Step 1: The Shape
Start by placing four blocks of any source on the ground.
Step 2: The Structure
Now build your way up to your desired hight, the taller your volcano the longer the eruption takes.
Step 3: Spawn Some Mobs
Grab a spawn egg (I used sheep but most eggs work) and spawn at least one hundred mobs, your world should start to lag, this means your volcano is ready to use.
Step 4: Add Water
Place a bucket of water on the very top of the volcano. Now stand back.
Step 5: Wait...
You should see a a black thing moving up the volcano when it gets to the top your volcano will erupt.
Step 6: The Eruption
Watch as your mobs burst out the top of the volcano.