Why are carbon anodes consumed during aluminum smelting?

In the Hall-Héroult process, carbon anodes are consumed because they participate in the chemical reaction required to extract aluminum from alumina.

During electrolysis, a direct current is passed through a molten bath of cryolite and dissolved alumina. The oxygen atoms from the alumina react with the carbon of the anode, resulting in the evolution of carbon dioxide gas. This chemical reaction causes the carbon anode to be gradually worn away or consumed as the aluminum is deposited at the bottom of the cell. In modern smelting operations, approximately 450 grams of carbon are consumed for every kilogram of aluminum produced.1