How is the symbol of the fire ironic?

The "fire on the mountain" represents creation and destruction. On one hand, fire becomes a tool that the boys can use for cooking food, signal, warmth, security...On the other hand, it also can become a force of destruction. Golding uses animal imagery as an allegory to point out that fire breathes, eats, and moves. The boys unleash this animal and cannot control it. It is ironic that their final inferno to smoke Ralph out, and completely destroy the island, ends up being noticed by a ship leading to rescue.

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