Plants and animals become agents of mechanical weathering when their growth, activities or movements expose rocks to the weathering actions of wind, rain and ice. The roots of plants, particularly large trees, can shift the soil and lift or crack rocks that block their paths. Animals often dig tunnels that cause the same effects, but their burrowing, foraging and den-making activities can also cause rocks to become exposed.
Examples of plants moving heavy items can be seen on many old driveways and sidewalks. As large trees grow, and their roots undermine these concrete structures, they cause them to move or crack. The same principle occurs when roots move under or around large rocks in a natural setting. Trees and large plants do not cause these kinds of changes quickly. They take many years or decades to do so.
Animals that are agents of weathering are almost always burrowing or tunneling creatures. Moles, voles, rodents and frogs may dig near an area’s bedrock and expose the rock to the air inside the tunnel. Merely exposing the rocks to the air causes some weathering over time. However, if this tunnel later fills with water or freezes, the damage is even more severe.