It's a form of weathering. If water gets into a crack in a rock and then freezes, it will expand, causing the crack to get bigger, which can happen continuously.
Water seeps into cracks in rocks, freezes, then expands (since ice occupies a slightly larger volume than liquid water). This expansion pushes the sides of the cracks further apart. ... Rocks can slowly break down by a mechanism known as freeze-thaw weathering, or alternatively, frost wedging.
