What determines the violence of an eruption?

2. What determines the violence ofan eruption? Composition of the magma determine the “violence” or explosiveness of a volcanic eruption

3. 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

4. What is Viscosity? • measure of a material’s resistance toflow Think of Syrup & Water

5. Factors affecting viscosity Viscosity – dependent on both silica content and temperature. • Temperature - Hotter magmas are less viscous (Compare difference between cold & hot syrup)

6. 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 )

7. Generalized Types Of Lava Basaltic Lava Andestic Lava Rhyolitic Lava

8. 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

9. A'a lava

10. Pahoehoe Lava

11. 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

12. Andestic Lava

13. Andestic Lava

14. 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

15. Refer to text p 510

16. ANY QUESTIONS?