What determines the violence of an eruption? Composition of the magma determine the “violence”...
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Transcript of What determines the violence of an eruption? Composition of the magma determine the “violence”...
What determines the violence of
an eruption?
Composition of the magmadetermine the
“violence” or explosiveness of a volcanic eruption
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
What is Viscosity?
• measure of a material’s resistance to flow
Think of Syrup & Water
Factors affecting viscosity
Viscosity – dependent on both silica content and temperature.
• Temperature - Hotter magmas are less viscous
(Compare difference between cold & hot syrup)
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 )
Generalized Types Of Lava
Basaltic LavaAndestic LavaRhyolitic Lava
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
A'a lava
Pahoehoe Lava
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
Andestic Lava
Andestic Lava
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
• Refer to text p 510
ANY QUESTIONS?