Mt Unzen Japan 1991 Dormant for 200 years
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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 km 2) 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=gewm. Ua. R 5 s. Qo 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 1 My – 3 in North America (Yellowstone, WY. and Long Valley, CA. ) » Classified as resurgent • Produce large amounts of pyroclastic debris (1, 000 km 3)
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
Pyroclastic Hazard A nueé ardente: Mt. St. Helens
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 – H 2 O most abundant gas emitted – CO 2 next – About 90% of emitted gases • Hazardous gases seldom reach population centers – CO 2 (hazardous) • Lake Nyos, Cameroon, West Africa • Killed 1, 700 people and 3, 000 cattle • Other gases – Most in small quantities • SO 2, NOx, HF, H 2 S – 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/km 2 • Herculaneum (79 A. D); 10 m 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
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