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What determines the violence of an eruption?. Composition of the magma determine the “violence” or explosiveness of a volcanic eruption. Composition of Magma . Temperature: most rock melt at 800-1200C

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

Composition of the magma

determine the “violence” or explosiveness of a volcanic eruption


Composition of magma
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
What is Viscosity?

  • measure of a material’s resistance toflow

    Think of Syrup & Water


Factors affecting viscosity
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 viscosity1
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
Generalized Types Of Lava

Basaltic Lava

Andestic Lava

Rhyolitic Lava


Basaltic 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




Andestic 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




Rhyolitic 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



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