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Our Hazardous Environment
GEOG 1110Dr. Thieme
Lecture 9: Magma Types and Volcanic Hazards
Types of MagmaBasaltic = most "primitive" (Fe- and Mg-rich)
Andesitic = "intermediate"Granitic = most "evolved"• enriched in
Si• precipitates "calc-alkaline" mineral assemblages (Al, K, Ca,
Na)
• has greater viscosity (resistance to flow)
• exists as a liquid at temperatures as low as 700oC
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"Violent" Eruptions• How "violent" or explosive a given eruption
will
be is determined by:• Composition of the magma• Temperature of
the magma• Dissolved gases in the magma
• These three factors together determine the viscosity of the
magma
• resistance to flow (Higher viscosity materials flow with great
difficulty)
• Factors affecting viscosity• Temperature - Hotter magmas are
less
viscous• Composition - Silica (SiO2) content
• Higher silica content = higher viscosity (e.g., felsic lava
such as rhyolite)
• Lower silica content = lower viscosity or more fluid-like
behavior (e.g., mafic lava such as basalt)
Viscosity
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• Dissolved Gases• Gas content affects magma mobility• Gases
expand within a magma as it nears the Earth’s surface due to
decreasing pressure
• The violence of an eruption is related to how easily gases
escape from magma
To Summarize:• Fluid basaltic lavas generally produce quiet
eruptions
• Highly viscous lavas (rhyolite or andesite) produce more
explosive eruptions
Factors affecting Viscosity
Vesicular Texture
• Pumice
• Scoria
• Vesicular Basalt
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What regions have a high risk for volcanic activity?
Composite volcanoes
Shield volcanoesCalderas
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• High Risk in Cascade Mtns
• High Risk in Yellowstone
• Moderate Risk in Areas of Southwest
• Caldera eruption could impact entire U.S.
Mount Lassen• east of
Redding, CA• erupted in
June, 1914
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Mount St. Helens (1980 eruption)
Mount St. Helens
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Mount St. Rainer threatens Seattle
06.14.c2
Hazards posed by Mount Rainier to Tacoma, Washington
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Volcanic Hazards• gases
• CO2• 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
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Carbon Monoxide (CO) is poisonous
Mt Unzen, Japan
• 1991 eruption
• pyroclastic flows killed 43
• more than 600 homes and one school destroyed
• claimed the lives of volcanologists Maurice and Katya
Kraft
Pyroclastic Flow(nuée ardentes)
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Pyroclastic Flows along the Mitzunashi River
A lahar triggered by rainfall on the Nima II River in Guatemala.
The lahar developed on the slopes of Santiaguito volcano
Damage caused by a lahar on the Toutle River triggered by the
Mount St. Helens eruption in Washington state.
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Pompeii - pyroclastic flows killed over 16,000
06.13.a-d
How Do We Monitor Volcanoes?
Increased seismic activity
Increased gas activity
Changes in topography Changes in temperature
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Volcanoes and climate
• Explosive eruptions emit huge quantities of gases and
fine-grained debris into the atmosphere which filter out and
reflect a portion of the incoming solar radiation
• Examples of volcanism affecting climate• Mount Tambora,
Indonesia – 1815• Krakatau, Indonesia – 1883• Mount Pinatubo,
Philippines - 1991