(´・_・｀)When large amounts of sediment are deposited on a particular region, the immense weight of the new sediment may cause the crust below to sink.
(´・_・｀)Similarly, when large amounts of material are eroded away from a region, the land may rise to compensate.
(´・_・｀)Therefore, as a mountain range is eroded, the (reduced) range rebounds upwards (to a certain extent) to be eroded further.
(´・_・｀)Some of the rock strata now visible at the ground surface may have spent much of their history at great depths below the surface buried under other strata,
(´・_・｀)to be eventually exposed as those other strata eroded away and the lower layers rebounded upwards.
(´・_・｀)An analogy may be made with an iceberg?it always floats with a certain proportion of its mass below the surface of the water.
(´・_・｀)If more ice is added to the top of the iceberg, the iceberg will sink lower in the water.
(´・_・｀)If a layer of ice is somehow sliced off the top of the iceberg, the remaining iceberg will rise. Similarly, Earth's lithosphere "floats" in the asthenosphere.
(´・_・｀)The formation of ice sheets can cause Earth's surface to sink.
(´・_・｀)Conversely, isostatic post-glacial rebound is observed in areas once covered by ice sheets that have now melted, such as around the Baltic Sea and Hudson Bay.
(´・_・｀)As the ice retreats, the load on the lithosphere and asthenosphere is reduced and they rebound back towards their equilibrium levels.
(´・_・｀)In this way, it is possible to find former sea cliffs and associated wave-cut platforms hundreds of metres above present-day sea level.
(´・_・｀)The rebound movements are so slow that the uplift caused by the ending of the last glacial period is still continuing.
(´・_・｀)In addition to the vertical movement of the land and sea, isostatic adjustment of the Earth also involves horizontal movements.
(´・_・｀)It can cause changes in Earth's gravitational field and rotation rate, polar wander, and earthquakes.
(´・_・｀)Eustasy is another cause of relative sea level change quite different from isostatic causes.
(´・_・｀)The term eustasy or eustatic refers to changes in the volume of water in the oceans, usually due to global climate change.
(´・_・｀)When Earth's climate cools, a greater proportion of water is stored on land masses in the form of glaciers, snow, etc.
(´・_・｀)This results in falling global sea levels (relative to a stable land mass). The refilling of ocean basins by glacial meltwater at the end of ice ages is an example of eustatic sea level rise.
(´・_・｀)A second significant cause of eustatic sea level rise is thermal expansion of sea water when Earth's mean temperature increases.
(´・_・｀)Current estimates of global eustatic rise from tide gauge records and satellite altimetry is about +3 mm/a.
(´・_・｀)Global sea level is also affected by vertical crustal movements, changes in Earth's rotation rate, large-scale changes in continental margins and changes in the spreading rate of the ocean floor.
(´・_・｀)When the term relative is used in context with sea level change, the implication is that both eustasy and isostasy are at work, or that the author does not know which cause to invoke.
(´・_・｀)Post-glacial rebound can also be a cause of rising sea levels. When the sea floor rises, which it continues to do in parts of the northern hemisphere, water is displaced and has to go elsewhere.