Geography Reference
In-Depth Information
in 1883 killing 33,000 people. When a tsunami travels across the
deep ocean it can typically only have a height of a few centimetres
(60 centimetres after two hours on 26 December 2004) but they
travel quickly at around 500-1,000 kilometres per hour. When
they reach shallower water and approach the land a tsunami wave
will begin to shoal, causing the water to suddenly retreat a long
way from the beach and eventually when the wave arrives it can
reach tens of metres in height.
Tides
Tides are driven by the gravitational pull of the Moon and the Sun
on the Earth. Tides have a predictable daily and monthly cycle.
The Earth and Moon exert a gravitational pull on each other,
which is counterbalanced by forces associated with the Earth's
orbital rotation. In theory, the force exerted due to the Earth's
rotation is equal on all parts of the Earth's surface except where the
gravitational pull of the Moon allows for a slight reduction. This
reduction allows the oceanic surface to bulge. This bulge will occur
on that part of the Earth closest to the Moon. In addition there is a
balancing bulge on the opposite side of the Earth as a product of
the forces exerted by the rotation of the Earth. Consequently there
are two bulges and thus two tides per day. The tide rises and falls at
a point on the Earth's surface as that point rotates away from and
towards the direct line of the gravitational pull of the Moon. The
gravitational pull of the Sun introduces an extra monthly dimen-
sion to the tidal sequence. As the Moon's position relative to the
Sun changes over the lunar month (the Moon takes 28 days to
revolve around the Earth) the two astronomical bodies either pull
in alignment or opposition to each other.
During the full and new
moon phases they pull along the same direction allowing the
oceanic surface to bulge further resulting in larger magnitude tides
known as
spring tides
. During the half moon phases the Sun and
Moon pull in opposite directions resulting in subdued tidal bulges
and these are known as
neap tides
. The actual impact of the grav-
itational pull on tides depends on the shape and topography of the
coast. In some locations the tide can rise and fall by several metres
whereas in others there is a barely noticeable tide. The largest tidal
ranges seem to be associated with constricted areas where there are
