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Question: How did Kirk die? He got hit by a lot of debris but from what was seen, not enough to do real damage.

Answer: Kirk also fell a considerable distance, clinging onto the platform as it dislodged from the cliff and rolled several times. In addition to being crushed/hit by debris, the fall and the rolling would have caused further serious trauma.

Sierra1

In addition to this statement, Kirk would have had servere internal bleeding as seen by the blood coming from his mouth.

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