What determines the violence of an eruption? Composition of the magma determine the “violence”...
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![Page 1: What determines the violence of an eruption? Composition of the magma determine the “violence” or explosiveness of a volcanic eruption.](https://reader037.fdocuments.net/reader037/viewer/2022102622/56649e9d5503460f94b9ddb7/html5/thumbnails/1.jpg)
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What determines the violence of
an eruption?
Composition of the magmadetermine the
“violence” or explosiveness of a volcanic eruption
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Composition of Magma • Temperature: most rock melt at 800-1200C • Pressure: increases with depth, as pressure
increases temperature rock melts increases• Dissolved gases: H2O vapor, CO2, SO2, H2SO4
As amount increases, exclusivity increases
The above three factors control the viscosity of magma
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What is Viscosity?
• measure of a material’s resistance to flow
Think of Syrup & Water
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Factors affecting viscosity
Viscosity – dependent on both silica content and temperature.
• Temperature - Hotter magmas are less viscous
(Compare difference between cold & hot syrup)
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Factors affecting viscosity
• Composition - Silica (SiO 2 ) content
More silica, > viscosity : (thick & stick) gases trapped, explosive eruptions (Rhyolitic Lava)
Less silica, < viscosity : (thin & runny) flow easily, nonexplosive eruptions (Basaltic Lava )
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Generalized Types Of Lava
Basaltic LavaAndestic LavaRhyolitic Lava
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Basaltic Lava
• low viscosity• flow rapidly for great distances • nonexplosive eruption • primarily from shield volcanoes (Example
Hawaiian Volcanoes)• Subdivided into flow types–A'a lava: rough, jagged blocky texture–Pahoehoe Lava: resembles a twisted or
rope texture
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A'a lava
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Pahoehoe Lava
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Andestic Lava
• 50-60% Silica • Massive explosions, huge volumes of ash &
debris • generally produces blocky lava • Composite (stratovolcanoes) forming the
Aleutian islands in Alaska, the crest of the Cascade Mountains in the Pacific Northwest
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Andestic Lava
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Andestic Lava
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Rhyolitic Lava
• More than 60% Silica• high viscosities and high gas contents • generally ooze out of the volcano's• The best known examples Yellowstone• Most recent eruptions 640,000 years ago
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• Refer to text p 510
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ANY QUESTIONS?