Beaches are generally defined as a narrow, gently sloping strip of land, usually situated between a standing body of water and the ocean. They can be made of sand, rocks, or other materials. The shape of a beach depends on the nature of the material, the type of waves, and the prevailing tides. Some sandy beaches are constructed with manmade infrastructure, such as hotels, lifeguard posts, or showers. Others are not, and may have been formed from local bedrock. Some are made up of mud, and even contain shells or seaweed. Some are polluted, and may have bacteria that can be harmful to humans.
In the past, beaches were formed by natural processes. Today, however, human forces have significantly altered beaches around the world. These impacts include bad construction practices, pollution, coastal erosion from sea level rise, and indirect effects such as plastic pollution. These indirect effects also contribute to climate change. Nevertheless, there are many examples of beaches that still remain in excellent condition.
In the United States, the Sea Gate community in Coney Island, New York built a series of seawalls to protect it from powerful storms. Similarly, the Brighton beach in England became a centre of upper-class pleasure and frivolity in the nineteenth century. At the beginning of the twentieth century, surfing began to develop in Australia and Hawaii. In the early 1960s, surfing began to spread to southern California.
Typically, beaches are composed of sand, but they can also be made up of rock fragments, mud, or other materials. The amount of sand on a beach is dependent on the size of the sea, the prevailing tides, and the wind. Some beaches are actively restored by practices such as beach nourishment. If the sand is too concentrated, it can cause scouring behind it.
The majority of the beach materials are produced by weathering and headland erosion. These sediments can be transported hundreds of kilometers from the ocean by the ocean currents. They can also be derived from biologic components of the nearby ocean. Some of these materials can be very toxic, and can take days to wash out to the ocean. Some of them can also be eroded before they can integrate into the established vegetation.
In the United States, the beaches have changed significantly since the late 19th century. Commercial sea bathing, and beach resorts, grew in popularity. These changes were driven by a trend that evolved in the early 1900s: an elevated, romantic ideal of picturesque landscape. Other gems are present on many beaches around the globe. For example, Punaluu Beach on the island of Hawaii is made of basalt. These sediments are cooled instantly when they enter the ocean, but they are eroded by waves and sand before they can be preserved in the sedimentary rock.
The beaches on Tuvalu in the 1990s were highly retreating. This was due in part to an El Nino-Southern Oscillation (ENSO) in the Pacific. Some beaches were reshaped by a freak wave event.