1. Volcanoes

2. What is a volcano?
A volcano is an opening in the Earth’s crust through which molten rock, gases, ash, dust, and cinders can erupt

4. Where do volcanoes occur?
1. Divergent Plate Boundaries
Examples: Iceland, Mid-Ocean Ridges
2. Subduction zones at Convergent Plate Boundaries
Examples: Mt. St. Helens, the Ring of Fire
3. Hot Spots
Examples: Hawaii

5. How does a Hot Spot form volcanic islands?

6. Ring of Fire

7. Eruptions and Magma

8. Two types of eruptions:
1. Explosive
2. Non-Explosive (Quiet)
Pyroclastic Flow (clouds of gas and ash that race down the sides of a volcano) during an explosive volcanic eruptions

9. Two factors that determine whether an eruption will be explosive or non-explosive:
The type of magma – felsic (granitic) or mafic (basaltic)
The amount of water vapor and other trapped gases in the magma.
more trapped gases = more explosive eruption

10. Two types of Magma

11. Felsic Magma Also known as granitic magma Light in color
Composed primarily of Silicon and Oxygen
Low density
High viscosity (thick, slow moving)
Explosive eruptions

12. Mafic Magma Also known as basaltic magma Dark in color
Composed primarily of Iron and Magnesium
High density
Low viscosity (runny, flows easily)
Quiet eruptions
2 types – pahoehoe and aa

13. Two types of Mafic Magma
AA (spiny, rough)
Pahoehoe (smoother)

14. Three Types of Volcanoes

15. Shield Volcano Shape – gentle slope, broad base
Composition – successive layers of lava
Features – quiet eruptions, lots of lava
Locations – hot spots and mid-ocean ridges
Examples – Hawaii and Iceland

16. Shield Volcano

17. Cinder Cone Shape – cone-shaped, very steep sides
Composition – tephra (ash, dust, cinders)
Features – very violent eruptions
Locations – subduction zones
Examples – Paracutin (Mexico) and
Krakatoa (Indonesia)

18. Cinder Cone

19. Composite (Stratovolcano)
Shape – cone-shaped, steep sides
Composition – alternating layers (tephra and lava)
Features – both explosive and quiet eruptions
Locations – on or near continents, subduction zones
Examples – Mt. St. Helens and Mt. Pinnatubo

20. Composite (Stratovolcano)

21. Intrusive Igneous Rock Structures (Plutons)

22. Igneous Intrusions
Batholith – the largest intrusive igneous rock structures
Sill – forms when magma squeezes into horizontal cracks in rocks and hardens
Dike – forms when magma squeezes into vertical cracks in rocks and hardens
Laccolith – forms when a sill pushes up on the rock layers above it to form a dome

23. Other volcanic structures:
Volcanic neck – the magma core of an eroded volcano (very resistant to erosion)

24. Other volcanic structures:
Caldera – forms when the top of a volcano collapses into a partially emptied magma chamber

25. Crater Lake in Oregon

26. Intrusive Formations
Sill

27. Dike

28. Columnar Jointing

29. Laccolith

30. Batholiths

31. Caldera

32. Pyroclastics from an explosive volcanic eruption

33. Lapilli

34. Blocks and Lava Bombs