Coasts and beaches

Walk to the edge of the land and you reach the sea.

That edge is the coast. Some coasts have tall rocky cliffs. Some have soft, sandy beaches. Waves splash on them all day and all night.

A coast is the place where the land meets the sea. Some coasts are tall, rocky cliffs. Others are long, sandy beaches, marshes or islands made of sand.

Coasts are always changing. Waves crash against them day and night. Tides make the sea rise and fall, so the waves can reach higher and lower parts of the shore. Rivers bring sand and mud down to the sea. Together, these forces wear some coasts away and build others up.

Coasts are dynamic boundaries shaped by waves, tides, currents and rivers. The British Geological Survey (BGS) explains that wind-driven waves erode loose sand from beaches, that the rise and fall of the tides lets waves work at higher and lower levels of the shore, and that currents sweep sand along the coast and drop it where they slow down, leaving some stretches exposed and building up others, so the shoreline evolves naturally.

Where low-energy waves can no longer carry their load, sediment is deposited as beaches, spits and shoals; beaches gain and lose sand with the seasons, and sometimes lose it entirely, leaving cobbles (BGS). Rocky coasts, by contrast, are dominated by erosion of cliffs and headlands.

Most waves are made by the wind.

Wind blows across the water and makes it ripple. The ripples grow into waves. At the beach, the waves get tall and crash down.

Most waves are made by wind blowing across the water. NOAA explains something surprising: the water itself does not travel across the ocean in a wave. The water mostly moves around in circles. What travels is energy, and a wave can carry its energy across a whole ocean.

Big waves need three things: strong wind, wind that blows for a long time, and a long stretch of open water for it to blow over. When a wave reaches shallow water near the beach, the sea floor slows its bottom, the wave gets steeper and steeper, and it breaks.

NOAA describes waves as energy passing through water, which moves in a circular motion; waves transmit energy, not water, and can cross an entire ocean basin. Most are wind-driven surface waves, formed by friction between wind and water. Their height depends on wind speed, duration and fetch, the distance over water the wind blows in one direction; large waves form only when all three combine.

Approaching shore, friction with the sea floor steepens the wave until it becomes unstable and breaks. Waves also bend toward the shape of the coast as different parts reach shallow water first, and usually arrive at a slight angle, the angle of wave approach, which matters for how sand moves along the beach. Other waves have other causes: storm surge comes from severe storms, tsunamis from sudden underwater disturbances, and tides from the pull of the Moon and Sun (NOAA).

Beach sand is made of tiny pieces.

Some are bits of old rock, worn down very slowly. Some are black bits of volcano rock. Some white sand comes from parrotfish poop!

Pick up a handful of sand and you are holding tiny pieces of rock, shell or reef. NOAA explains that much sand begins as rock, often thousands of miles from the sea. Weathering breaks it down, rivers carry the pieces toward the ocean, and waves and tides grind them smaller, over thousands or even millions of years.

The color tells you where it came from. Most tan beaches get their color from the minerals quartz and feldspar. Black sand comes from volcanic rock, like the beaches of Hawaii. Pink sand in Bermuda comes from tiny shelled creatures. And some of Hawaii's white sand is made by parrotfish, which grind up coral and pass it out as sand.

NOAA traces sand to many sources. Most forms as rocks weather and erode over thousands to millions of years, especially durable quartz (silica) and feldspar; the fragments often begin thousands of miles inland, break down as rivers carry them, and are further worn by waves and tides. Iron oxide tints quartz light brown, and feldspar is naturally brown to tan, giving most beaches their color.

Other sands are local. Black sand comes from eroded lava, basalt and other dark volcanic material and is common in Hawaii, the Canary Islands and the Aleutians. Bermuda's pink beaches come from the shells of single-celled foraminifera. Hawaii's famous white sand comes largely from parrotfish, which scrape algae from rock and dead coral, grind the calcium-carbonate reef material in their guts and excrete it as sand, hundreds of pounds a year per fish. By definition, sand is material mostly between 75 microns and about 0.18 inches across; finer is silt or clay, coarser is gravel.

