There are already some small cracks in boulders, water gets in the cracks, when that water expands the rock is crunched instead of breaking apart.
(the rock; of breaking; apart) (water; gets; in the cracks; T:when that water expands the rock is crunched instead of breaking apart)
the rock water
of breaking gets
apart
in the cracks T:when that water expands the rock is crunched instead of breaking apart