1. VOLCANOES form where molten rock is vented at Earth’s surface. Where do volcanoes form in the context of plate tectonics? Volcanoes aren’t equally dangerous.... how do their hazards differ, and why? BIG Questions

3. What comes out of a volcano? Ash

4. What comes out of a volcano? Gas Most common: H 2 O CO 2 SO 2 HCl

5. What comes out of a volcano? Lava

6. Mauna Loa (Hawai’i): A typical shield volcano

7. Mt. St. Helens: A typical composite volcano

8. Mt. St. Helens after its 1980 eruption

9. Composite Volcano

10. Cinder Cone Built from ejected lava fragments Steep slope angle Rather small size Sunset Crater, near Flagstaff, AZ

11. A size comparison of the three types of volcanoes

12. Explosive Non-explosive (“Clogged”) (“Runny”) Andesitic composition Basaltic composition Composite volcanoes Shield volcanoes Volcanologists recognize many types of volcanoes, but we only need to worry about two:

13. Volcanic eruptions Factors that determine the violence of an eruption Composition of the magma Temperature of the magma Dissolved gases in the magma Viscosiy of magma Viscosity is a measure of a material's resistance to flow

14. Volcanic eruptions  Viscosity of magma – factors Temperature (hotter magmas are less viscous) Composition [Si0 2 (silica) content] - High silica – high viscosity (e.g., rhyolitic lava) - Low silica – more fluid (e.g., basaltic lava) Dissolved gases - Mainly water vapor and carbon dioxide – - Gases expand near the surface More silica = more linkage = more viscous

15. Magma compositions affect properties

16. Volcanic Hazards Lava flows Ash fall Pyroclastic flows Mudflows Volcanic Gases Tsunami

17. Plate-tectonic setting of volcanism Explosive (andesitic) volcanoes form at subduction zones.

18. Plate-tectonic setting of volcanism At spreading centers, low pressure triggers mantle melting—fluid basaltic magma rises.

19. Plate-tectonic setting of volcanism Within plates, rising plumes of hotter mantle feed hot spots; varied volcanoes result (basaltic on Hawaii).