Geology Reference
In-Depth Information
Fig. 1.4
Tropical diurnal model. (
a
) Model of the Moon in its
orbit around the Earth (see Fig.
1.3a
). (
b
) Graph showing the
1994 predicted relative high tides (mixed, predominantly diur-
nal) for the Barito River estuary in Borneo (NOAA
1993
). Note
the passages of the Moon above the Earth's equator perfectly
track the neap tides and spring tides to the maximum declinations
of the Moon in its orbit around the Earth, a pattern not predicted
by equilibrium tidal theory. Such neap-spring tidal cycles are
termed “tropical neap-spring tides” (Kvale
2006
). (
c
) Photograph
of a portion of a core from the Pennsylvanian Brazil Formation,
Daviess County, Indiana, USA.
Arrows
indicate lamina depos-
ited with the Moon was above the Earth's equator. (
d
)
Bar chart
of lamina thicknesses measured from core obtained from the
Brazil Formation. This unit also is mixed, predominantly diurnal.
Note the diurnal inequality of the semidiurnal component goes to
zero only in the neap tide deposits. This corresponds to the Moon
above the Earth's equator during deposition (From Kvale and
others (
1998
) and used by permission from SEPM)
The semimonthly inequality of the spring tides disappears
when the Moon lies along the minor axis of the lunar
orbit and the difference in lunar distance is minimized
during subsequent spring tides. The time it takes for the
Moon to move from perigee to perigee is called the
anomalistic month, which is at present 27.55 days.
the dashed line in Fig.
1.6
). In the equilibrium tidal
model, the date of this tidal maximum is a function of
latitude that is related to the declinational effects of the
Moon and Sun. An annual inequality has been docu-
mented in several ancient tidal rhythmite successions
(Kvale et al.
1994
). This inequality is interpreted to be
climatic (non-tidal) in origin.
1.2.6
Semiannual (182.6 Days)
1.3
Dynamic Tidal Theory
The synodic, tropical, and anomalistic periods have
slightly different values. Because of this, these periods
will interact constructively twice each year causing tidal
forces at these times to reach a maximum (as shown by
As noted in the introduction, the equilibrium tidal
model explains the driving forces that cause tides but
does not explain real-world tides. For instance, the
