What does isostasy refer to in terms of geology?

Isostasy refers to the concept of vertical movements in the Earth's crust as a response to changes in load, such as the weight of ice sheets during glaciation or sediment deposition. The principle suggests that the Earth's crust is in gravitational equilibrium, meaning that it will rise or sink to maintain balance in response to various forces acting upon it, such as the addition or removal of mass.

For example, when large ice sheets melt, the weight on the crust is removed, leading to a gradual rebounding or rising of the land in a process known as glacial isostatic adjustment. Similarly, if sediments accumulate in a location, the crust may sink slightly in response to the additional weight. This dynamic balance is fundamental to understanding geological processes and the structure of the Earth's surface over time.

The other options, such as changes in river base levels, weathering processes of rocks, and the formation of sand bars, pertain to different geological processes and do not encompass the concept of isostasy, which specifically deals with vertical adjustments of the crust.