pyroclastic flows or ash flows
– Cloud of rock fragments, glass, and hot
gases flowing rapidly down slope
– Hot avalanche, ignimbrite, nuee
ardentes
Volcanic Hazards
Pyroclastic Hazard
A nueé ardente:
Mt. St. Helens
Pyroclastic Hazards
• Ash Fall
– Cover large areas
• Thousands of square kilometers
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Vegetation
Surface water
Structural damage
Health hazards
Transportation problems
Pyroclastic
Hazards
• Ash Flows
– Travel at high speeds
– Contain very hot materials
– Examples
• Martinique (Pelee, and St. Pierre)
• Mt. Unzen
• Montserrat (Plymouth)
Volcanic Gases
• Gases emitted
– H2O most abundant gas emitted
– CO2 next
– About 90% of emitted gases
• Hazardous gases seldom reach
population centers
– CO2 (hazardous)
• Lake Nyos, Cameroon, West Africa
• Killed 1,700 people and 3,000 cattle
• Other gases
– Most in small quantities
• SO2, NOx, HF, H2S
– May be injected high in the atmosphere
Lake Nyos
Debris Flows
and Mud Flows
• Debris Flows
– 50%+ of the particles are 2 mm or greater
– Snow and ice melted by eruption
– Mt. Redoubt, Alaska flow equals the Mississippi at flood stage
• Mudflows
– 50% of more of particles are smaller than 2 mm
• Examples
– Mt. Rainier's old flows threaten large cities in Washington State
(Tacoma and Seattle)
– Armero, Nevada del Ruiz, Colombia (22,000 dead)
– Potential large "landslides" along the north coast of the Island of
Hawaii may generate large tsunamis
Debris-Mudflow
Hazard Map
Case Histories
• Mount Pinatubo, Philippines
• Mount St. Helens, Washington
• Pompeii-Herculaneum, Italy
Modern Vesuvius
• 3 around Bay of Naples
• Population density about
15,000/km2
• Herculaneum (79 A.D); 10m ash
• 5,000 residents, mostly escaped
Prediction of Volcanic
Activity
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Seismic activity
Geophysical monitoring
Topographic monitoring
Emitted gas monitoring
Geologic history
Volcanic Alert or Warning
Adjustment/ Perception
of Volcanic Hazard