1. Chapter 12 Section 2 Types of Volcanoes

2. What Controls eruptions Two factors control the type of eruption. 1. Amount of water vapor and other gases. 2. How much silica is present in the magma

3. Trapped Gases Trapped gases and water vapor are trapped in the magma. As magma rises to the top there is less pressure so gases and water vapor escape. Quiet eruption- little gas or it is under less pressure Explosive eruption- more gas or it is under high pressure

4. Water vapor If there a great deal of water vapor especially at an oceanic and continental plate boundary then there will be an explosive eruption.

5. Composition of magma Silica rich or silica poor Magma that is low in silica is Basaltic magma. Magma that is high in silica is Granitic magma

6. Quiet eruptions Low in silica Fluid Pours from the vents and runs down the sides of the volcano Pahoehoe (pa HOY hoy) lava cools and forms a rope like structure If this lava flows a a lower temperature then it will form AA lava (AH ah)

7. Explosive magma Rich in silica Granitic magma Thick Many gases trapped in it Pressure builds up

8. Andesitic magma It is in between the other two magmas It has more silica than Basaltic magma but less than Granitic. The eruption is not necessarily quiet but it is not explosive either. It is somewhere in between.

9. Forms of volcanoes Shield Cinder Cone Composite volcano

10. Shield Volcanoes Broad volcano with gently sloping sides. Hawaiian islands are examples Shield volcanoes generally form from quiet eruptions and basaltic magma

11. Cinder cone volcanoes Explosive eruptions throw lava and rock high into the air. Tephra- bits of solidified rock or lava Forms a steeped sided, loosely packed cinder cone volcano

12. Composite volcano Some eruptions vary between quiet and explosive. An explosive period releases tephra. A quiet period releases lava which covers the tephra. These alternating layers of tephra and lava form a composite volcano.

13. Composite volcano picture