Geology Reference
In-Depth Information
Going with the flow: Currents and tides
Ruled by forces more distant than the wind,
tides
move ocean water toward and away
from land in response to the pull of the moon's gravity. As the moon orbits the earth, its
gravity pulls on the water at earth's surface just slightly. The result of the moon's pull is
that the water on earth bulges out toward the moon as illustrated in Figure 15-4.
Figure 15-4:
The
pull of the moon
creates a bulge,
resulting in tides.
The earth continues to rotate while the water bulges out, resulting in two high
tides and two low tides every day. The
high tides
occur in the places that are
closest to and farthest from the moon, where the bulging of water is the greatest.
Standing on the beach, the experience of tides is less abstract:
Lowtide
is when the wa-
ter is moving away from the land, and
high tide
is when it moves toward land. The flow
of water with the tides creates
tidal currents.
When the tides are moving from low tide to
high tide, the tide is “coming in” and the tidal current is called a
flood current.
When the
tide is “going out,” or switching to low tide, the movement of water away from the shore
is called an
ebb current.
The areas along the shore that are covered and uncovered by
the cycle of tidal currents are called
tidal flats.
While tidal currents move in and out with the tides, other types of currents are moving
water along the shoreline with the waves. When waves hit the beach at an angle, their
motion creates
longshore currents.
Longshore currents move water (and sediments) par-
allel to the shore. A longshore current is generated by the energy of breaking waves
moving back out and hitting incoming waves. The water is then forced to move along
parallel to the shore. This process is illustrated in Figure 15-5.
