Explain how a Coral reef both living and dead?

Only the outer layer of the reed is alive, as coral colonies grow and expand and new corals build their skeletons on those dead corals from earlier generations.

Coral reefs are made by small animals the look like flowers. These animals make tubes and shells to live in. The secrete a concrete like material. This part of the animal is completely and totally dead. The rest of the animal is alive living.