1. Warm Up Why was the Mt. St. Helens Eruption so destructive?
What about the explosion was not predicted by most scientists?

2. Volcanoes
Earth Science

3. What is a Volcano?
Volcano – A structure which moves magma onto the surface of the Earth
Magma – liquid rock formed from heat inside the Earth
Lava – Magma that erupts onto the Earth’s surface

4. Critical Thinking #1
Considering the prior definitions, explain how a volcano first forms.

5. How Volcanoes Form
1. Plate Boundaries – Subduction and Seafloor Spreading
2. Hot Spots – Extremely hot intraplate regions where mantle rises

6. Where are Volcanoes Located?
Most located along the “Ring of Fire” - zone of volcanic activity along edge of Pacific Ocean

7. Critical Thinking #2
Why would oceanic-continental convergent boundaries produce a great number of volcanoes?

8. Types of Lava
Mafic Lava – forms oceanic crust, dark colored, generally runny
Felsic Lava – forms continental crust, light colored, generally thick

9. Two Forms of Mafic Lava
Pahoehoe – means “ropy”, formed when mafic lava cools slowly

10. Two Forms of Mafic Lava
AA – jagged chunks, formed when mafic lava cools rapidly

11. Critical Thinking #3
What factors would determine the speed at which mafic lava hardens?

12. Pyroclastics
Pyroclastics – Rock fragments made of hardened felsic lava, types based on size
Volcanic Dust – Less than 0.25mm in diameter
Volcanic Ash – mm in diameter

13. Pyroclastics Lapilli – 2-64mm in diameter
Volcanic Bombs - >64mm in diameter

14. Critical Thinking #4
What factors would determine the size of pyroclasts produced?

15. Viscosity and Gas Content
Viscosity – Ability to resist flow
Determined by amount of Silica
Low Viscosity = Thin lava
Medium Viscosity = Medium Thickness
High Viscosity = Thick lava
Gas Content – Determines Explosiveness
High Gas Content = Most Explosive
Low Gas Content = Least Explosive

16. Critical Thinking #5
What is the average viscosity of mafic and felsic lava? What does this tell you about the silica content of mafic and felsic lava?

17. Critical Thinking #6
What is the relationship between gas content and viscosity?

18. Three Types of Volcanoes
1. Shield Volcano
2. Cinder Cone
3. Composite Volcano (Stratovolcano)

19. Shield Volcano Broad, gently slopped (900)m High)
Eruption of fluid low viscosity mafic lava through multiple vents
Ex: Kilauea (Hawaii)

20. Cinder Cone Steep cone-shaped volcano (500m high)
Medium-viscosity Lava fragments (Cinders) ejected from single vent
Ex: Paricutin (Mexico)

21. Composite Volcano Large, often snow-capped peaks (3000m high)
Most explosive eruptions, high viscosity lava and pyroclasts
Ex: Mt. St. Helens (Washington)

22. Critical Thinking #7
Which volcano type would likely be the most dangerous? Explain why.

23. Craters and Calderas
Crater – Funnel-shaped pit at top of volcanic vent
Formed when material is blown out of a volcano by explosions
Caldera – Large basin-shaped depression
Formed when magma chamber is emptied and entire volcanic cone is collapsed

24. Critical Thinking #8
In what situation would a crater form? In what situation would a caldera form?

25. Mount St. Helens
Mount St. Helens – Composite Volcano (Stratovolcano) in Washington State
Erupted in 1980 after being dormant – “inactive” for 120 years
North face of mountain experiences largest ever recorded debris landslide
Pyroclastic flow from eruption destroyed 230 square miles
57 people killed from the blast

26. Critical Thinking #9
What factors would create the greatest landslide in a volcanic eruption?

27. Critical Thinking #10
What aspect of the eruption of Mount St. Helens caused the greatest damage and casualties?