The cone is formed by alternate layering of lava flows and ash (or cinder) emissions. This means the type of eruption can vary from one episode to the next; from a relatively "quiet" extrusion of lava, to a more eruptive type emitting ash, gas, and steam. So, it's called a stratovolcano because of the characteristic layering, as much as it is called a "composite" volcano for the same reason.

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