Mt. Unzen, Japan, 1991

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