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

ANY QUESTIONS?