Download - What determines the violence of an eruption?
What determines the violence ofan 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
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