Waves crash on rocky cliffs.

They wear a hole in the rock. The hole becomes a cave. Then an arch, like a bridge. When the arch falls, a tall rock is left standing alone.

On rocky coasts, waves throw water, sand and even rocks against the cliffs. Over time, this wears away the bottom of the cliffs until pieces of rock fall into the sea.

OneGeology, a project of the world's geological surveys, explains how waves carve a headland, a piece of rock that sticks out into the sea. First the waves wear a cave into its side. The cave grows until it breaks through and becomes an arch. When the top of the arch falls down, a lonely pillar of rock is left: a sea stack. Soft rocks wear away much faster than hard rocks.

Shoreline erosion dominates rocky coasts. Waves armed with sand and rock undercut cliffs until large sections collapse into the sea (OneGeology); BGS describes cliff retreat at Newbiggin near Filey in North Yorkshire, where crashing waves loosen the rock face and the sea later redistributes the debris.

Waves bending around a headland erode caves into its flanks; a cave enlarges until it becomes an arch, and when the arch roof collapses an isolated stack remains (OneGeology). Different rock types erode at different rates, so soft rocks are worn away much faster than hard ones. The cliffs, arches and sea stacks page and its journey look at famous examples.

Waves often hit the beach at a slant.

They push the sand a little way along the beach. Wave after wave, the sand moves along the shore, like a slow river of sand.

Waves usually reach the beach at a slight angle, not straight on. NOAA explains that this makes a current that runs along the shore, called a longshore current. As the water washes up the beach and back, it picks up sand and carries it along. This is called longshore drift, and it can move a lot of sand.

Where the current slows down, it drops the sand, building beaches and long fingers of sand called spits. In some places, waves pile up sand in long islands just off the coast, called barrier islands, like the Outer Banks of North Carolina. Their dunes and grasses help protect the mainland from storm waves.

Waves breaking at an angle release energy that drives a longshore current parallel to the shore; it runs faster with steeper approach angles, steeper beaches and higher waves (NOAA). Water moves on and off the beach in a sheet, capturing sediment and moving it along, a process called longshore drift that can cause significant beach erosion where sand is removed and growth where it is deposited (NOAA; BGS).

Barrier islands form as waves repeatedly deposit sediment parallel to the shoreline; they move, erode and grow with changing wind and waves, and can disappear. NOAA notes that they are common off the U.S. Gulf and East coasts, near river deltas and where tectonic activity is low and coastal terrain flat, but not on the cliffed, tectonically active West Coast. Their dunes and grasses absorb wave energy, generally reducing storm surge and flooding; damming, dredging, sea level rise and storms can speed their erosion.

Sometimes water rushes away from the beach in a narrow stream.

It is called a rip current. Always swim with a grown-up. If the water pulls you, swim sideways, along the beach.

Breaking waves make currents near the shore. Sometimes water rushes away from the beach in a narrow, fast stream called a rip current. NOAA says rip currents usually move at 1 to 2 feet per second, but some have been measured at 8 feet per second, faster than an Olympic swimmer.

Most rip currents are less than 80 feet wide. So NOAA's advice is: if you are caught in one, do not panic and do not fight it. Swim sideways, parallel to the shore, out of the narrow stream, or float and let it carry you until it weakens, then swim back.

Rip currents are localized currents flowing away from shore, perpendicular or at an acute angle to it, that form around low spots or breaks in sandbars and near structures such as jetties and piers; they are often wrongly called rip tides (NOAA). They usually break up not far from shore and are generally no more than about 25 m (80 ft) wide. Typical speeds are 1 to 2 feet per second, but speeds up to 8 feet per second have been measured.

NOAA's guidance follows from their shape: swim parallel to the shore to leave the narrow current, or let it carry you until it weakens just beyond the breaking waves, and above all do not panic or exhaust yourself fighting it. Light wave action tends to produce several weaker rips; heavier wave action fewer, more concentrated ones.