"Why did God take the form of a burning bush to speak with Moses?"

Exodus 3:1-5 Now Moses was keeping the flock of his father-in-law, Jethro, the priest of Midian, and he led his flock to the west side of the wilderness and came to Horeb, the mountain of God. And the angel of the Lord appeared to him in a flame of fire out of the midst of a bush. He looked, and behold, the bush was burning, yet it was not consumed. And Moses said, “I will turn aside to see this great sight, why the bush is not burned.” When the Lord saw that he turned aside to see, God called to him out of the bush, “Moses, Moses!” And he said, “Here I am.”

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In this well-known passage God communicates directly with Moses, and chooses to do so by taking the form of a burning bush. The question is why? Why a burning bush? Would a burning rock have worked just as well? Why did the bush need to be burning? Is there a natural explanation for this phenomenon? There's also some confusion in the passage as to who is actually speaking to Moses. Initially we are told "an angel of God" appears, but then soon after we are told it is God himself speaking. Does God appear along with the angel or is it just his voice?  

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Referenced Verses:
Exodus 3:1, Exodus 3:2, Exodus 3:3, Exodus 3:4, Exodus 3:5