Spray the inside of a large saucepan with cooking spray.

Heat butter in saucepan on low, stirring constantly until melted.

Add marshmallows. Stir constantly until melted.

Add Cocoa Pebbles cereal and remove pan from heat. Stir until mixed well.

Pour onto waxed paper (or parchment paper sprayed with cooking spray). Cool completely.

Shape into a volcano.

Use Fruity Pebbles as the "lava" for the volcano, adding to the top and down the sides if desired.

Eat immediately or store in tightly covered dish.