The water makes the fractures, and the sand keeps them open. There’s some other stuff in that fracking blend, too: biocides, for one thing, not very different from what’s in your swimming pool, to keep bacteria and algae and other gunk from growing in the water and clogging up the works. There are also some friction reducers, because water and sand moving at speed can produce a lot of wear and tear (cf. the Grand Canyon), and the occasional jolt of 7 percent hydrochloric acid solution for boring out holes in the concrete. The mix is 99+ percent water and sand, and the rest of the stuff is mostly run-of-the-mill industrial chemicals (those friction-reducers use a polymer that also is used in children’s toys, for example). Real concerns, but not exactly an insurmountable environmental challenge.