Geology Reference
In-Depth Information
( ) Spilling
a
( ) Plunging
b
Breaking position
Still water level
()Collapsing
c
( ) Surging
d
Figure 13.3
Kinds of breaking waves derived from high-speed moving pictures.
Source:
Adapted from Komar (1998, 210)
between rip currents. Even where waves approach a
coastline head on, a nearshore circulation of long-
shore currents, rip currents, and onshore currents may
evolve.
period 1990-99 eighty-two were reported, ten of which
were generated by earthquakes associated with plate col-
lisions around the Pacific Rim and killed more than
4,000 people (Box 13.1).
Tsunamis
Tides
Tsunamis
are commonly produced by faulting of the sea
floor, and much less commonly by volcanic eruptions,
landslides or slumping, or by impacting asteroids and
comets. They are also referred to as tidal waves, although
they bear no relation to tides and are named after the
Japanese word meaning 'harbour wave'. The pushing up
of water by sudden changes in the ocean floor gener-
ates a tsunami. From the site of generation, a tsunami
propagates across the deep ocean at up to 700 km/hr.
While in the deep ocean a tsunami is not perceptible as
it is at most a few metres high with a wavelength about
600 times longer than its height. On approaching land,
a tsunami slows down to around 100 km/hr and grows
in height by a factor of about ten. It rushes ashore, either
as a tide-like flood, or, if wave refraction and shoaling
allow, a high wall of water.
Tsunamis occur on a regular basis. The historical
average is fifty-seven tsunamis per decade, but in the
Tides are the movement of water bodies set up by the
gravitational interaction between celestial bodies, mainly
the Earth, the Moon, and the Sun. They cause changes
of water levels along coasts. In most places, there are
semi-diurnal tides
- two highs and two lows in a day.
Spring tides
, which are higher than normal high tides,
occur every 14-75 days when the Moon and the Sun are
in alignment.
Neap tides
, which are lower than normal
low tides, alternate with spring tides and occur when
the Sun and the Moon are positioned at an angle of
90
◦
with respect to the Earth. The form of the 'tidal
wave' depends upon several factors, including the size
and shape of the sea or ocean basin, the shape of the
shoreline, and the weather. Much of the coastline around
the Pacific Ocean has mixed tides, with highs and lows
of differing magnitude in each 24-hour period. Antarctic
coasts have diurnal tides with just one high and one low
every 24 hours.
