Lecture 8: Volcanoes and Volcanic

Hazards

Our Hazardous Environment

GEOG 1110Dr. Thieme

Volcanoes• volcano - a conical or dome-shaped

landform created when lava and/or tephra accumulates on the earth’s surface

• lava - molten material on the surface• magma - molten material below the

surface• tephra - igneous material, ranging in

size from dust to boulders, that is explosively ejected from a volcano

Distribution of Volcanoes

1. Most frequently associated with subduction zones2. Also common along mid-ocean ridges3. Some at hot spots, but much more rare

• magma chamber - an accumulation of molten rock beneath the Earth’s surface

• vent - the opening magma uses to move from the magma chamber to the Earth’s surface

• crater - the opening through which lava and tephra issues• caldera - a very large crater created by explosion or collapse• cone - a build-up of lava and/or tephra around a vent, creating a

hill or mountain

Types of Volcanoes

• There are 3 basic types of volcanoes1) Shield volcanoes - largest2) Stratovolcanoes (also called composite cones)3) Cinder cones - smallest

Shield Volcanoes• Created by mafic magma

• Thin and flow easily in comparison to felsic magmas (less “viscous”)

• Usually hold little gas in comparison to felsic magmas• Eruptions of volcanoes consisting of mafic magmas are relatively

quiet because of the thin, fluid nature and the absence of gas under pressure

• Mafic lava flows tend to travel long distances across the surface, and form broad-based volcanoes with gently sloping sides

• Fissure eruptions on the sides of the volcanoes are common, called flank eruptions

• Calderas often develop

Shield Volcanoes• Composed of multiple stacked lava flows• Famous examples include Hawaii, Iceland, and the

Azores.• Hawaii is formed over a hot spot, Iceland and the

Azores over the Mid-Atlantic Ridge

• hot spot - an area where a plume of magma is

welling up from deep within the asthenosphere

’a ’a

Pahoehoe

Stratovolcanoes• Created by intermediate to felsic

magmas– thick and gummy magma/lava– magma/lava frequently high in gas content– eruptions can be very explosive– cones are composed of interlayered lava and tephra– tall, steep-sided cones

Stratovolcanoes• also called composite volcanoes or composite cones• stratovolcanoes are known for their scenic beauty• famous examples include:

– Mount Rainier and Mount St. Helens in Washington– Mount Shasta in California– Mount Fujiyama in Japan

• volcanoes that most concern scientists– explosive eruptions– glowing avalanches, scorching hot clouds of ash and debris– glowing avalanche from Mount Pelée on the Caribbean island of

Martinique killed all but 2 of the 30,000 inhabitants of the city of St. Pierre in 1902

Composite Cone, Mount Shasta, northern California

Mount Lassen• east of

Redding, CA• erupted in

June, 1914

Formation of Crater Lake

Figure 4.13

Long Valley• caldera

eruption• 700,000

years ago• tephra

used to date other rocks and sediments ("tephro-chronology")

“Peléean” eruptions

• Silica-rich (“felsic”) magma• form domes• glowing avalanches• ash flows

Cinder Cones

• made up entirely of tephra• form when frothy mafic magma is ejected from a

vent under high pressure• rarely get more than a few hundred meters high,

and they are the most easily eroded volcanic cone

Volcanic Hazards• gases

• CO22

• SO2

• H2S• F2

• lava flows• bombs, ash

• pyroclastic flows• landslides• mudflows (“lahars”)

Molten Sulfur and Sulfur Gases in an

Indonesian volcano

Lake Nyos, Cameroon, West Africa

Carbon Monoxide (CO) is poisonous