Joints form free space in rock by which other agents of chemical or physical weathering can enter. Crystal Growth - As water percolates through fractures and pore spaces it may contain ions that precipitate to form crystals. As these crystals grow they may exert an outward force that can expand or weaken rocks. Plant and Animal Activities - Plant roots can extend into fractures and grow, causing expansion of the fracture. Growth of plants can break rock - look at the sidewalks of New Orleans for an example.
Animals burrowing or moving through cracks can break rock.
As the water freezes it expands and exerts a force on its surroundings. Frost wedging is more prevalent at high altitudes where there may be many freeze-thaw cycles. Since many rocks and minerals are formed under conditions present deep within the Earth, when they arrive near the surface as a result of uplift and erosion, they encounter conditions very different from those under which they originally formed. Because of these different conditions, minerals in rocks react with their new environment to produce new minerals that are stable under conditions near the surface. These are igneous minerals that crystallize from a liquid.