Here's a recipe from ScienceArts (by Kohl, Bright Ring Publishing). But this
is certainly not an original idea!!
soda bottle, baking pan, moist soil
1 tablespoon of baking soda
1 cup vinegar
red food coloring
Place the baking pan on the grass, and set the soda bottle in the center f
the pan. Mound and shape the moist soil or sand around the bottle to form a
mountain volcano. Bring the soil right up to the top of the bottles opening,
but not inside the bottle.
Pour 1 T. baking soda into the bottle.
Color 1 cup vinegar, and pour it into the bottle. Stand back and watch thew
red foam spray out the top and down the sides of the volcano.
I have also seen this done with "mini volcanoes" in film canisters with less
of everything. I don't have exact measurements on hand.