Weathering refers to the process of wearing away, dissolving or breaking down rocks that are found near or at the surface of the Earth. National Geographic notes three types of weathering processes: mechanical, chemical and biological or organic.
According to the Plant and Soil Sciences eLibrary, mechanical weathering happens when rocks are broken down into smaller and smaller pieces without changes in their chemical components. One of the causes of mechanical weathering is the constant freezing and thawing of water. In its liquid form, water can seep into cracks and crevices in rocks. As the temperature drops, the water freezes and expands. The ice then acts as a wedge, slowly widening the cracks and splitting the rocks.
When the water thaws, it penetrates deeper into cracks and crevices where it can freeze again. As water thaws and freezes over and over, the process slowly creates larger holes and cracks in the rocks. Eventually, the rocks crumble.
Salt water can sometimes seep into rocks, according to National Geographic. As salt water evaporates, salt crystals are left behind. Over time, the salt crystals build up, putting pressure on the rock and causing it to split and crack.
Changes in temperature also cause weathering. Rock expands as it is heated and contracts when cooled. As this happens repeatedly, the rock weakens and eventually crumbles.
Chemical weathering refers to the weakening and the subsequent breakdown of rocks by chemical reactions. According to National Geographic, these reactions cause changes in a rock's chemical composition. Carbonic acid, formed when water in the air or soil combines with carbon dioxide, breaks down minerals in rocks. Carbonic is particularly effective at dissolving lime. Limestone caves are formed through this process.
Another chemical reaction that causes weathering is oxidation. Oxidation occurs when rock minerals react with oxygen. Iron is a common mineral found in rocks. Iron oxidizes to form rust. Rust weakens the rock and helps to split it apart.
Humans, plants and animals can wear away rocks, notes National Geographic. This is called biological weathering. For instance, a rabbit can burrow into a crack in a rock, causing it to split or crumble. Plant roots grow into cracks in a rock and exert pressure on it, eventually causing the rock to break apart. Humans cause weathering by walking over paths. Over time, the many feet that tread over the rocks eventually wear them down.