FIND OUT MORE. Forces 164 • Moon 17 • Oceans 40–41 • Sun 15
Coasts are border zones where the land meets the
ocean. There is about 502,000 km (312,000 miles)
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.
Once or twice daily, coastlines are washed
by the tide – a regular rise and fall in
the sea level. Some coastlines experience
powerful tides, with rises and falls of 15 m
(50 ft) or more a day. On other shores,
the water level changes only by a few
centimetres, so the tide is barely noticeable.
WHY ARE SOME TIDES STRONGER THAN OTHERS?
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 the Earth and their pulls
largely cancel each other out.
HOW DO WAVES CHANGE COASTAL LANDSCAPES?
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.
WHAT CAUSES TIDES?
Tides are caused mainly by the pull of the Moon’s
gravity on the Earth. This gravitational pull creates
a bulge of water, or a high tide, on the sea’s surface.
As the Earth spins round eastwards on its axis, the
bulge moves westwards, causing a high tide in
different parts of the world.
1 SEA STACKS
The Twelve Apostles rock
formation in Victoria, Australia,
was created by waves scouring
away the bases of cliffs to form
caves. In time, the cave roofs
collapsed, forming arches of rock.
Eventually, the tops of the arches
crumbled, leaving behind a series
of isolated pillars called stacks.1 HIGH TIDE
At high tide, boats bob on the waves as water fills the harbour. Tides
rise and fall once or twice every 24 hours and 50 minutes, so high and
low tides occur 50 minutes later each day.1 LOW TIDE
Boats lie stranded on the shore at low tide, when the harbour at
Polperro in Cornwall, Great Britain, empties of water. The tides follow a
28-day cycle, linked to the Moon’s orbit around the Earth.
TIDES
Coasts
1 SALT MARSH
Formed on flat land at river
mouths, salt marshes are
regularly covered by tides.1 FJORD
Fjords form where glaciers
gouge out valleys that are
later flooded by the sea.1 SPIT AND LAGOON
A sandy spit may form where
waves deposit sediment,
with a calm lagoon behind.coasts