It's a laser, lasers don't cause random elements to become combustible.
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Technically, it's a bolt of hyper-energized particles, more akin to a plasma shot than a laser.
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No real difference in this case.
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Theoretically, it's a huge difference. The nature of a Blaster sounds is that it rapidly disperses on impact with a surface. So whatever the Death Star is firing, it's causing subsequent reactions throughout the planet, thus the uniform detonation pattern.
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There is no evidence that blasters cause things to become combustible.
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I'm not saying that a standard blaster causes things to magically become combustible. What I'm saying is that the Death Star main cannon, which appears to be a significantly different type of weapon from "standard" blasters, appears to destroy planets from some form of internal reaction.
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It looks to be a laser. It has never been hinted that it causes chain reactions. So we can't assume it does.