Shorelines and Shoreline Processes - The Changing Earth: Exploring Geology and Evolution - page 410

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Figure 16.3 Tidal Bulges The gravitational attraction
of the Moon and Sun cause tides. The sizes of the tidal
bulges are greatly exaggerated.

Earth

Moon

Tidal bulges if only the Moon caused them.

a

Lunar tide Solar tide

Sun

Full
moon

New
moon

Spring tides

b When the Moon is new or full, the solar and lunar tides reinforce one another, causing spring
tides, the highest high tides and lowest low tides.

First quarter moon

Lunar tide

Solar tide

Sun

Third quarter moon

Neap tides

During the Moon's fi rst and third quarters, the Moon, Sun, and Earth form right angles, causing
neap tides, the lowest high tides and highest low tides.

c

factors that control wave size. High-velocity wind blowing
over a small pond will never generate large waves regardless
of how long it blows. In fact, waves on ponds and most lakes
appear only while the wind is blowing; once the wind stops,
the water quickly smooths out. In contrast, the surface of the
ocean is always in motion, and waves with heights of 34 m
have been recorded during storms in the open sea.
The reason for the disparity between wave sizes on
ponds and lakes and on the oceans is the fetch , which is the
distance the wind blows over a continuous water surface.
Fetch is limited by the available water surface, so on ponds
and lakes it corresponds to their length or width, depending

on wind direction. To produce waves of greater length and
height, more energy must be transferred from wind to water;
hence large waves form beneath large storms at sea.
Waves with different lengths, heights, and period may
merge, making them smaller or larger. Under some circum-
stances, two wave crests merge to form rogue waves that are
three or four times higher than the average. These waves can
rise unexpectedly out of an otherwise comparatively calm sea
and threaten even the largest ships. During the last two de-
cades, more than 200 supertankers and container ships have
been lost at sea, many of them apparently hit by these huge
waves. As recently as April 16, 2005, the Norwegian cruise