Physical weathering takes place without causing chemical changes in the composition of the rock. Sand, rocks or other particles carried by streams, wind or glaciers can scrape the surface of rocks and abrade them to make them smaller. Since ice is less dense than water, when water periodically seeps into cracks in rocks and freezes it expands and broadens the cracks. Some minerals within rocks also expand at different rates with changing temperatures; the resulting pressure can gradually fracture the rock.
Unlike physical weathering, chemical weathering involves reactions that change the composition of rock. Oxygen in the atmosphere can react with iron and other metallic elements to form oxides like iron(III) oxide or rust; typically these oxides are weaker than the parent rock and crumble easily. Acids, like the weak carbonic acid in rainwater, can react with some rocks like limestone (calcium carbonate) to form compounds that can dissolve in water. Sometimes living organisms, too, can play a role in weathering: Plants and bacteria can release acids that react with the rock; lichens can take hold on bare rock surfaces and gradually break them down; and tree roots or other plants can force their way into crevices and split the rock over time.
Weathering plays an important part in the phosphorus cycle. Phosphorus is an essential nutrient for life; the molecules in cell membranes, the nucleotides in DNA and the nucleoside triphosphates that cells use to store energy all contain phosphate groups. Most of earth's phosphorus, however, is stored in rocks. Over time, weathering releases phosphorus in the form of phosphate ions (a phosphorus atom covalently bonded with four oxygens and a negative -2 charge, PO4). Organisms like plants and bacteria can take up phosphates and pass this essential nutrient up the food chain. Once phosphate compounds are dissolved in lakes, rivers or oceans they eventually precipitate, settle to the bottom and are compressed with other sediments to form sedimentary rock, starting the cycle all over again.
As weathering breaks rocks down into smaller particles like sand and dirt, given the right conditions, biological activities can add organic material to form soil. Without the soil we use to grow our crops, it would be difficult if not impossible to feed Earth's human --- let alone animal --- population.
The same processes that act on minerals and rocks in nature can also deface human structures, particularly those built of stonework, and cause urban weathering. Dissolved carbon dioxide and sulphur dioxide can form acids in rainwater that slowly eat away at stone buildings and statues; frost wedging can fracture and crack pavements and roadways; and lichens and moss can colonize and alter sheer surfaces. According to "Essential Environment," anthropogenic air pollution has accelerated the rate of urban weathering.