Geology Reference
In-Depth Information
Fig. 1.1
Semidiurnal equilibrium model. (
a
) Two oceanic tidal
bulges are produced on opposite sides of the Earth by the gravita-
tional forces of the Sun and the Moon. (
b
) Two tides are produced
each day by the spin of the Earth through these bulges. The diur-
nal inequality is produced when the tidal bulges are not centered
above the Earths equator. Semidiurnal tides can be recognized in
the rock record by the coupling of thick and thin lamina (
c
) and
graphically in the thickness measurements of laminated sequences
(
d
) as preserved in the tidal rhythmite succession from the
Pennsylvanian Mansfield Formation (Hindostan whetstone beds)
from Orange County, Indiana, USA (From Kvale and others
(
1998
) and used by permission from SEPM)
tidal theory of five of the six tidal periods; a bar chart
of tidal height data (high tide elevations) from a modern,
real-world setting that shows how the astronomical
effects are reflected in cyclic changes in daily high
tides; a core from an ancient tidal rhythmite succession
showing how these cyclic tidal effects might be mani-
fested in a laminated tidal rhythmite; and a bar chart of
laminae thicknesses interpreted in the context of the
modern tidal cycle.
diurnal inequality), as one tide is higher (dominant)
than the other (subordinate) because the Moon's
orbital plane and the Earth's equatorial plane are not
parallel. The angular difference between the two
planes is termed lunar declination.
1.2.2
Synodic (29.53 Days)
Daily high tides are higher when the Earth, Moon, and
Sun are nearly aligned (full or new moon); this is
referred to as “syzygy” (Fig.
1.2
). Conversely, lower
tides occur when the Sun and Moon are at right angles
to the Earth (first or third quarter phase), also known as
“quadrature”. Tides during full or new moon are
referred to as spring tides: “spring” in this context
refers to “lively” or “energetic” rather than implying a
seasonal connotation. Tides at quarter phases are
referred to as neap tides. The neap-spring tidal period
1.2.1
Semidiurnal (12.42 h)
Within the equilibrium tidal model, the interaction of
tidal forces from the Moon and Sun produce two oce-
anic bulges on opposite sides of the Earth (Fig.
1.1
).
The rotation of a point on the Earth through these
bulges once a day produces two tides (the semidiur-
nal tide). Typically, these tides are not equal (termed
