Weathering breaks rock down into smaller pieces. Granite resists weathering more than layered and bedded sedimentary rocks do. Rock outcrops are constantly subjected to the elements, which gradually cause chemical changes in the rock's minerals. Oxygen and carbon dioxide in runoff waters produce chemical weathering . Physical weathering occurs when water, lodged in fractures in the rock, freezes and expands in all directions, forcing the sides of the fractures apart.
Tree roots have the same effect.
Rocks exposed to long periods of alternating heat and freezing disintegrate into sand and clay. To preserve the Mount Rushmore sculpture, experts needed to predict which of the granite blocks would shift. First, they shot a series of 300 overlapping photographs of the monument from precision cameras mounted on an airplane and a helicopter. From data provided by these photographs, a computer created 3-D projections of the internal fracture system. With these images, preservationists can estimate the potential for damage to the monument.