Just make a copy of the Meteor and Earthquake spell and negate it's effects. Then let a dummy unit cast them. For the Volcano, you just make a unit with the volcano model, set flying height so the volcano is just below terrain, and then create the volcano somewhere and slowly set the flying height to 0. Then it will raise from the ground. You can make it play the animation to erupt (don't know which animation that is, but I assume you can figure that out yourself).