A eustatic movement denotes a worldwide change in sea level caused by a marine transgression or regression. It is also called sea level change. Sea level changes are caused by two processes: a change in the volume of seawater or a change in the volume of the marine basin. Changes in the volume of seawater are mainly caused by changes in the continental ice sheets. When the continental ice sheets grow, the amount of seawater decreases, whereas when the ice sheets retreat, the amount of seawater increases. Similarly, when the volume of the marine basin increases, sea level will fall, whereas when the basin volume decreases, sea level rises. It is still uncertain whether changes in marine basin volume are due to expansion or filling up of the basin or to morphological changes of the continents.