As gravity rain passes thru the planet it loses energy that is evidenced by the heat build up we all know is down there
..they say that its there and I suspect it is
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Well gravity rain comes up quicker
(seeking a simple escape path) when the Moon shields that side of the planet and with it has more chance of driving the hot stuff to the surface ..The hot stuff, fast bits, moves quicker and encounters more contact with gravity rain which in turn adds energy or heat or faster bits, so more energy is available to cause an erruption.
Dismiss this and you will never understand the process
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But actually I am not joking here. So all has a greater chance of being "pushed"to the surface
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alex