<P> When continents collide, the continental crust may thicken at their edges in the collision . If this happens, much of the thickened crust may move downwards rather than up as with the iceberg analogy . The idea of continental collisions building mountains "up" is therefore rather a simplification . Instead, the crust thickens and the upper part of the thickened crust may become a mountain range . </P> <P> However, some continental collisions are far more complex than this, and the region may not be in isostatic equilibrium, so this subject has to be treated with caution . </P> <P> 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 . </P> <P> 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 . </P>

The concept of isostasy is related to what type of movement of the crust