Changes in sea level occur daily due to tides. Drastic sea-level changes occur mainly due to glaciations, de-glaciations and tectonic forces.
During glacial periods, extreme low temperatures caused a large volume of water to freeze. During those times, water stored as ice was probably three times more than there is today. Consequently, there was a smaller volume of water. This caused a worldwide fall in sea level of about 100-150m.
When temperatures rose after the period of glaciation, the large volume of ice started to melt. This lead to a rise in sea level.
During glaciations, large amounts of ice may accumulate on one part of a landmass. This exerts great pressure on this part of the continent, thus pushing it down. The earth’s crust tends towards equilibrium (it acts like a see-saw), so when one part sinks, the other part will rise.
During de-glaciations, the melting ice releases some of the pressure. The land that formerly sank, will now start to rise slowly. The other side will then find its equilibrium and thus move down.
Tectonic activities, especially at destructive plate margins can also cause changes in sea-level.
Sometimes land is uplifted by mountain forming (orogenesis) or faulting. This can be seen by the seashells and fossils trapped in rock which are now found above sea-level. E.g. Dingli Cliffs, and Rdum Qammiegh (Mellieha).
Land is also downthrown by tectonic activities. Example Maghlaq Fault and South Comino Graben (the series of faults between Malta and Comino).
When sea level rises, the land is submerged (it drowns) and fjords, ria coasts, dalmation coastlines and estuaries occur.
Fjords are most common in Scandinavia especially Norway.
Ria coasts are mainly found in N.W. of France and Spain. In Malta, rias are found in the Grand Harbour area.
Dalmation coastlines can be seen along Ex-Yugoslavia (Slovenia and Bosnia Herzegovina)
A fall in sea level causes the emergence of coastal features, example the base of cliffs will be uncovered. In this way, the cliffs seem to be higher (Example: Dingli Cliffs).
Raised beaches, such as at Rdum Qammiegh, are another formation.
Global warming caused by the burning of fossil fuels, can effect sea level changes in two ways:
a) Warmer water is less dense, so it expands and occupies a greater volume. It thus contributes to sea-level rise.
b) Higher temperatures will lead to the melting of glaciers and ice- caps, causing a rise in sea level.
Low lying ocean states (example Maldives), harbours (like Tokyo) and coastal conurbations (such as Calcutta) are world examples which are threatened by sea-level rise. In Malta, Marsa and other coastal areas, example Msida, may also be affected. This would lead to severe human and economic problems.
