1. VOLCANIC Magma Types
Chapter 18

2. 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

3. 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

5. 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

6. Animation
Types of Volcanic Activity

7. Animation
Crater Lake

8. Three types of Magma
The composition of magmas and lavas is controlled by the most abundant elements in the Earth Si, 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

10. 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)‏

11. 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

12. 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)‏

13. 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

15. Relative size of Volcanoes
9 km
150 km
Shield volcano (e.g. Hawaii)‏
3 km
15 km
Composite volcano (e.g. Vesuvius)‏
0.3 km
1.5 km
Cinder cone (e.g. Sunset crater)‏

16. Volcanic Features Craters and vent Volcanic cones
Caldera: Collapsed craters typically from explosive eruptions
Hot springs and geysers
Fissure line: Basaltic lava flow

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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)‏

26. 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

27. 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

28. 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

30. 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