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A composite volcano, such as Krakatoa,  is formed when runny lava escapes through a fissure in the earths crust and spreads out .  This occurence happens everytime the  volcano erupts creating layers of cooled lava and ash.  These types of volcanos are typicaly tall and have steep sides.  This piticular type of volacano also is commonly found near convergent boudries in the earth's crust.  This type of volcano can also be called a stratovolcano.
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Check out the simulation at this webstie: http://kids.dicovery.com/games/pompei/pompei.html
Some key terms:

Pyroclastic flow: Hot gas and rocks that rush away from the volcano after an eruption.

Stratovolcano: A tall, steep volcano, also called a composite volcano.

Convergent boundry: A place in the earth's crust wher two plates collide.