The boulder particle system has 12 different particle systems, its has mesh emission and it has world collision. In really only has 6 different particle system, but they are all copied once. It has 5 different particle systems, all emitting different rock variations. The rocks emit with a random start size between two numbers. I think it would be better to make this effect with only one particle system that emits a random mesh instead of 5 different ones. 

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Each rock emitted has world collision, which means it will collide with anything that get in its way.

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There are two smoke particle systems and the bottom.

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