1. VOLCANOES

2. What is a volcano?

3. A volcano is a vent or fissure in the Earth's surface through which magma and gases are expelled.

4. Where are volcanoes located?

5. MOST of the world's active volcanoes occur at the plate boundaries as shown in the map below.
The blue lines below show the plate boundaries. The yellow dots show recent earthquakes and the red triangles show active volcanoes. 5

6. Volcanism at Plate Boundaries
A. Spreading-Center Volcanism: occurs at divergent boundaries
B. Subduction Zone Volcanism: occurs at convergent boundaries
C. Intra-plate (Hot Spot) Volcanism: occurs within the plates 6

7. A. Spreading Center Volcanism
As plates move apart at divergent boundaries, magma seeps through fissures in the ocean floor and creates new oceanic crust. 7

8. B. Subduction Zone Volcanism
Volcanoes at subduction zones can form island arcs or continental volcanic arcs.
Ex: Aleutian Islands, Alaska 8

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11. C. Intraplate (Hot Spot) Volcanism
Hot spots are areas that are thought to be caused by abnormally hot plumes in the mantle. 11

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13. How do volcanoes erupt?

14. Volcanic eruptions are driven by the ascent of magma (molten rock) from deep beneath the surface. There are different types of magma. Magma is classified based on the amount of silica it contains. 14

15. Magma = molten rock beneath the Earth's surface
Lava = magma that flows on Earth's surface

16. Non-explosive Eruptions
There are two types of volcanic eruptions.
Non-explosive Eruptions
Explosive Eruptions
The type of eruption depends on several factors including viscosity, temperature, and the amount of dissolved gasses in the magma. 16

17. Explosive Eruptions
Explosive eruptions are violent eruptions of rocks, dust, gas, and pyroclastic material.
They are rare, but can be very destructive.
Ash from explosive eruptions can remain in the atmosphere for years causing climate change.
Mount St Helens – May, 18, 1980 17

18. Nonexplosive Eruptions
Nonexplosive eruptions are relatively calm and do not typically involve a lot of pyroclastic material. 18

19. Are all volcanoes the same?

20. There are three types of volcanoes
Shield Volcanoes
Cinder Cone Volcanoes
Composite Volcanoes (Stratovolcanoes) 20

21. Shield Volcanoes
Characterized by runny liquid lava that comes from a large central vent and spreads out over a large area.
Non-explosive eruptions
Examples: Hawaiian Islands 21

22. Cinder Cone Volcanoes Made by moderately explosive eruptions.
Usually small but can build up to form shield volcanoes, erode quickly
Pyroclastic material forms steep sides. 22

23. Stratovolcanoes (Composite)
Made by alternating explosive and nonexplosive eruptions.
More viscous lava flows from the central vent
Lots of pyroclastic material from explosive eruptions.
Usually large, broad bases and steep sides. 23