1. Our Hazardous Environment GEOG 1110 Dr. Thieme
Lecture 8:
Volcanoes and Volcanic Hazards

2. 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

3. Distribution of Volcanoes
Most frequently associated with subduction zones
Also common along mid-ocean ridges
Some at hot spots, but much more rare

6. 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

7. Types of Volcanoes There are 3 basic types of volcanoes
1) Shield volcanoes - largest
2) Stratovolcanoes (also called composite cones)
3) Cinder cones - smallest

9. 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

10. 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

11. ’a ’a
Pahoehoe

12. 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

13. 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

14. Composite Cone, Mount Shasta, northern California

16. Mount Lassen
east of Redding, CA
erupted in June, 1914

18. Formation of Crater Lake

19. Figure 4.13

20. Long Valley
caldera eruption
700,000 years ago
tephra used to date other rocks and sediments ("tephro-chronology")

21. “Peléean” eruptions Silica-rich (“felsic”) magma form domes
glowing avalanches
ash flows

22. 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

23. Volcanic Hazards
gases
CO2
SO2
H2S F2
lava flows
bombs, ash
pyroclastic flows
landslides
mudflows (“lahars”)

24. Molten Sulfur and Sulfur Gases in an Indonesian volcano
FIGURE 5.30c

25. Lake Nyos,
Cameroon,
West Africa

26. Carbon Monoxide (CO) is poisonous