A beach is a large area of loose rock (sand) that gathers along the shore of a body of water. The sand comes from erosion of rocks both far away and close to the sea; in addition, coral reefs are a significant source. Beaches are created by waves and currents that carry sand particles, but the shape of a beach varies over time due to forces that are difficult to see.
The grain size of sand determines the slope of a beach, with sandy beaches having gentler slopes than gravelly ones. The sand is also affected by the amount of water that flows up from the beach. This water carries the particles of sand in a process called backwash, and this causes the beach to flatten and widen. The amount of water that moves up and down the beach is determined by wind conditions. On windy days, the beach can be much narrower than on calm days.
Many beaches are popular places for recreation, and they may have amenities such as lifeguard posts, changing rooms, showers, shacks and bars, and food vendors. These activities often cause the beach to be crowded, especially in summer and during holidays when vacationers crowd the coastlines. Beaches may be patrolled by beach guards or by local police, and they may have signs warning visitors of dangers, including rip currents.
Sand is a white, black or tan color; this depends on the type of rock from which it comes and its mineral composition. Beach sand typically includes quartz and feldspar, but it may also include a small percentage of minerals such as titanium, aluminum or magnesium. Some beaches are composed of calcareous rock, consisting of the skeletal remains of marine organisms and precipitated calcium carbonate; these are often found in the tropics.
Beach sand usually erodes very slowly, and erosion accelerates if the beach is exposed to wave action, especially in strong winds. This is because the softer sediments are more easily carried away by the waves. To slow erosion, sand can be replenished by dumping it from boats or by a process called beach nourishment. If the new material is not placed properly, it can encourage scouring in front of the beach or may introduce foreign materials that can alter the morphology of the beach.
If erosion reaches the base of the beach, it can create an ocean bank, which cuts off the beach from open water. This typically occurs where the current pushing from the coast meets the current flowing toward the coast. Some beaches are composed of a series of ocean banks separated by bay-mouth bars.