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CHAPTER 6: VOLCANOES AND OTHER MOUNTAINS
SUMMARY
This chapter begins with a description of the impact of science and society through a case
study of the Nyiragingo eruption, Congo. You are introduced to the potential threat that
volcanoes pose in order to spark your interest to learn more about the characteristics of
volcanoes. The concept of viscosity is introduced using a simple analogy. That concept is
extended to magmas and the influence of viscosity on gas release. The viscosity is then related to
silica content and temperature. You are led through a series of examples to reinforce the idea that
higher silica content and lower temperatures increase viscosity, and thus hazards from volcanic
eruptions. The chapter continues as the volcano location, and resulting magma type, is related
directly to plate tectonics. The concepts are merged and illustrated with another example of
science and society and the nature of science as applied to the 1980 Mount St. Helens eruption.
This is followed by a more general discussion of blast effects, tephra fallout, gas release, lava
flows, pyroclastic flows, and lahars. Finally, volcanoes are discussed in the context of landforms,
shield volcanoes, stratovolcanoes, cinder cones, calderas, lava plateaus, geysers, and other
related features.
Figure Above: distribution of world's active volcanoes. Most are associated with
subduction zones located around the rim of the Pacific Ocean and the northeast Indian Ocean.
Some volcanoes are concentrated along the ocean ridge in the Atlantic Ocean on Iceland or
along the beginning of a divergent boundary in East Africa. Isolated volcanoes in the interiors
of plates are typically associated with hot spots above mantle magma sources.
6.2: Magma Viscosity (page 142):
Viscosity: defined as "resistance to flow" (low viscosity fluids flow easily). Magma and
lava have viscosity and its viscosity is influenced by different factors including temperature (the
viscosity of magma increases as it temperature decreases) and silica content (more silica means
higher viscosity). See figure below for a comparison of viscosities; notice the three main
volcanic igneous rocks and where they plot on the diagram (the difference between low viscosity
basalt lava and high viscosity rhyolite lava is similar to the difference between water and peanut
butter!). Also, in general the higher the viscosity, the more violent the eruption during volcanic
activity (because gases cannot escape as easily from higher viscosity magma so pressure builds
up into violent eruptions).
Silica (silicon and oxygen) content also plays a role in viscosity of magma (it plays a
bigger role than temperature); as stated above, the higher the silica content, the higher the
viscosity (see page 144 for more).
Table Above: Comparison of viscosities. The differing viscosities of the three types of
volcanic igneous rocks are shown in relation to the viscosities of some common household
materials.
The table below is a comparison of the three magma types and shows typical
characteristics of each type.
Table Above: basaltic magma comprises approximately 80% of the volume of magma
that reaches the Earth's surface (remember, it has relatively low viscosity so it can reach and
flow along the Earth's surface easier). Compared to other magma it contains the lowest amount
of silica and volatiles (water), and it occurs at the highest temperatures; these reasons result in
peaceful eruptions (e.g., Hawaii, Iceland). Study the table carefully to determine how silica
(SiO2) content, temperature of magma, and percentage of volatiles (mainly water) vary for each
magma type and result in characteristic viscosities and eruptive styles.
6.3: Magma Sources and Magma Composition (page 144):
Most of the worlds active volcanoes are located along convergent plate boundaries, above
hot spots or are associated with divergent plate boundaries. Examples: Mt St Helens and Mt
Vesuvius are products of convergent plate boundaries; Hawaii and Yellowstone are products of
hot spot activity; Iceland and Nyiragongo are located above divergent plate boundaries. As
stated on page 151 "different plate boundaries produce different magma compositions because
each setting generates magma from melting a different source rock". See diagram below.
Figure Above: Formation of different magmas and volcanic features. Basalt (the least
explosive) is produced by partial melting of the mantle below volcanic islands in the interior of
oceanic plates and oceanic ridges. It is also present in eruptions that occur in continental rift
zones. Andesite is derived from the partial melting of the mantle wedge (continental lithosphere)
above descending plates of oceanic lithosphere. Rhyolite (the most explosive) is generated by
the partial melting of the continental crust.
6.5: Products of volcanic eruptions (page 152):
These products can be divided into airborne elements (i.e., lateral blast, tephra, volcanic
gases) and flows on the ground which are made up of mixtures of molten rocks, gases and/or
water (i.e., lava, pyroclastic flows, lahars). Please read this section to understand these products
of eruptions and the hazards they present; which product do you feel is most hazardous to
people??
6.6: Volcanoes and volcanic landforms (page 160):
The three basic types of volcanic cones are first presented in this section: Shield
volcanoes, Stratovolcanoes and Cinder (Scoria) Cone volcanoes. Other volcanic landforms are
presented next (not typically cone-shaped); these include Calderas, Lava Plateaus (Food Basalts)
and Geysers, Hot Springs, Fumeroles and Mud Volcanoes. With exception of Geysers/Hot
Springs/Fumeroles/Mud Volcanoes you should be able to find these landforms in the table below
which shows how the three V's (Viscosity, Volatiles such as water vapor and Volume) play a
role in determining what type of volcanic landforms develops.
Table Above: Volcanic landforms are related to the characteristics of the magma that
produced them. Going from top to bottom of this table magma changes from basaltic and
peaceful eruptions (shield volcanoes; example, Hawaiian Islands) to rhyolitic and explosive
eruptions (calderas; example, Yellowstone).
The next part of this chapter deals with the connection of mountains with plate tectonics.
This chapter focuses on the global change implication of mountain building rather than their
structural styles. The concepts of density and isostasy are developed, as is their connection to
mountain building. Global change and the Earth system are revisited as a discussion of the
influence of erosion on mountains and are linked to their elevation through isostatic
compensation. Lastly, the important link of global climate change and mountain building is
discussed in the context of the effect of the Himalayas.
We are skipping section's 6.7 and 6.8 with exception of the last subheading "Mountains
and Climate" beginning on page 169. See diagram and caption below.
Figure Above: the Himalayas, continuing to be pushed upwards while India collides with
Eurasia (conitinental-continental collision). Monsoon weather patterns develop as warm, wet
air is drawn in from the ocean and rises along the front of the Himalayas to produce heavy rains
that feed the Ganges and Brahmaputra rivers. The rivers flow into the Bay of Bengal, producing
the largest delta complex on Earth in Bangladesh. What is the impact of these heavy monsoon
rains on global climate?? (see your book for more!).
LEARNING OBJECTIVES
1. Students will explain concepts related to volcanic activity and mountains.
2. Students will explain the relationship between viscosity, eruption violence, and magma
composition.
3. Students will describe the relationship between magma source and magma composition.
4. Students will interpret the eruption of Mount St. Helens in terms of the scientific method.
5. Students will describe products of volcanic eruptions.
6. Students will compare and contrast three types of volcanoes.
7. Students will describe volcanic landforms.
8. Students will explain the process of mountain building, erosion of mountains, and the effect
of mountains on Earth’s climate.