Weathering is the process in which rocks are broken into smaller pieces. Small pieces may be moved away from the original rock when erosion occurs. There are three main types of weathering: physical, chemical and biological.
Physical forces can break rock into smaller pieces. This may be caused by expansion and contraction of rock as it heats and cools. If water seeps into a crack or pores in a rock, it expands when frozen and can break apart the rock.
This volcanic bomb was broken apart by frost wedging in Alaska. (MFitz, Katmai National Park and Preserve 2010, Public domain)
Wind and waves shape rocks at seashores and in deserts into dramatic shapes. The process occurs more quickly as the wind or water carries sand and other sediments. The sediment pieces act as tiny hammers, bashing away at the rock and creating more sediments (broken rock material). Over time, running water can carve dramatic landscapes.
The bluffs at Torrey Pines State Natural Reserve are shaped by wind and waves.
The most common causes of chemical weathering are acidic water and oxygen. Acidic water moves through cracks in limestone or marble, gradually dissolving rock and creating extensive cave systems like Jenolan Caves.
Jenolan Caves were carved out by chemical weathering of limestone due to acidic water. (CSIRO Science Image)
Oxidation is the reaction of oxygen with minerals to create new substances such as the iron oxide which can be found on the surface of many of Australia’s rocks. You may be able to spot oxidation and other signs of chemical weathering on the surface of broken rocks. Weathering causes colour to change.
This sandstone has a weathered outside that is much darker in colour than the inside.
- How do limestone caves form? (Jenolan Caves)
Living things are responsible for biological weathering. This may have a chemical component, like the weathering caused by plants. Plants release a weak acid that dissolves minerals in the rock. The minerals can be used by the plant for nutrients and the acid makes little holes for rootlets to grow into.
Lichens and moss release weak acids that gradually dissolve minerals in the rock.
Animals and plants also cause physical weathering of rock. Tree roots grow into cracks in rock and pry the rock apart. Some animals dig holes in rocks by scraping away the grains. This may be to create protective holes or as a side effect of feeding behaviour. Animals also cause physical weathering by tracking. Rock is worn away on tracks that are often used like stone stairways in popular parks.
Chitons have a rasping tongue with teeth on it called a radula that scrapes algae off rocks, but also scrapes away some of the rock.
Sandstone steps in the Blue Mountains have been worn down by thousands of hikers.
The most extreme example of biological weathering is a type of shipworm described in 2019. This ‘worm’ is actually a clam that eats limestone.
Explore weathering at home
BBC Bitesize Science provides a written summary of weathering and the rock cycle.
Observe the effect of oxidation.