BUT, as the mass of rock is many times more than the mass of water, at any given depth, the pressure on the rock should be the same multiple of rock to water.......
I.e., wouldn’t the rock pressure a mile deep be greater than the water pressure in the ocean a mile deep?
Yes, but only if the rock were a liquid. As it is a solid, the pressure is probably nil at any given point, being distributed over a huge volume.
Have to ask my structural engineer son......I’m electrical......
Haha, me too, but I also took strengths of materials and fluids. But that was back...way back! You raise good points. I guess the liquid needs to be forced into the cracks to further fracture the rock. The solids prolly don’t harbor any gas anyway, so who cares. It’s in those pesky cracks.