Page 179 - Sedimentology and Stratigraphy

166 The Marine Realm: Morphology and Processes

If the land areas are ignored the effect of these 11.2.2 Tidal ranges
bulges is to create an ellipsoid of water with its long
axis oriented towards the position of the Moon. As the The tidal bulge created in the open ocean is only a few
Earth rotates about its axis the bulges move around tens of centimetres, but of course the difference
the planet. At any point on the surface the level of the between high and low tide is many metres in some
water will rise and fall twice a day as the two bulges places, so there must be a mechanism to amplify the
are passed in each rotation. This creates the daily vertical change in sea level. The tidal bulge can be
or diurnal tides. During the daily rotation, a point considered as a wave of water that passes over the
on the Earth will pass under one high bulge and surface of the Earth. In any waveform resonance
a slightly lower bulge 12 hours or so later: this is effects are created by the shape of the boundaries of
referred to as the diurnal tidal inequality, the two the ‘vessel’ the wave is moving through. In oceans
high tides in a day are not of equal height. The two and seas the shape of the continental shelf as it shal-
tides in the diurnal tidal cycle are just over twelve and lows towards land, indentations of the coastline and
a half hours because the Moon is orbiting the Earth as narrow straits between seas can all create resonance
the planet is rotating, changing its relative position effects in the tidal wave. These can increase the ampli-
each day. tude of the tide and locally the tidal range is increased
The Moon rotates around the Earth in the same to several metres by tidal resonance effects. The high-
plane as the Earth’s orbit around the Sun. The Sun est tidal ranges in the world today are in bays on
also creates a tide, but its strength is about half that continental shelves, such as the Bay of Fundy, on
of the Moon despite its greater mass, because the the Atlantic seaboard of Canada, which has a tidal
Sun is further away. When the Sun and Moon are range of over 15 m (Dalrymple 1984).
in line with the Earth (an alignment known as In addition to the influence of land masses, the
syzygy) the gravitational effects of these two bodies movement of water between high- and low-tide con-
are added together to increase the height of the tidal ditions is also affected by the Coriolis force (6.3):
bulge. When the Moon is at 90˚ to the line joining the water masses moving in the northern hemisphere
Sun and the Earth (the quadratic alignment), the are deflected to the right of their path and in the
gravitational effects of the two on the water tend southern hemisphere to the left. These effects break
to cancel each other. During the four weeks of the up the tidal wave into a series of amphidromic cells
Moon’s orbit, it is twice in line and twice perpendicu- and at the centre of each cell there is an amphidro-
lar. This creates neap–spring tidal cycles with the mic point around which the tidal wave rotates
highest tides in each month, the spring tides, occur- (Fig. 11.4). At the amphidromic point there is no
ring when the three bodies are in line. (The term change in the water level during the tidal cycle. All
‘spring’ in this context is not referring to the season oceans are divided into a number of major amphidro-
of the year.) A week either side of the spring tides mic cells and there are additional, smaller cells in shelf
are the neap tides, which occur when the Moon and areas such as the North Sea and small seas such as
Sun tend to cancel each other and the tidal effect is the Gulf of Mexico. Tidal ranges are therefore very
smallest. variable and within a body of water the pattern of
Superimposed on the diurnal and neap–spring cycles tides can be very complex: in the North Sea, for
is an annual tidal cycle caused by the elliptical example, the tidal range varies from less than a
nature of the Earth’s orbit around the Sun. At the metre to over 6 m (Fig. 11.4). For sedimentological
spring and autumn (Fall) equinoxes, the Earth is clo- purposes it is useful to divide tidal ranges into the
sest to the Sun and the gravitational effect is stron- following categories: up to 2 m mean tidal range the
gest. The highest tides of the year occur when there regime is microtidal, between 2 and 4 m range it is
are spring tides in late March and late September. In mesotidal and over 4 m is macrotidal.
mid-summer and mid-winter the Sun is at its furthest
away and the tides are smaller. This pattern of three
superimposed tidal cycles (diurnal, neap–spring and 11.2.3 Characteristics of tidal currents
annual) is a fundamental feature of tidal processes
that controls variations through time of the strength The horizontal movement of water induced by tides is
of tidal currents. a tidal current: tidal currents are weak in microtidal