Eustatic and Isostatic Sea Level

Eustatic sea level change occurs on a global scale, either due to a change in the volume of water in the ocean basins, or a change in the volume of ocean basins themselves. One cause of eustatic sea level change is variation in mid-ocean ridge (tectonic) activity: when the ridges are at their most active they are regionally elevated over large areas in response to the thermal expansion caused by the generation of new hot oceanic lithosphere. Regional elevation of the ridges causes a decrease in the volume of the oceans basins and flooding of the continents occurs as a consequence. Global climate change may also lead to eustatic sea level change. Widespread glaciation as a result of global cooling may cause sea levels to fall as liquid water in the oceans is locked up in ice sheets and glaciers. A global increase in temperature has the opposite effect - glaciers and ice sheets melt, adding water to the oceans.

Isostatic sea level change occurs on a local scale, caused by vertical movement of land relative to the sea. Isostatic sea level change is most often caused by loading of the crust with sediment or ice - as sediment or ice accumulates on top of the crust, the crust subsides and relative sea levels increase. When the sediment is eroded or the ice melts, the pressure is reduced and the crust rises relative to the sea level.

Rising and falling sea levels also create submergent landforms (e.g. rias and fjords), and emergent landforms (e.g. raised beaches) respectively