Microscopic Spaces and Erosion
Microscopic spaces between the grains of a rock are numerous. These spaces act like microscopic caves, allowing water to flow freely through them.
As water travels through these spaces, it causing weathering, as chemicals are carried with it that can loosen the bonds between grains, and help to break the rock into smaller pieces. In some cases, the rocks become so weak, that they can be crumbled with the force of a bare hand.