1. Ch.12 - Volcanoes
Earth Science

2. Section 1 – Volcanoes and Earth’s Moving Plates
Learning Objectives:
Describe how volcanoes can affect people
List conditions that cause volcanoes to form.
Identify the relationship between volcanoes and Earth‘s moving plates.

3. Section 1 – Volcanoes and Earth’s Moving Plates
Volcano – opening in Earth that erupts gases, ash, and lava.

4. B. Volcanoes can kill people, destroy property, and disrupt the environment.
1. Lava and pyroclastic flows (very hot ash and debris) can bury cities and towns in their paths.

5. 2. Sulfurous gases from volcanoes can mix with water vapor and create acid rain, which can kill organisms and pollute water.

6. How do volcanoes form? (Not in notes)
Magma is less dense than the rock around it, so it is forced slowly toward Earth’s surface.

8. C. Volcanoes form when magma flows out of a surface opening called a vent.
A steep-walled depression around the vent is called a crater.

10. Where do Volcanoes form?
D. Volcanoes often form where plates are moving together, or moving apart, or at hot spots.

11. 1. The Mid-Atlantic Ridge is a divergent plate boundary that forms rifts through which lava can flow

12. 2. At convergent plate boundaries, volcanoes tend to erupt more violently than they do in other areas.

13. 3. At the boundary between Earth’s mantle and core, unusually hot areas form hot spots.

14. Example of a hot spot  Hawaiian Islands
The Pacific plate is moving over a stationary hot spot

15. Discussion Question Where do volcanoes tend to form?
Hint – 3 main areas

16. Discussion Question Where do volcanoes tend to form? Answer –
Where plates are moving together (convergent boundaries)
or moving apart (divergent boundaries)
or at hot spots

17. Section 2 – Types of Volcanoes
Learning Objectives:
Explain how the explosiveness of a volcanic eruption is related to the amount of silica and water vapor in its magma
List three forms of volcanoes

18. (Add to the top or bottom of your notes packet)
Two major factors that determine how violent a volcano will erupt…
1. Amount of gas or water vapor in magma
2. Amount of silica in magma
 Silica – compound made of the elements
silicon (Si) and oxygen (O)

19. A. The amount of water vapor and other gases present is one factor that determines whether a volcanic eruption will be quiet or explosive.

20. 1. Gases can be trapped in magma by pressure of surrounding magma and rock; eventually they cause an explosive eruption.
2. Magma at convergent boundaries can contain a lot of water vapor that can cause explosive eruptions.

21. B. The composition of magma
(specifically, the amount of silica) is a second factor affecting the nature of a volcano’s eruption.

22. 1. Low-silica magma, called basaltic, is fluid and produces a quiet, nonexplosive eruption.

23. a. Pahoehoe (Pa-HOY-hoy) lava runs down the side of a volcano  ropelike structure
b. Aa (AH-ah) lava is a stiff, slow moving lava.
 Same kind of lava, just different speeds

24. 2. High-silica magma is called granitic
Intermediate-silica magma is called andesitic
 They both produce explosive eruptions.

25. C. Three types of volcanoes form from the three types of lava.
 Lava flows, hardens, and builds up over time forming volcanoes.

26. 1. As quiet eruptions of basaltic lava spread out in flat layers, they form a broad volcano with gently sloping sides called a shield volcano.

27. 2. As tephra (bits of rocks or solidified lava) falls to the ground, it forms a steep-sided, loosely packed cinder cone volcano.
Formed from explosive eruptions

28. 3. A composite volcano forms from alternating layers of quiet lava and more explosive tephra. (Found at subuction zones)

29. Review: Cinder cone volcano Shield volcano Composite volcano
Basaltic lava)  Low silica
Quiet eruptions
Composite volcano
Low and high silica lava
Alternates between quiet and explosive eruptions
Cinder cone volcano
Granitic + intermediate lava)  High silica
Explosive eruptions

30. Composite
Shield

31. Discussion Question
What two factors account for the varying force of volcanic eruptions?

32. Discussion Question
What two factors account for the varying force of volcanic eruptions?
1. Amount of gas or water vapor
2. Amount of silica in magma

33. Section 3 – Igneous Rock Features
Learning Objectives:
Describe intrusive igneous rock features and how they form
Explain how a volcanic neck and a caldera form

34. Intrusive Features – are formed underground
Extrusive Features – are formed at Earth’s surface

35. A. Many intrusive igneous features form underground and are later exposed
1. Batholiths – rock bodies formed when magma bodies that are being forced upward from inside Earth cool slowly and solidify before reaching the surface.

37. Example of a batholith  Granite domes of Yosemite National Park
 Remember, Granite is a type of igneous rock which is formed from cooled magma

38. 2. Dike – magma that hardens after being forced into a crack cutting across rock layers

39. Sill – magma that hardens after being forced into a crack parallel to rock layers.

40. B. A volcanic neck forms when the cone of an old volcano is eroded away, leaving the solid igneous core. (Devil’s Tower, Wyoming)

41. Example of a volcanic neck 
Ship Rock, New Mexico

42. 1. Caldera – large depression formed when the top of a volcano collapses.

43. Example of a caldera 
Crater Lake, Oregon

44. 2. Weathering and erosion wear down surface rock and expose igneous rock features.

45. Discussion Question
How are a dike and sill different?

46. Discussion Question How are a dike and sill different?
Dike – forms from magma that hardens after being forced into a crack cutting across rocks
Sill – forms from magma that hardens after being forced into a crack running parallel with rocks