Geology Reference
In-Depth Information
Fig. 1.6 Semiannual equilibrium model. ( a ) View of the con-
figuration of the Earth, Moon, and Sun representing the maxi-
mum spring tides formed when the Moon is at perigee, maximum
northern declination and new. Such spring tides occur every
182.6 days. ( b ) 1992 predicted high tides from Saint John, New
Brunswick, Canada (NOAA 1991 ) showing the effects of the
semiannual convergence of maximum spring tides. ( c ) Photograph
of a core from the Pennsylvanian Lead Creek Limestone, Indiana,
USA. In this core the neap-spring cycles thicken and thin in a
semiannual pattern. ( d ) Graph showing the thicknesses of
individual lamina from the Brazil Formation, Indiana. These
thicknesses are also organized into semiannual tidal cycles. Each
number records an individual neap-spring cycle (From Kvale
et al. ( 1998 ) and used by permission from SEPM)
constituent as a phantom “satellite” that has its own
mass (that of the Moon, Sun, or a combination of the
two). Each phantom “satellite” has a motion within a
plane or is fixed relative to the stars and each generates
its own tide with a unique period, response time, and
amplitude (Pugh 1987 ) (Table 1.1 ). For instance S 2
represents the twice-daily tide generated at a fixed
point on the Earth by a “satellite” that has the mass of
the Sun in a perfectly circular orbit around the Earth's
equator. O 1 represents the daily tide generated at a
fixed point on the Earth by a “satellite” with a mass of
the Moon and a motion above the Earth's equator. For
each of the tidal constituents, the subscript indicates
if the tide is diurnal ( 1 ) or semidiurnal ( 2 ).
The relative intensity for each of these tidal constitu-
ents along any oceanic coastline in the world can be
determined by a harmonic decoupling of an extended
hourly tidal record. These measurements typically are
recorded in most major harbors and other tidal stations
around the world. More than 100 tidal constituents have
been identified from a harmonic extraction of Earth's
tides, however, seven of these (Table 1.1 ) account for
more than 80% of any real-world tide (Defant 1961 ).
The resonate amplification or destruction of these tidal
constituents determines the resulting tide for a specific
area within the Earth's oceans (Fig. 1.8 ).
As noted above, each of these tidal constituents
corresponds to a unique tidal wave. These waves do
not travel around the world as predicted by equilibrium
tidal theory, but rather rotate around a point (referred
to as an “amphidromic point”) within a region of the
ocean at a speed determined by their constituent's
orbital periodicity or the periodicity of the Earth's spin
(Fig. 1.7 ). The location of these points is determined
by basin geometries and the Coriolis force.
Ideally, amphidromic circulation should be counter-
clockwise in the Northern Hemisphere and clockwise
in the Southern Hemisphere and never on the equator
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