VOLCANIC Magma Types

Chapter 18

Introduction (1)

•~ 1,500 active volcanoes on Earth

•400 erupted in the last century

•~ 50 eruptions per year

•Most activity concentrated along major plate boundaries

•Impact risks depend on the types of volcanoes

Introduction (2)

•~ 500 million people living near volcanoes

•100,000 deaths during the last 125 years

•23,000 in the last 20 years

•Densely populated countries in the volcanic zones (e.g., Philippines, Indonesia, Japan, Mexico)

•Some major cities are located near volcanoes

Volcanism in Space (1)

•Highly related to plate tectonic movement

•Concentrated along the Pacific ring of fire

•In the United States: Alaska, Cascades, and Hawaii

Animation

• Types of Volcanic Activity

Animation

• Crater Lake

Three types of Magma• The composition of magmas and lavas is controlled

by the most abundant elements in the EarthSi, Al, Fe, Ca, Mg, Na, K, H, and O

• Three distinct types of magma are more common than others: – Basaltic, containing about 50 percent SiO2

– Andesitic, about 60 percent SiO2

– Rhyolitic, about 70 percent SiO2

Types of Volcanoes (1)

•Volcanic eruption style Depending on

lava’s viscosity and amount of dissolved gas content

Viscosity: Liquid’s resistance to flow Determined by:

silica content (lava composition) and lava temperature

Quiet flow (low viscous basalt flow) Violent explosion (high viscous lava eruption)

Magma Viscosity• The internal property of a substance that offers

resistance to flow is called viscosity

• The more viscous a magma, the less easily it flows

• Viscosity of a magma depends on temperature and composition (especially the silica and dissolved-gas contents)

• The higher the temperature, the lower the viscosity, and the more readily magma flows

Viscosity• The greater the silica content, the larger is the

polymerized group

• For this reason, rhyolitic magma (70% silica) is always more viscous than basaltic magma (50% silica)

• Andesitic magma has a viscosity that is intermediate between the two (60% silica)

Nature of Volcanic Eruption• Factors affecting viscosity continued

– Lower silica content = lower viscosity or more fluid-like behavior (e.g., mafic lava such as basalt)

– 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

Relative size of Volcanoes

9 km

3 km

0.3 km1.5 km

15 km

150 km

Shield volcano (e.g. Hawaii)

Composite volcano (e.g. Vesuvius)

Cinder cone (e.g. Sunset crater)

Volcanic Features

•Craters and vent

•Volcanic cones

•Caldera: Collapsed craters typically from explosive eruptions

•Hot springs and geysers

•Fissure line: Basaltic lava flow

Volcanic Hazard• Eruptions present five kinds of hazards:

– Hot, rapidly moving pyroclastic flows and laterally directed blasts can overwhelm people before they can evacuate; e.g.,• Mont Pelee in 1902• Mount St. Helens in 1980

– Tephra and hot poisonous gases can bury or suffocate people• 79 Mount Vesuvius in A.D. 79

Volcanic Hazard• Mudflows, called lahars, can be devastating

– In 1985, the Colombian volcano Nevado del Ruiz experienced a small, nonthreatening eruption. But, when glaciers at the summit melted, massive mudflows of volcanic debris moved swiftly down the mountain , killing 20,000

• Violent undersea eruptions can cause powerful sea waves called tsunamis– Krakatau, in 1883, killed more than 36,000 on

Java and nearby Indonesia islands• A tephra eruption can disrupt agriculture, creating

a famine

Volcanic Impact Risks (1)

•Lava flows: From the vent of a crater or along a line of fissure

•Most common and abundant type: Basaltic lava low

•Pahoehoe lava: Less viscous, higher temp, with a smooth ropy surface texture

•Aa lava: More viscous, lower temp, with a blocky surface texture

Volcanic Impact Risks (2)

•Pyroclastic flow Enormous amount of rock fragments, volcanic

glass fragments, and volcanic bombs Associated with explosive volcanic eruptions More deadly if lateral blast Pyroclastic avalanches Hot temperature and fire hazards

Volcanic Impact Risks (3)

•Ash flow Covering large area, 100s or 1,000s of km2

Wider impact if ash flows reach upper atmosphere

Hot temp (nueé ardentes) ash and moving at rapid speed (100 km/h)

Harm to human health and structures Blocking away solar radiation Hazardous for air traffic

Nuée ardente• Pyroclastic flows are also known as nuée ardente

(glowing cloud)

• Historic observations indicate that pyroclastic flows can reach velocities of more than 700 km/h

• In 1902, a pyroclastic flow rushed down the flanks of Mont Pelee Volcano at an estimated speed of 200 KM/h, instantly killing 29,000 people

Volcanic Impact Risks (4)

•Poisonous Gases Volcanic gases: H2O, CO2, CO, SO2, H2S Floating in air Dissolved in water Dangerous for health, plants, and animals Producing smog air, acid rain, and toxic soil

Volcanic Impact Risks (5)

•Debris and Mudflows Collapse of volcano slopes Sudden melting of snow caps and glaciers at the

top of a volcano Rapid downslope flow at the speed of 50 km/h Long flowing distance: Tens of miles from

volcano

Case Study (1)

Mount Pinatubo• June 15–16, 1991 • Killed 350 people and destroyed a U.S. military base• Nearly 1-ft depth of ash covered buildings over a 40-

km radius• Huge cloud of ash 400 km wide into nearly 40 km

elevation• Affected global climate (cooler summer the next

year; global temp differences −0.5°C, ~1°F)

Case Study (2)

Mount St. Helens• May 18, 1980, erupted after a 120-year dormancy• Earthquake (4–5 magnitude) precursor, triggered

massive landslide displacing water in Spirit Lake and traveling an 18-km distance down the Toutle River

• Lateral blast impacted 19 miles at 1000 km/h

• Mudflows reached nearly 100 km (60 miles) away Cowlitz and Columbia Rivers

Case Study (2)

Mount St. Helens (continued)• Ash/tephra materials spread over WA, ID, and west

MT• Its maximum altitude (peak) reduced 450 meters

(over 1476 ft)• Killed 54 people, damaged 100 homes, 800 million

feet of timber: Total cost $3 billion

Forecasting Volcanic Activity

•Seismic Activities: Earthquakes as precursors

•Thermal, magnetic and hydrologic conditions

•Amount of volcanic gas emission

•Topographic monitoring: Tilting and special bulging

•Remote sensing: Radar 3-D interferometry

•Geologic history of a volcano

Public Perception and Adjustment

•Perception of the volcanic hazards Age and residence near a volcano affects one’s

knowledge of volcanic activity and possible adjustment

•Adjustment Public awareness and education Improvement in education Better scientific info dissemination Timely and orderly evacuation