A beach is a section of the seashore where unconsolidated sediment, or grains of worn-down rock, has collected. Unconsolidated sediment is a sediment in which the individual grains are clearly separated and can move freely, like grains of rice. In contrast, consolidated sediment is a sediment in which the individual grains cling together, like particles of moist flour or mud. Most beaches are composed primarily of sand (grains of quartz and other hard minerals between 0.063 mm/0.025 in. and 2 mm/0.08 in. in size), although some are composed primarily of pebbles or fragments of seashells. The unconsolidated and small sediments that make up a beach are easily moved by the action of waves and wind. Consequently, the shape of a beach is constantly changing as sand is removed from or deposited along the shoreline.

Sources of Sediment

The sands that form a beach are produced through erosion, or the wearing away of rocks by wind and water. In some cases, the erosion takes place far inland, where winds and water erode mountains and plains, producing sediment grains of many sizes. Rivers carry the sediment to the coast. Waves and water currents move the sediment along the coast, depositing some of it on beaches. For many beaches sediment from inland is the most important source of sand. However, beach sand may also result from erosion that occurs at the shore, especially at bluffs, as a result of the action of wind and waves.
Sand comes in a variety of colors, which indicate where the sand originated. The most common sand material carried in rivers is quartz, which ranges in color from pale yellow to brown. In tropical regions, the erosion of coral reefs may produce brilliant white beach sand. In contrast, beaches around volcanic islands, such as Hawaii, may have black sand, derived from the erosion of a black volcanic rock called basalt.

Transport of Sediment

The shape of a beach constantly changes as sediments are eroded from or deposited along the shoreline. The main mechanisms for moving sediment along a beach are longshore drift and wind transport.

Longshore Drift

Waves move much sand when they break on the shore. The direction in which the sand is moved depends on the direction at which the waves approach the shore. As an ocean wave approaches the shore, the size of the wave increases until the wave breaks. When the wave breaks, its forward motion creates a powerful surge of water, called the swash, that runs up onto the beach carrying sediment with it. Like a ball rolling up an incline, the surging water eventually comes to rest and reverses direction, rolling back down the beach under the pull of gravity. This return flow, called the backwash, also carries sediment. If waves approach a shoreline straight on, the cycles of swash and backwash repeatedly trace the same path up and down the beach. After each cycle, the sand is approximately back where it started.
When waves approach a shoreline at an angle, however, the swash advances up the slope of the beach at an angle, carrying sediment diagonally across the beach slope. The backwash, however, tends to retreat straight down the beach slope, carrying the sediment straight toward the sea. The next swash cycle picks up the sediment and moves it up the beach at an angle. After each cycle, the sand has moved a small distance along the seashore. The repeated cycle of swash and backwash advances the sand on a saw-tooth path along the shoreline. The resulting transport of sediment along the shoreline is known as longshore drift.

Wind Transport

Swift currents of air can move sand, and this movement often creates mounds of sand called sand dunes. A sand dune is started by an obstruction, such as a boulder or a clump of vegetation. The obstruction blocks the wind, creating a region of quieter air, called a wind shadow, downwind of the obstruction. The movement of air within the wind shadow is too weak to keep the sand grains aloft, so any windblown sand that enters the wind shadow quickly falls to the ground. The accumulation of sand in the wind shadow forms a dune.
Once formed, dunes migrate. On the upwind face, where the wind meets the dune head-on, the wind pushes sand grains up the dune slope. These grains are eventually pushed over the top of the dune and accumulate in the wind shadow behind the dune. The persistent movement of sand from the front face to the rear face causes the dune to migrate downwind.

Where Sediment Ends Up

Just as the wind and the waves can bring sand to a beach, and just as they can move it along a beach, they can also remove it from a beach. Wind can blow sand inland or out to sea. Longshore drift can move sand past the end of a beach, carrying the sand into shallow water. Currents in shallow water may continue to move sand along, perhaps depositing it on the next beach down the coast. Ultimately, much of the sand on a shore is carried to the head of a submarine canyon, a steep-sided valley underwater that leads down to the deep ocean. The sand travels down the canyon's slope and accumulates on the floor of the deep ocean.

Beach Equilibrium

A beach may appear the same from week to week. However, the individual sand grains that make up the beach are not the same because the grains are steadily transported along the shore by longshore drift and the wind. The beach is maintained because the longshore drift or the wind supplies sand at the updrift, or upwind, end of the beach about as rapidly as it removes sand from the downdrift, or downwind, end. A beach may be regarded as a system, or an assembly of individual elements (in this case, sand grains) that make up a complex whole. A system, such as a beach, whose individual elements (sand grains) are constantly changing while the system as a whole (the beach profile) remains constant is said to be in dynamic equilibrium.
The dynamic equilibrium of a beach can be disrupted if the longshore drift is cut off by a human-made obstruction such as a groin. A groin is a barrier that juts out into the water. The groin halts the longshore drift so that sediment accumulates on the updrift side of the barrier. While this accumulation benefits the beach located directly updrift of the groin, it hurts the beach located downdrift by depriving it of an essential source of sand. Longshore drift continues to remove sand from the downdrift beach, which erodes because no new sand arrives from the updrift direction.