Geology Reference
In-Depth Information
Fig. 1.5
Anomalistic equilibrium model. (
a
) Polar view of the
Moon in orbit around the Earth. Note that lunar orbit is not
perfectly circular but somewhat elliptical (greatly exaggerated
in the diagram) and that the Earth is not position in the direct
center of the orbit path. The time it takes for the Moon to go
from perigee (closest approach) to apogee (furthest from the
Earth) and return is called the “anomalistic month”, which is
27.55 days long at present. (
b
)
Graph
showing the 1992 pre-
dicted high tides for Saint John, New Brunswick, Canada
(NOAA
1991
) showing the effects of the anomalistic month on
the Saint John tides. Note the semimonthly inequality goes to
zero when the Moon and Sun are aligned with the Moon's
minor orbital axis (termed “phase flip”). (
c
) Photograph of a
core from the Mississippian Tar Springs Formation, Indiana,
USA showing the effects of the anomalistic month on neap-
spring tidal deposition. (
d
) Graph illustrating thicknesses as
measured between neap-to-neap tide deposits from the Tar
Springs Formation core, a portion of which is shown in
Fig.
1.5c
. Note the position of the “phase flip” (From Kvale
et al. (
1998
) and used by permission from SEPM)
world does not spin through two tidal bulges. Instead,
oceanic tides rotate as waves around fixed (amphidro-
mic) points within individual ocean basins (Fig.
1.7
).
Equilibrium tidal theory indicates that diurnal tides
should exist only at very high latitudinal positions,
which is not the case. For example, the Gulf of Mexico
and large tracts in the Indian and western Pacific oceans
are dominated by diurnal tides. Tides like those found
in Immingham, England, where the semidiurnal tides
have minimal diurnal inequality, cannot be explained
by equilibrium tidal theory, which requires such tides
to exist only in equatorial positions. Finally, equilib-
rium tidal theory does not explain neap-spring tidal
cycles which are synchronous with the 27.32 tropical
monthly period such as illustrated in Fig.
1.4
.
The difficulties in understanding and explaining
real-world tides can be addressed by a dynamic tidal
model. This model is built around the concept of a
harmonic analysis of the components that compose
real-world tides. For instance, the Moon and Sun each
generate their own tide within the Earth's oceans. Since
the orbits of the Earth around the Sun and the Moon
around the Earth are not perfectly circular, the ampli-
tude of the tides generated by each of these bodies, in
part, fluctuates depending on the Earth's proximity
to the Sun and, much more importantly, the Moon's
distance from the Earth. Periodically each of these
tides will constructively or destructively interact with
each other. The tides associated with changes in Moon-
Earth distance or Earth-Sun distance can be considered
to be a constituent of the overall tide, which can affect
any coastline.
To model these tidal constituents (also known as
tidal “species”) oceanographers conceptualize each
