Coasts are border zones where the land meets the ocean. There is about 312,000 miles (502,000 km) of coastline worldwide. TIDES, waves, and currents endlessly wear away at the land to form a variety of coastal landscapes, from sheer cliffs and rugged headlands to sandy coves and wide, lonely mudflats.
Pounding waves continually hurl sand, pebbles, and boulders against rocky coastlines, scouring away the land. As waves wear away coastal cliffs, the coastline gradually moves inland. Elsewhere, however, tides and rivers deposit sand, mud, and pebbles to build new land in the form of river deltas, beaches, and spits.
Once or twice daily, coastlines are washed by the tide--a regular rise and fall in sea level. Some coastlines experience powerful tides, with rises and falls of 50 ft (15 m) or more a day. On other shores, the water level changes by only a few inches, so the tide is barely noticeable.
Tides are caused mainly by the pull of the Moon’s gravity on Earth. This gravitational pull creates a bulge of water, or a high tide, on the sea’s surface. As Earth spins around eastward on its axis, the bulge moves westward, causing a high tide in different parts of the world.
Extra-strong tides called spring tides occur twice a month, when the Sun and Moon line up so that their combined gravitational pull produces an even bigger bulge on the ocean’s surface. Weak tides called neap tides also occur twice every month, when the Sun and Moon are at right angles to Earth and their pulls largely cancel each other out.