1

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

2

"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

3

• 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

4

What regions have a high risk for volcanic activity?

Composite volcanoes

Shield volcanoesCalderas

5

• 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

6

Mount St. Helens (1980 eruption)

Mount St. Helens

7

Mount St. Rainer threatens Seattle

06.14.c2

Hazards posed by Mount Rainier to Tacoma, Washington

8

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

9

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)

10

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.

11

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

12

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