Underwater, there are insane implosions at 25K feet below sea level. At that same depth on land, we have caves.
Why doesn’t the earth compress things like water does? Does gravity do different things to different materials?
In: Planetary Science
The difference you’re asking about is to do with density. The pressure of fluids like water and air is greater when the density of the fluid is greater.
Right now, I have a column of air sitting on top of me. The column of air goes to the top of the sky, 62 miles up, and it puts constant pressure on my body.
How much pressure? The equivalent of 15 pounds pressing against every square inch of the surface of my body. Like I’m covered with hundreds of bowling balls that press into me from all directions.
I don’t notice the pressure because it’s fluid. It compresses me from all directions and the forces cancel out.
Same reason that I don’t feel gravity most of the time: I’m standing on a solid floor or sitting in a solid chair that pushes me up with the same force that gravity pulls me down. To feel gravity, I just need to lift up my hand, let it go limp, and feel it fall.
To feel the weight of a few bowling balls of pressure, just turn on a vacuum cleaner and cup the hose with your hand. You’ve removed atmospheric pressure from one side of your hand, and now you feel the weight of the atmosphere pushing your hand toward that side.
Now that we’ve talked about that, we can compare pressure changes in air and water.
Let’s say I board an elevator and ride it deep down into the earth. I start out sea level, where the amount of air pressure on me is equal to 1 standard atmosphere, or 1 atm.
To reach double that air pressure, or 2 atm, I’d need to take that elevator down the shaft 20,000 ft.
To reach 10 atm, I’d go down 80,0000 ft… But I’m already feeling very uncomfortable at 2 atm, so I wouldn’t go any deeper without protective equipment. The same sort of equipment deep sea divers would use.
Air has low density. Water has greater density than air. As a result, fluid pressure on my body increases at much shorter distances in water.
To reach 2 atm of pressure in water, I need only dive down 33 feet.
To reach 10 atm in water, I could dive to 297 feet.
To recap:
– 20,000 ft down a dry shaft to double atmospheric pressure at sea level
– 33 ft down in water to double atmospheric pressure at sea level
And the difference is proportional to the difference in density between air and water.
One more fun comment:
Gravity doesn’t do different things to different materials. Water, rock, air, and anything else on earth with mass speeds up toward the center of the earth at the same rate.
To demonstrate this experimentally, drop a bowling ball and a feather from the same height, at the same time. The bowling ball drops quickly, but the feather drifts down. Now drop them both again, but in a vacuum chamber that has pumped out all the air. What do you think will happen?
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