1. Eruptions and Forms of Volcanoes

2. Types of Eruptions Violent and explosive Quiet and flowing
Depends on trapped gases and magma composition

5. Trapped Gases Water vapor and carbon dioxide are trapped in magma
At low pressure, they escape quietly when they reach the surface
At high pressure, they escape violently when they reach the surface

6. Magma Composition Two types:
Basaltic – less silica and very fluid; produces quiet eruptions
Granitic – lots of silica, high water vapor content, and very thick; produces very violent eruptions

7. Forms of Volcanoes
Three forms:
Shield
Cinder Cone
Composite

8. Shield Volcano Formed by quiet eruptions
Basaltic lava builds up in flat layers
Gently sloping sides
Ex: Hawaiian Islands

10. Cinder Cone Volcano Caused by explosive eruptions
Granitic lava thrown high into the air
Lava cools into different sizes of volcanic material called tephra
Steep-sided, loose slopes

12. Composite Volcano A mix of the other two types Quiet or violent
Basaltic or granitic
Steep or gentle slopes
Layered or tephra