Chemical weathering transforms the makeup of rock and sediments to a softer, weaker material. Heat is often the cause of chemical weathering, as warm molecules undergo important physical changes. Wood that is hard and green becomes dry and seasoned, perfect for burning, after being exposed to the sun for some time. Rock wears in a similar way because of chemical changes.
The causes of physical weathering include abrasion, crystallization, thermal insolation, wetting and drying, and pressure release. Some forces originate within the sediment, while others are applied externally, causing the physical breakdown of rock. The wetting and drying pattern of rock, known as slaking, causes layers of moisture to build up between rock grains. Eventually the interior pressure is too great and causes breaking and cracking, which leads to deterioration.
"Biological weathering involves the disintegration of rock and mineral due to the chemical and/or physical agents of an organism. The types of organisms that can cause weathering range from bacteria to plants to animals," according to the Physical Geography Fundamentals eBook. The process of chelation is an example of biological weathering where organic substances, known as chelates, are produced with the ability to decompose minerals and rocks.