With respect to sea level change, what is the difference between eustacy and isostasy?

Eustacy and isostasy are ways of describing relative sea level change which is a measure of changes in the level (height) of the sea, relative to the land.

Eustatic sea level change describes a change in the volume of the sea in ocean basins. This is caused by either a change in the amount of water in the basins (for example, an addition of water from melting ice sheets) or by a change in the size of the ocean basins (such the formation of new continental crust, reducing the capacity of the basins).

Eustatic change causes a global rise or fall in sea level.

Isostatic sea level change occurs due to the movement of the land in relation to the sea in specific areas and hence causes a local change in sea level. Isostasy is a process by which the Earth’s crust attempts to reach an equilibrium balance with the mantle it is floating on. Hence isostatic sea level change occurs when the Earth’s crust rises of falls relative to the sea, often due to an increase or decrease of mass on top of the crust.

For example, during an ice age, the crust becomes heavier due to the mass of ice on top of it, and sinks into the mantle to remain buoyant and in equilibrium, leading to an apparent increase in sea level relative to the land.

When measuring change in sea level, both isostatic and eustatic sea level changes must be taken into consideration to measure the total amount of change.  

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