Geography Reference
In-Depth Information
extinct, the island erodes and also sinks back into the ocean because
the oceanic crust on which the volcano rests will cool and sink
over millions of years as the crust moves away from the seafloor
spreading centre at the mid-ocean ridge. However, coral growth
may be able to keep up with the rate of island sinking and so there
may be a large depth of limestone formed on top of the volcanic
base. The key is whether vertical reef growth can keep pace with
falling land levels or rising sea levels.
Atolls
are reefs that surround
a central lagoon and most of these are found in the Indian and
Pacific Oceans. They tend to be circular in shape and range from
75 kilometres width to less than 1 kilometre. Many are believed to
have developed around the rim of a volcanic island. As the volcano
has itself sunk back into the ocean, the coral has been able to keep
up growth but only around the edges where the coral had previ-
ously developed. Thus the central portion of the island is largely
devoid of reef and a lagoon forms. The nutrients in the lagoon are
poor as wave action is restricted and thus growth is limited.
Coastal management
In addition to sea level change coastal managers have to deal with
natural erosion and depositional processes and those processes that
have been created by human action (e.g. delta erosion due to
upstream dam construction; beach erosion due to sand abstraction
somewhere further down the coast which starves longshore drift of
sediment). Many coastal management solutions have involved
engineering solutions such as sea walls, breakwaters and
groynes
.
Sea walls are large and costly concrete, steel or timber structures
often with a curved face. However, as sea walls provide a limit to
the beach zone they may stop the usual process of beach profiles
changing (lengthening/flattening) during stormy periods and steep-
ening during calm periods. Sea walls also reflect waves and instead
of dissipating energy more erosion takes place at points further along
the coastline beyond the extent of the wall. A common response is
to then extend the seawall further but this just shifts the erosion
problem further along the coast. A sea wall was built in the nine-
teenth century at the seaside resort of Blackpool, England and 150
years later almost 80 kilometres of coastline in the area had a sea
wall as erosion developed along the coast from the Blackpool wall.
