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.