1. Break out the chicken wire to make the shape. Staple to a wooden base. You can make a dip on the top to place a plastic cup inside to hold the "lava". If you wnat the "lava" to run down the sides you might want to create a "catch" area around the base to catch the mixture.
Feel free to try out the lava mixture as a dry run.
mixture: 1. steam only, place dry ice into a smal amount of water, or 2. baking soda and water mixture if you want it to cascade down the sides
2. Cut up strips of newspaper that are about 1-1.5 inches wide and as lon as you would like
3. Mix up paper mache or a flour/water mix, place newspaper strips into this and lay on wire frame. Repeat process until covered.
4. Once dry paint.
5. Decorate base with moss, small trees, etc
6. You can have them steam during the event or place the baking soda in a separate cup that can be poured into the water during the event.
HTH.