Composite Volcanoes
Review: What characteristics make a
composite volcano?
Answer: High Gas, High Silica, High
Viscosity
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?
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.
Example
• Ring of Fire. Volcanoes formed around the
pacific plate are referred to as the “Ring of
Fire”.
Ocean vs. ocean
• Plates have same density
• The OO subduction zone:
– Rich in silica = viscous lava flowing.
– How does this work???
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?)
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.
Examples
Mt. Shasta
Mt. St. Helens