These deposits, called travertine, are formed at the rim of the spring pool. A time passes, a series of terraced, shallow pools results. Here's a Web site example of travertine terraces. Then, when they cool upon reaching the surface, the silica is dropped as a spongy material. This is very similar to the mechanism of mid-ocean ridge tower building.
Geologists are a prolific bunch when it comes to web publishing.
Have your students search for the different forms and make comparisons to see the similarities among them. There are numerous experiments for middle through High School grade that explore solutions and precipitation of solids from solution. You can begin with the simple evaporation of a saucer of salt water to form large salt crystals. I particularly recommend number 12, Solutions, as an introduction to solutions. These crystals form hollow towers of silicate of various colors as they grow in a solution of sodium silicate.