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