The movement of material due to differences in density that are caused by differences in tempeture is called?

Convection. It occurs when a cooler, denser mass of gas or liquid replaces a warmer, less dense mass of gas or liquid by pushing it upward. A good example would be steam rising from a pot of boiling water on a stove top.
When the water is boiled, the water moves in roughly circular patterns because of convection. This motion is due to density differences that result from temperature differences. Also referred as a convection current.

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Answer carbon dioxide ice;