Weathering, which is the process by which rocks and minerals are broken down, causes deterioration of otherwise strong materials. Weathering is caused by water, ice, temperature changes, salt, chemical processes, plants and animals.
Weathering damages national monuments, historic buildings and landmarks. It causes rust, cracks, crumbling and other imperfections to stone, marble, wood and other materials.
Mechanical weathering breaks rocks into smaller pieces. Temperature change causes thermal stress on rocks as they expand and contract, which leads to crumbling and cracks forming. Water collects inside these cracks and then freezes, causing further damage. Likewise, saltwater crystallizes and creates more fractures. Plants grow through the cracks and animals dig in the soil, further breaking apart and dislodging structures.
Chemical processes, like the oxidation of iron, cause unsightly rust. Carbonic acid, a result of carbon dioxide mixing with water, dissolves limestone. Acid rain also wears down stone.