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Volcanism
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Volcanism
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Eruptive Style
Volcanic Materials
Volcanoes
Other Volcanic Landforms
Plate Tectonics and Igneous Activity
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Eruptive Style
Explosive
Effusive
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Eruptive Style
• Why do volcanoes have different eruptive
styles?
– Pressure vs. resistance
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Eruptive Style
• A volcano is like a giant pop can that’s
been shaken up
– Pressure builds up
– When pressure is released,
material is pushed out
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Eruptive Style
• Pressure comes from gases dissolved in
magma within the volcano
– Mostly water vapor and carbon dioxide
– When magma rises toward vent, gases come
out of solution and expand
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Eruptive Style
• Resistance to this pressure comes from
magma’s viscosity
• Viscosity = resistance to flow (“thickness”
or “stickiness”)
• Higher viscosity = thicker, more resistant
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Eruptive Style
• Explosive eruptions
– High-viscosity magma
resists gas pressure
• Effusive eruptions
– Low-viscosity magma
offers little resistance
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Eruptive Style
• Why are some magmas more viscous than
others?
– Temperature
– Silica content
• Silica = silicon and oxygen dissolved in magma
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Eruptive Style
• Why are some magmas more viscous than
others?
– Temperature
• Hotter = less viscous (runnier)
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Eruptive Style
• Why are some magmas more viscous than
others?
– Silica content
• More silica = more viscous (thicker)
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Eruptive Style
This volcano is fed by
high-silica, low-temp
magmas with high
viscosity
This one’s magma is
low-silica and high-temp,
and low viscosity
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Eruptive Materials
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Eruptive Materials
• Lava
Aa lava flow
Pahoehoe lava flow
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Eruptive Materials
• Gases
– Water vapor
– Carbon dioxide
– Smaller amounts of other gases
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Eruptive Materials
• Pyroclastics
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Ash and dust – fine, glassy fragments
Lapilli – walnut-sized material
Cinders – pea-sized material
Blocks – hardened or cooled lava
Bombs – ejected as hot lava
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Eruptive Materials
• Pyroclastic flow
– Hot, fast-moving cloud of
pyroclastic material
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Eruptive Materials
• Lahar
– Volcanic mudflow
– Mixture of water, soil, and ash
– Triggered by melting of snow during eruption
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Eruptive Materials
• Three Japanese lahars
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Volcanoes
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Volcanoes
• Shield volcanoes
– Largest type
– Dome-shaped
– Effusive eruptions
– Much lava, few pyroclastics
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Volcanoes
Mauna Kea, a Hawaiian shield volcano
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Volcanoes
• Composite cones
– Smaller than shield volcanoes
– Classic “volcano shape”
– Explosive eruptions
– Much pyroclastic material, little lava
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Volcanoes
Mt. Fuji, a composite cone in Japan
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A Composite Volcano
Interbedded pyroclastic deposits
and small lava flows
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Figure 4.11
Volcanoes
• Cinder cones
– Smallest type
– Loose pyroclastic materials
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A Size Comparison of the
Three Types of Volcanoes
Figure 4.14
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Other Volcanic Landforms
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Other Volcanic Landforms
• Calderas
– Pits caused by magma chamber collapse
– Three types
• Hawaiian-type
• Crater Lake-type
• Yellowstone-type
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Other Volcanic Landforms
• Hawaiian-type calderas
– On shield volcanoes
Olympus Mons
Kilauea
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Other Volcanic Landforms
• Crater Laketype calderas
– Catastrophic
eruptions
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Other Volcanic Landforms
• Yellowstone-type calderas
– Largest
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Caldera outline
(diameter
approx. 30 mi)
Other Volcanic Landforms
• Basalt plateaus
– Very large, flat areas covered with basalt
– Outpourings of low-viscosity lavas from
fissure eruptions
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Other Volcanic Landforms
Fissure eruption in Hawaii
Basalt plateau in Washington
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Plate Tectonics & Igneous
Activity
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Plate Tectonics & Igneous Activity
• Most volcanism occurs along tectonic
plate boundaries
– Divergent boundaries: decompression melting
– Subduction zones: hydration melting
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Plate Tectonics & Igneous Activity
The “Ring of Fire” is a chain of active volcanoes around the Pacific
Ocean. It marks the boundaries of tectonic plates.
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Plate Tectonics & Igneous Activity
• Hot spot (intraplate) volcanism
– Not near plate boundaries
– Fed by magma reservoirs beneath the crust
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Plate Tectonics & Igneous Activity
• Hawaiian Islands were formed by hot spot volcanism
North
Kauai
5.1 my
Oahu
3.7 my
Molokai
1.9 my
Maui
1.3 my
Hawaii (Big Island)
< 1 million yrs old
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Plate Tectonics & Igneous Activity
• Hot spot animation
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End of Chapter 6
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