Geologists recognize two categories of weathering processes Physical Weathering - disintegration of rocks and minerals by a physical or mechanical process. Chemical Weathering - chemical alteration or decomposition of rocks and minerals. Although we separate these processes, as we will see, both work together to break down rocks and minerals to smaller fragments or to minerals more stable near the Earth's surface. Physical Weathering Physical weathering takes place by a variety of processes. Joints form as a result of expansion due to cooling or relief of pressure as overlying rocks are removed by erosion.
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.