A beach is a landform along the shoreline of a sea or lake. It is usually made of sand, gravel, cobbles, rock and seashells. Beach material accumulates during periods of accretion and moves away during periods of erosion.
Beaches are constantly changing, and this is partly why they are so interesting. Coastal processes can alter the shape of a beach within hours, and even over the course of a few years. Beaches are usually made up of sediment – particles smaller than grains of sand – that has been eroded from other landforms and carried by waves and currents.
There are many ways that a beach can be changed: attrition, in which rocks rub together; hydraulic action, in which water is driven into cracks; and corrosion, in which the chemicals of weathering (e.g., sulfates and nitrates from fertilizers and sewage) wear away at the beach’s surface.
Another way a beach can change is by the formation of a sandbar. A sandbar is a submerged or partly exposed ridge of sand built up offshore from a beach by the swirling turbulence created as waves break over it. When the current pushing from the coast meets a sandbar, sand is deposited on its flank, and over time a bar forms.
Beaches can also be eroded by the constant beating of waves against rocky cliffs. This erodes the softer rock at a faster rate than the harder rock above it. The result is that the cliff becomes less steep and the beach below it extends farther into the ocean.
The most common kind of beach is a continental shelf beach, which is a sandy shoreline that separates a lagoon from the open sea. These beaches are often home to large numbers of marine species. A beach can also be a dangerous place, as it is a major route for chemical and sewage pollution to reach the ocean. This contamination may cause sickness in humans and wildlife.
There are three kinds of beaches: a strip of sediment bordering a rocky or cliffy coastline, a large sandy plain of marine or fluvial accumulation (free beaches) and a narrow sediment barrier stretching for dozens to hundreds of kilometers along a coast (barrier islands). The shape and contour of these differ according to the kinds and quantities of sediments involved, the rates of their delivery, and the actions of waves and currents. Well-stabilized beaches tend to accrete, while unstabilized ones tend to erode. Freak wave events can substantially change the shape and composition of a beach within hours.