1. Volcanism

2. Volcanism Eruptive Style Volcanic Materials Volcanoes
Other Volcanic Landforms
Plate Tectonics and Igneous Activity

3. Eruptive Style
Explosive
Effusive

4. Eruptive Style Why do volcanoes have different eruptive styles?
Pressure vs. resistance

5. Eruptive Style A volcano is like a giant pop can that’s been shaken up
Pressure builds up
When pressure is released,
material is pushed out

6. Eruptive Style
Pressure comes from gases dissolved in magma within the volcano
Mostly water vapor and carbon dioxide
When magma rises toward vent, gases come out of solution and expand

7. Eruptive Style
Resistance to this pressure comes from magma’s viscosity
Viscosity = resistance to flow (“thickness” or “stickiness”)
Higher viscosity = thicker, more resistant

8. Eruptive Style Explosive eruptions Effusive eruptions
High-viscosity magma
resists gas pressure
Effusive eruptions
Low-viscosity magma
offers little resistance

9. Eruptive Style Why are some magmas more viscous than others?
Temperature
Silica content
Silica = silicon and oxygen dissolved in magma

10. Eruptive Style Why are some magmas more viscous than others?
Temperature
Hotter = less viscous (runnier)

11. Eruptive Style Why are some magmas more viscous than others?
Silica content
More silica = more viscous (thicker)

12. Eruptive Style
This volcano is fed by high-silica, low-temp magmas with high viscosity
This one’s magma is low-silica and high-temp, and low viscosity

13. Eruptive Materials

14. Eruptive Materials
Lava
Aa lava flow
Pahoehoe lava flow

15. Eruptive Materials Gases Water vapor Carbon dioxide
Smaller amounts of other gases

16. Eruptive Materials Pyroclastics Ash and dust – fine, glassy fragments
Lapilli – walnut-sized material
Cinders – pea-sized material
Blocks – hardened or cooled lava
Bombs – ejected as hot lava

17. Eruptive Materials Pyroclastic flow Hot, fast-moving cloud of
pyroclastic material

18. Eruptive Materials Lahar Volcanic mudflow
Mixture of water, soil, and ash
Triggered by melting of snow during eruption

19. Eruptive Materials
Three Japanese lahars

20. Volcanoes

21. Volcanoes Shield volcanoes Largest type Dome-shaped Effusive eruptions
Much lava, few pyroclastics

22. Volcanoes
Mauna Kea, a Hawaiian shield volcano

23. Volcanoes Composite cones Smaller than shield volcanoes
Classic “volcano shape”
Explosive eruptions
Much pyroclastic material, little lava

24. Volcanoes
Mt. Fuji, a composite cone in Japan

25. A Composite Volcano Figure 4.11 Interbedded pyroclastic deposits
and small lava flows
Figure 4.11

26. Volcanoes
Cinder cones
Smallest type
Loose pyroclastic materials

27. A Size Comparison of the Three Types of Volcanoes
Figure 4.14

28. Other Volcanic Landforms

29. Other Volcanic Landforms
Calderas
Pits caused by magma chamber collapse
Three types
Hawaiian-type
Crater Lake-type
Yellowstone-type

30. Other Volcanic Landforms
Hawaiian-type calderas
On shield volcanoes
Olympus Mons
Kilauea

31. Other Volcanic Landforms
Crater Lake-type calderas
Catastrophic eruptions

32. Other Volcanic Landforms
Yellowstone-type calderas
Largest
Caldera outline
(diameter approx. 30 mi)

33. Other Volcanic Landforms
Basalt plateaus
Very large, flat areas covered with basalt
Outpourings of low-viscosity lavas from fissure eruptions

34. Other Volcanic Landforms
Fissure eruption in Hawaii
Basalt plateau in Washington

35. Plate Tectonics & Igneous Activity

36. Plate Tectonics & Igneous Activity
Most volcanism occurs along tectonic plate boundaries
Divergent boundaries: decompression melting
Subduction zones: hydration melting

37. Plate Tectonics & Igneous Activity
The “Ring of Fire” is a chain of active volcanoes around the Pacific Ocean. It marks the boundaries of tectonic plates.

38. Plate Tectonics & Igneous Activity
Hot spot (intraplate) volcanism
Not near plate boundaries
Fed by magma reservoirs beneath the crust

39. Plate Tectonics & Igneous Activity
Hawaiian Islands were formed by hot spot volcanism
North
Oahu
3.7 my
Molokai
1.9 my
Kauai
5.1 my
Maui
1.3 my
Hawaii (Big Island)
< 1 million yrs old

40. Plate Tectonics & Igneous Activity
Hot spot animation

41. End of Chapter 6