Page 80 - (DK) Ocean - The Definitive Visual Guide

78 WAVES AND TIDES

Time (hrs)
0 6 12 18 24
Tides If no continents existed and the Moon 3m/10ft
Tidal Patterns
orbited in the Earth’s equatorial plane, the 2m/6ft Tide heights
TIDES ARE REGULAR RISES AND FALLS in sea level, sweeping of the tidal bulges over the oceans 1m/3ft
accompanied by horizontal flows of water, that are would produce two equal daily rises and falls 0m/0ft
in sea level (a semidiurnal tide) everywhere
caused by gravitational interactions between the Moon, SEMIDIURNAL TIDE
on Earth. In practice, landmasses interfere
Sun, and Earth. They occur all over the world’s oceans with the movement of the tidal bulges, and 0 6 12 18 24
but are most noticeable near coasts. The basic daily the Moon’s orbit tilts to the equatorial plane.
Consequently, many parts of the world 3m/10ft
pattern of high and low tides is caused by the Moon’s
experience tides that differ from the semi- 2m/6ft
influence on the Earth. Variations in the range between diurnal pattern. A few have just one high and 1m/3ft
high and low tides over a monthly cycle are caused by one low tide a day (called diurnal tides), and 0m/0ft
many experience high and low tides of
the combined influence of the Sun and Moon. DIURNAL TIDE
unequal size (known as mixed semidiurnal
tides). In addition, the tidal range, or difference 0 6 12 18 24
High and Low Tides in sea level between high and low water, varies 3m/10ft
considerably across the globe.
Although the Moon is usually thought of as orbiting the Earth, in fact 2m/6ft
both bodies orbit around a common centre of mass – a point located KEY GLOBAL PATTERNS 1m/3ft
inside the Earth. As the Earth and Moon move around this point, two diurnal This map shows the general 0m/0ft
forces are created at the Earth’s surface: a gravitational pull towards the pattern of tides (diurnal,
mixed semidiurnal, or mixed) and MIXED SEMIDIURNAL
Moon, and an inertial or centrifugal force directed away from the Moon. TIDE
semidiurnal size of tidal range (average
Earth gravitational pull of These forces combine to produce difference between high and
Moon creates tidal two bulges in the Earth’s oceans: small tidal range low water) around the world.
bulge
one towards the Moon, and the medium tidal range
other away from it. As the Earth large tidal range
spins on its axis, these bulges sweep
Moon
over the planet’s surface, producing
inertial force
creates second high and low tides. The cycle
tidal bulge
repeats every 24 hours 50 minutes
(one lunar day) rather than every
centre of 24 hours (one solar day), because
mass of during each cycle, the Moon
Earth–Moon P A CIFIC
system moves round a little in its orbit. A T L A N T I C O C E A N
OCEAN
INDIAN
DAILY TIDES OCEAN
Earth’s spin causes The two ocean bulges caused by the
bulges to sweep gravitational interaction between the
bulges due to
combined over surface Earth and Moon are shown (much S O U T H E R N O C E A N
forces exaggerated) here.

INTERTIDAL LIFE

Compared to permanently submerged
plants and animals, organisms living in
the intertidal zone have to cope with
many extra stresses. They need to
adapt, for instance, to the problem of
becoming dried out (desiccated) when
the tide is out. They may also have to
endure extreme cold on frosty winter
nights and even predation by land
animals. Mussels, for example, often
have to wait for hours between high
tides to feed. At low tide, their shells
close tightly to prevent desiccation
INTRODUCTION
and to protect against predators.