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

4. 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 viscosity the viscosity of magma

5. What is Viscosity? measure of a material’s resistance to flow Think of Syrup & Water

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

7. Factors affecting viscosity Composition 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 )

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

9. Basaltic Lava low viscosity flow rapidly for great distances nonexplosive eruption primarily from shield volcanoes (Example Hawaiian Volcanoes)shield volcanoes Subdivided into flow types – A'a lava: rough, jagged blocky texture – Pahoehoe Lava: resembles a twisted or rope texture

10. A'a lava

11. Pahoehoe Lava

12. 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

13. Andestic Lava

15. Rhyolitic Lava More than 60% Silica high viscosities and high gas contents generally ooze out of the volcano's The best known examples YellowstoneYellowstone Most recent eruptions 640,000 years ago

16. Refer to text p 510

17. ANY QUESTIONS?