volcanoes. volcanic hazards smith ch 7 another tectonic hazard occurrence controlled by plate...
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Volcanoes
Volcanic Hazards
• Smith Ch 7• Another tectonic hazard• Occurrence controlled by plate tectonics
Types of Volcano
• Subduction (edge of continent)• Rift (oceanic)• Hot-Spot (oceanic)
Subduction Volcanoes
• Silica-rich magma, viscous– steep slopes– gassy, explosive, intermittent– ash/lava layered volcanoes
Mt St Helens
• Explosive eruption
Pompeii
Rift Volcanoes
• Mg & Fe-rich magma, basaltic, less viscous– Lava flows important– eruptions more continuous– gently-sloping volcanoes
Hot-Spot Volcanoes
• Like Rift volcanoes• Found on oceanic “hot spots” like Hawaiian
islands
Primary Volcanic Hazards
• Pyroclastic flows• Airfall tephra• Lava• Gases
Pyroclastic Flows
• Dense clouds of hot, glowing solid lava fragments ejected from volcano– “nuee ardente” or glowing cloud
• flows fast downhill – 30m/sec for 30-40 km
• especially dangerous if blast is lateral– responsible for 70% of C20th volcano deaths
Mt Pelee, Martinique, 1902
• Eruption of Mt Pelee• Nuee ardente
– travelled 6 km at 33m/sec (approx. 120 km/h)– temperature: 700 degrees C– Destroyed town of St Pierre– Killed 29,000 people in 3 minutes– 2 survivors in St Pierre
Air-fall Tephra
• Tephra: volcanic dusts and solids– A feature of explosive eruptions especially
• Dust-fall can cover 100s of Kms• Dust can rise into upper atmosphere
– affect climate globally
Mt St Helens 1980
• Ash-fall over 400 km• Night-like darkness in Yakima & Spokane• Traffic disrupted• Vehicle engines clogged• Breathing problems
Mt Pinatubo 1991
• Ash-fall disrupted 500,000 farming people• Ash-fall thick for 30km radius• 1 million affected by ash-fall• significantly reduced global temperatures
1991-2
Pintaubo: crater
From Clark AFB
Clark AFB
After ash fall and lahar
Karakatoa 1883
• Explosive eruption audible at 5000 km• Global dustcloud created golden sunsets in
1880s
Tambora 1815
• Killed 12,000 directly• blew 1400m off the top• 80,000 die in famine and disease locally• global cooling caused global crop failure
– riots and famine in Europe
Tambora 519 CE
• Eruption caused global cooling for a decade• Global crop failures, famine, social
breakdown
Lava
• Can kill, usually doesn’t– silica-rich lavas solidify too quickly– basaltic lavas move faster 15 m/s (54 km/h)
Nyirangongo Zaire 1977
• Volcano sides crack draining lava lake– kills 72
Goma volcano 2002
Lava
• Ruins agricultural land– 1783 eruption in Iceland causes death of 10,521
in famine
Mauna Kea
Kilauea
Kilauea
Gases
• Potentially fatal• Carbon Dioxide the most likely to kill
1979 Java eruption
• 142 people being evacuated from eruption asphyxiated by carbon dioxide
Cameroon tragedies
• 1984 Lake Monoun– Carbon dioxide released by eruption kills 37
• 1986 Lake Nyos– Carbon dioxide released by eruption kills 1746
1986 Lake Nyos disaster
• International effort shipped inappropriate relief supplies:– 22000 blankets supplied (5 each!)– 1430 tents, 5000 gas masks– 5000 kg of jam– 11000 frozen chickens
Secondary Volcanic Hazards
• Ground deformation– Lahars– Flooding– Tsunamis
Ground Deformation
• Pre-eruption buildup of magma distorts ground– triggers avalanches, debris flows
Mt St Helens 1980
• Triggers debris avalanche• 20km down North Fork of Toutle R• fills valley to 40m depth
Lahars
• Mudflows triggered by volcanoes– saturation of old and new ash-falls– melting snowpack
1919 Kelut Volcano, Java
• 5500 killed by Lahar
Nevado del Ruiz 1985
• Lahar brings largest loss of life in C20th since Mt Pelee, 1902
• 1985 Eruption generates lahar• Overwhelms town of Armero, 50km
downstream– 3-8 m depth of mud– 5000 buildings damaged– 22,000 killed in minutes
Nevado del Ruiz 1985
Tsunamis
• Volcanic-origin tsunamis can do significant damage
• Karakatoa eruption 1883– Tsunamis kill 36,000
Volcanic Hazards
• Normally have their worst effects nearby• Intermittent• Can have global impact
– serious with large populations depending on finite global farmland