1. Volcanoes

2. The Nature of Volcanic Eruptions Viscosity (resistance to flow) determines the “ violence ” or explosiveness of a volcanic eruption Factors which determine viscosity Composition of the magma Temperature of the magma Dissolved gases in the magma

3. Temperature - Cooler magmas are more viscous A volcano ’ s eruptions may get more explosive over time, as magma in chamber cools down Example: Crater Lake (formerly Mt. Mazama)

4. Magma Composition and Viscosity Granitic/andesitic lavas have greater silica (SiO 2 ) content and are more viscous –Convergent plate volcanism Basaltic lavas have less(SiO 2 ) content and are less viscous –Divergent plate volcanism – Intraplate (hot spot) volcanism

5. Dissolved Gases Content and Viscosity Gases expand within a magma as it nears the Earth ’ s surface due to decreasing pressure The violence of an eruption is related to how easily gases escape from magma “ Wet ” magma (oceanic subduction) has significant gas content

7. Materials extruded from a volcano Lava Flows –Pahoehoe lava (resembles a twisted or ropey texture) –Aa lava (rough, jagged blocky texture)

8. Figure 4.5a

9. Figure 4.3

10. Materials extruded from a volcano Dissolved Gases: Mainly water vapor and carbon dioxide Pyroclastic materials – “ Fire fragments ” – Ash and dust - fine, glassy fragments – Cinders – slightly larger than ash – Pumice - porous rock from “ frothy ” lava – Blocks and bombs – larger discrete pieces of lava

11. Shield volcanos Broad, slightly domed-shaped Composed primarily of basaltic lava Generally cover large areas Produced by mild eruptions of large volumes of lava Mauna Loa on Hawaii is a good example

12. Cinder Cones –Built from ejected lava (mainly cinder-sized) fragments –Steep slope angle –Rather small size –Frequently occur in groups

13. Composite cone (Stratovolcano) –Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens). –Large, classic-shaped volcano (1000 ’ s of ft. high & several miles wide at base). –Composed of lava flows alternating with large quantities of pyroclastic flow deposits.

14. Figure 4.1a

15. Figure 4.1b

16. A composite volcano Figure 4.7

17. Size comparison of volcano types

18. Formation of Crater Lake

19. Partial Melting and Magma Formation Formation of Basaltic magmas Most originate from partial melting of ultramafic rock in the mantle Basaltic magmas form at mid-ocean ridges by decompression melting or at subduction zones Formation of Granitic magmas Basaltic magma pools beneath granitic continental rock and melts it, forming granitic magma Granitic magma often does not reach the surface, but instead forms intrusive rocks at depth.

20. How Magma Rises

21. Formation of Plutons from Granitic Magma Formation of Granitic magmas Basaltic magma pools beneath granitic continental rock and melts it, forming granitic magma Granitic magma often does not reach the surface, but instead forms intrusive rocks at depth. Pluton – a large mass of intrusive rock Most plutons are granitic in composition Granitic magma forms at base of continental crust and rises up because it is less dense than the solid crust

22. Forming Igneous Features and Landforms

23. Fig. 8-15, p.179

24. Fig. 8-16, p.180

25. Figure 4.24

26. Figure 4.26

27. Figure 4.20

28. Figure 4.21

29. Plate Tectonics and Magma Generation

30. Figure 4.27

31. Tectonic Settings and Volcanic Activity