mt. unzen, japan, 1991
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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 – 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
• Pyroclastic materials (tephra)• Lava tubes• Calderas• Rift zones (normal faults)
Volcano Types
• Cinder cones– 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
Before
After
Composite Volcanoes commonly produce Andesites – a silica-rich igneous rock
http://www.youtube.com/watch?v=gewmUaR5sQo
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 • Debris flows• Mudflows • Floods • Fires
Lava Flows
• 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
A nueé ardente:Mt. St. Helens
Pyroclastic Hazard
Pyroclastic Hazards
• Ash Fall – Cover large areas
• Thousands of square kilometers
– 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-MudflowHazard 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
• Seismic activity• Geophysical monitoring• Topographic monitoring• Emitted gas monitoring• Geologic history• Volcanic Alert or Warning
Adjustment/ Perception of Volcanic Hazard