Reference:
Chapters 4, 15, 16, 19; Appendix A & B
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Unit 4:
Topic 5.2
 describing how the characteristics of a magma can affect the nature of a volcanic eruption.
Include; silica content, viscosity, & gas content.
 relating the eruptive style of each type of volcano to the associated plate boundary.
 identifying the rocks associated with each boundary.
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The Nature of Volcanic Eruptions
 What determines if an eruption is
- “violent”?
- “gentle”?
3 primary factors:
 magma’s composition more silica more viscous (chains)
 magma’s temperature hotter magma less viscous
 amount of gas in magma
Can increase fluidity
Magma
Viscosity magma mobility
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The Nature of Volcanic Eruptions
 Factors that determine the “violence” or explosiveness of a volcanic eruption include:
1) Composition of the magma
2) Temperature of the magma
3) Dissolved gases in the magma
 The above three factors actually control the viscosity of a given magma; which in turn controls the nature of an eruption.
Text Reference
Pages 89 - 95
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The Nature of Volcanic Eruptions
 Viscosity
 is a measure of a material’s resistance to flow
(e.g., Higher viscosity materials flow with great difficulty)
 Factors affecting viscosity Include:
1) Composition - Silica (SiO
2
) 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)
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The Nature of Volcanic Eruptions
 Viscosity
 is a measure of a material’s resistance to flow
(e.g., Higher viscosity materials flow with great difficulty)
 Factors affecting viscosity Include:
2) Temperature
 Hotter magmas are less viscous (fluid)
(e.g., Mafic magmas tend to be hotter)
 Cooler magmas are more viscous (thicker)
(e.g., Felsic magmas tend to be cooler)
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The Nature of Volcanic Eruptions
 Viscosity
 is a measure of a material’s resistance to flow
(e.g., Higher viscosity materials flow with great difficulty)
 Factors affecting viscosity Include:
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.
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The Nature of Volcanic Eruptions
In Summary:
 Fluid, low viscous lavas (basalt) generally produce quiet eruptions.
-- Ocean Floor.
 Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions.
-- Continental Crust.
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Materials Extruded from a Volcano
 Lava Flows
 Basaltic lavas are much more fluid
 Types of basaltic flows
 Pahoehoe lava (resembles a twisted or ropey texture)
 Aa lava (rough, jagged blocky texture)
 Dissolved Gases
 One to six percent of a magma by weight
 Mainly water vapor and carbon dioxide
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Materials Extruded from a Volcano
Basaltic (low Si content) pahoehoe flow
 flows in thin, broad sheets, ~10 - 300 meters/hr
 smooth & wrinkled outer skin with molten flow beneath
 (Hawaiian type)
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Materials Extruded from a Volcano
Basaltic (low Si content) aa flow
 thicker & slower flows
~5 - 50 meters/hr
 rough & jagged surface
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Plate Boundary Volcanism
 Volcanoes are associated with two of the three types of plate boundaries, these being convergent and divergent boundaries. Very little volcanic activity is seen at transform fault boundaries.
 Volcanism associated with plate tectonic activity are found in three areas on Earth ;
1) Ridges (or spreading centers)
2) Subduction zones
3) Interior of tectonic plates.
Text Reference
Pages 116-122
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 Rift volcanism can occur in two areas;
1) beneath the oceans (Mid Atlantic Ridges)
2) within continental plates (African Rift Zone)
 Rift eruptions are associated with Divergent Boundaries.
 Eruptive style is generally quiet with free flowing lavas.
This is because the magma/lava has low silica and gas content, and has a relatively low viscosity (fluid).
 Basaltic magmas are produced at spreading centers.
 Shield volcanoes are formed along ridges when basaltic lava flows on the ocean floor.
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 Subduction eruptions are associated with Convergent Boundaries.
 Eruptive style is generally violent and explosive . This is because the magma/lava has a medium - high silica and gas content, and has a relatively high viscosity (thick).
 Basaltic magmas are produced at ocean – ocean convergent boundaries.
 Andesitic and granitic magmas are produced at ocean – continent convergent boundaries.
 Composite volcanoes are formed along convergent plate boundaries. Example: Mt Fuji in Japan and Mt. St. Helens in USA.
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Subduction volcanism occur in two areas:
1) ocean – ocean convergent boundary
 Ocean crust subducts beneath ocean crust.
 Basaltic magmas are produced and burns upward toward the surface forming a chain of volcanoes called a “volcanic island arc” parallel to the trench.
 Examples include; Islands of Japan.
Ocean Plate
Asthenosphere
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Ocean Plate

Subduction volcanism occur in two areas:
2) ocean – continent convergent boundary
 Ocean crust subducts beneath continental crust.
 Andesitic and Granitic magmas burns upward into the continental crust adding to mountain systems.
 Most of the world’s volcanoes are of this type and border the
Pacific Ocean, called the Pacific Ring of Fire .
 Examples include; volcanoes in the Andes mountains.
Ocean Plate Continental Plate
Asthenosphere
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The Nature of Volcanic Eruptions - Summary
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Which rock type is associated with a divergent plate boundary?
(A) Andesite (C) Basalt
(B) Granite (D) Rhyolite
Which type of volcanic cone displays gentle, fluid, mafic eruptions?
(A) cinder cone
(B) composite cone
(C) shield cone
(D) strata volcano
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Which landform at location X and rock type at location Y is shown in the diagram below?
Location X
(A) island arc
(B) island arc
(C) volcanic arc
(D) volcanic arc
Location Y basalt gabbro basalt gabbro
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Take the time and complete the following questions . . .
(Solutions to follow)
Question:
Describe a spreading center and briefly describe what volcanic activity is present at these locations.
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Questions:
Describe a spreading center and briefly describe what volcanic activity is present at these locations.
Answer:
A divergent boundary, sometimes referred to as spreading centers, displays tensional forces caused by mantle convection causing oceanic plates to move apart. Basaltic magmas which are very fluid magmas with little dissolved gases erupt quietly onto the ocean floor adding to the diverging oceanic crust. New sea floor is formed at spreading centers.
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Overview of Points covered:
The nature of a volcanic eruption depends on:
1) Composition of the magma
2) Temperature of the magma
3) Amount of dissolved gases in the magma
Volcanic Activity is associated with two plate boundaries:
1) Divergent
2) Convergent
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