Albert Barnes Commentary 1 Kings 13:5

"The altar also was rent, and the ashes poured out from the altar, according to the sign which the man of God had given by the word of Jehovah." — 1 Kings 13:5 (ASV)

We do not need to imagine the altar was completely shattered. Instead, a crack or fissure likely appeared in its structure, extending from top to bottom. This break would have caused the embers and the pieces of the sacrificial offerings to spill out onto the ground.