1. Composite Volcanoes
Review: What characteristics make a composite volcano?
Answer: High Gas, High Silica, High Viscosity

2. Convergent boundary What are the 3 types of convergent boundaries?
Ocean – Ocean
Ocean – continental
continental – continental
Which boundaries produce a subduction zones?
Ocean – Ocean produce volcanic island arcs….. Why?
Ocean – continental produce continental volcanic arcs…… why?
What happens at continental – continental boundary??? Why?

3. Ocean vs. Continental Crusts
Continental crust (CC) is much thicker than Oceanic crust (OC) and composed of rocks with much higher silica content. As the silica rich rock melts in the subduction zone the magma type changes.
Andesitic rock – has a higher silica content than basaltic rock but lower than rhyolitic magma.
Ex; Juan de Fuca plate boundaries with the North American plate.

4. Example
Ring of Fire. Volcanoes formed around the pacific plate are referred to as the “Ring of Fire”.

5. Ocean vs. ocean Plates have same density The OO subduction zone:
Rich in silica = viscous lava flowing.
How does this work???

6. Characteristics 1. tall with steep slopes – nearly symmetrical.
2. violent loud eruptions
Pyroclastic flows (huge amounts released – hot gases and flowing , and large rock).
From Crater? (how caused)
Lava flow? ( viscosity?)

7. Dangers of composite volcano
Steep slopes allow lava to fall freely down
Mixed with water from craters, causing mudflows, called Lahars, ( which can happen even without volcanic eruption) form from volcanic debris being saturated in water.

8. Examples
Mt. Shasta
Mt. St. Helens