Geology Reference
In-Depth Information
Fig. 1.3
Tropical, semidiurnal equilibrium model. (
a
) Model of the
Moon in orbit around the Earth. The lunar declination is exaggerated
from its modern range of 18-28°. The tropical month (currently
27.32 days) is the time it takes for the Moon to move from its
maximum northern declination to its southernmost declination and
back to its northernmost declination in a single orbit. (
b
) Graph
of tidal heights of a portion of the same modern tidal record shown
in Fig.
1.2b
illustrating diurnal inequality of semidiurnal tides.
Note diurnal inequality goes to zero when the Moon passes
directly over the Earth's equator. (
c
) Image of core shown in
Fig.
1.2c
showing approximate position (labeled “C”) when Moon
was above the Earth's equator during deposition. Note the approx-
imate equal thicknesses of the lamina on either side of the
arrow
.
(d)
Bar chart
shown in Fig.
1.2d
with arrows denoting passages
of the Moon above the Earth's equator during deposition (From
Kvale et al. (
1998
) and used by permission from SEPM)
1.2.5
Anomalistic (27.55 Days)
is responsible for generating neap-spring cycles. In
contrast to the synodic system, tides in a tropical sys-
tem behave as though the Sun's gravitational effects
are dampened, which is impossible to explain in an
equilibrium tidal model (Fig.
1.4
). In such cases, the
dominant tidal force depends on the declination of
the Moon relative to the Earth's equator with the
force being greatest when the Moon is most directly
over the site in question. In these systems, the pre-
dicted and ancient tide data reveal that equatorial
passages of the Moon (crossovers) occur in phase
with the generation of neap-spring tides, in contrast
to the variable relationship exhibited by tropical
(semidiurnal) tides.
Another tidal effect arises from the changing distance of
the Moon relative to the Earth during the lunar orbit
(Fig.
1.5
). Because the lunar orbit forms an ellipse, with
the Earth slightly offset from the center, the Moon alter-
nates between perigee (closest approach to the Earth) and
apogee (the farthest distance from the Earth). During the
lunar synodic month there will be two spring tides (see
synodic periods described above). These spring tides,
however, will be of unequal magnitude producing alter-
nating high-spring and low-spring tides, which corre-
spond to spring tides during or near perigee (high spring)
and spring tides during or near apogee (low spring).
