Geology Reference
In-Depth Information
important now because in many areas sea level is rising, so
buildings that were far inland are now in peril or have already
been destroyed. Furthermore, hurricanes expend much of their
energy on shorelines, resulting in extensive coastal fl ooding,
numerous fatalities, and widespread property damage.
The study of shorelines provides another excellent ex-
ample of systems interactions—in this case, between part of
the hydrosphere and the solid Earth. The atmosphere is also
involved in transferring energy from wind to water, thereby
causing waves, which in turn generate nearshore currents.
And, of course, the gravitational attraction of the Moon and
Sun on ocean waters is responsible for the rhythmic rise and
fall of tides. As dynamic systems, shorelines continuously ad-
just to any change that takes place, such as increased wave
energy or an increase or decrease in sediment supply.
Both the Moon and the Sun have suffi cient gravitational
attraction to exert tide-generating forces strong enough to
deform the solid body of Earth, but they have a much greater
infl uence on the oceans. The Sun is 27 million times more
massive than the Moon, but it is 390 times as far from Earth,
and its tide-generating force is only 46% as strong as that
of the Moon. Accordingly, the tides are dominated by the
Moon, but the Sun plays an important role as well.
If we consider only the Moon acting on a spherical,
water-covered Earth, its tide-generating forces produce two
bulges on the ocean surface (
Figure 16.3a). One bulge
points toward the Moon because it is on the side of Earth
where the Moon's gravitational attraction is greatest. The
other bulge is on the opposite side of Earth; it points away
from the Moon because of centrifugal force due to Earth's
rotation, and the Moon's gravitational attraction is less.
These two bulges always point toward and away from the
Moon (Figure 16.3a), so as Earth rotates and the Moon's po-
sition changes, an observer at a particular shoreline location
experiences the rhythmic rise and fall of tides twice daily, but
the heights of two successive high tides may vary depending
on the Moon's inclination with respect to the equator.
The Moon revolves around Earth every 28 days, so its
position with respect to any latitude changes slightly each
day. That is, as the Moon moves in its orbit and Earth rotates
on its axis, it takes the Moon 50 minutes longer each day to
return to the same position it was in the previous day. Thus,
an observer would experience a high tide at 1:00
P
.
M
. on one
day, for example, and at 1:50
P
.
M
. on the following day.
Tides are also complicated by the combined effects of
the Moon and the Sun. Even though the Sun's tide-generating
force is weaker than the Moon's, when the two are aligned
every two weeks, their forces added together generate
spring
tides
about 20% higher than average tides (Figure 16.3b).
When the Moon and Sun are at right angles to each another,
also at two-week intervals, the Sun's tide-generating force can-
cels some of the Moon's, and
neap tides
about 20% lower than
average occur (Figure 16.3c).
Tidal ranges are also affected by shoreline configura-
tion. Broad, gently sloping continental shelves as in the Gulf of
Mexico have low tidal ranges, whereas steep, irregular shorelines
experience much greater rise and fall of tides. Tidal ranges are
greatest in some narrow, funnel-shaped bays and inlets. The Bay
of Fundy in Nova Scotia has a tidal range of 16.5 m, and ranges
greater than 10 m occur in several other areas.
Tides have an important impact on shorelines because
the area of wave attack constantly shifts onshore and offshore
as the tides rise and fall. Tidal currents themselves, however,
have little modifying effect on shorelines, except in narrow
passages where tidal current velocity is great enough to erode
and transport sediment. Indeed, if it were not for strong tidal
currents, some passageways would be blocked by sediments
deposited by nearshore currents.
◗
CURRENTS
In contrast to other geologic agents such as running water,
wind, and glaciers, which operate over vast areas, shore-
line processes are restricted to a narrow zone at any par-
ticular time. However, shorelines might migrate landward
or seaward depending on changing sea level or uplift or
subsidence of coastal regions. During a rise in sea level, for
instance, the shoreline migrates landward, and then wave,
tide, and nearshore-current activity shifts landward as well;
during times when sea level falls, just the opposite takes
place. Recall from Chapter 6 that during marine transgres-
sions and regressions, beach and nearshore sediments are
deposited over vast regions (see Figure 6.22).
In the marine realm, several biological, chemical, and
physical processes are operating continuously. Organisms
change the local chemistry of seawater and contribute their
skeletons to nearshore sediments, and temperature and salin-
ity changes and internal waves occur in the oceans. However,
the processes most important for modifying shorelines are
purely physical ones, especially waves, tides, and nearshore
currents. We cannot totally discount some of these other
processes, though; offshore reefs composed of the skeletons
of organisms, for instance, may protect a shoreline from
most of the energy of waves.
The surface of the oceans rises and falls twice daily in re-
sponse to the gravitational attraction of the Moon and Sun.
These regular fl uctuations in the ocean's surface, or
tides
,
result in most seashores having two daily high tides and
two low tides as sea level rises and falls anywhere from a few
centimeters to more than 15 m (
Figure 16.2). A complete
tidal cycle includes a
fl ood tide
that progressively covers more
and more of a nearshore area until high tide is reached, fol-
lowed by
ebb tide
, during which the nearshore area is once
again exposed (Figure 16.2). These regular fl uctuations in
sea level constitute one largely untapped source of energy
as do waves, ocean currents, and temperature differences in
seawater (see Geo-Focus on pages 416 and 417).
◗
You can see
waves
, or oscillations of a water surface, on all
bodies of water, but they are best developed in the oceans
Search WWH ::
Custom Search