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
Weight
Air doesn’t even compare to the amount of weight water has
Imagine you are laying on your back.
Picture air as balloons. Now you are covered in thousands of them. You’ll probably won’t feel a thing
Now picture water as water filled balloons.
Have thousands of these balloons on you and you’ll feel them
Think of it like this:
An arch of rocks? A bridge.
An arch of sand? A pile of sand.
An arch of water? A puddle.
A cave exists because the rock around it is rigid. This rock holds up the ceiling and keeps the space clear.
Sand or water both flow. Nothing is held up. It all flows into the cave, making it no longer a cave.
Answer: If I understand your question, and I’m not sure i do, you want to know why at 25,000 feet below sea level in the ocean, things crush whereas they don’t in a cave.
It is the weight of the water and being submerged in water. Every 33 feet of depth adds another atmosphere (15 psi.). Since water is a fluid, it pushes equally on all sides of the vessel.
>Underwater, there are insane implosions at 25K feet below sea level. At that same depth on land, we have caves.
No we don’t. I think you’re not understanding how deep that is. The deepest cave is only about 7,500 ft deep, so a third as deep. (Also, that cave actually begins at about 7,500 ft elevation and the bottom is at approximately sea level.)
>Why doesn’t the earth compress things like water does? Does gravity do different things to different materials?
The answer to the question you’re really asking is that rocks actually have shear strength and you can tunnel through them without having them collapse, depending on exactly what you’re tunneling through and how you build the tunnel. Water doesn’t have shear strength. It just flows everywhere it can. So the reason you can have a 7,000 ft deep cave with a void that people can go into and survive, but 7,000 ft underwater would crush you to death, is that in the cave, the rock around you is supporting all of the weight of the rock above it, and the only pressure increase you have to worry about is air. Air has very low density, so you don’t have to worry too much about the increased air pressure.
The deeper you go in the ocean, the more ocean above and around you that wants real bad to be in the space you currently occupy
you can drill a hole in a rock, you cant drill a hole in water. the earth is pretty much self supporting so it can have holes in it but the ocean is not so it tries collapse into any empty space.
Caves are openings in rocks that were (often, there’s also caves in result of tectonic shifts) dug by water or lava and hot gasses. They already applied a lot of pressure, and rocks that could withstand that pressure, are what’s left. Other rocks were dug out.
Underwater caves can have high water pressure – it’s about compressing gasses and liquid, not rocks. Air is much easier to compress, so caves filled with air, even deep ones, do not have high pressure. Because if you compress air, it’ll just escape into areas of lower-pressure air. Water cannot be compressed whatsoever, so it’s just holding all the weight. Also, air (our atmosphere) **is** what is keeping ocean surface intact and pressures in the bottom high. If we had no atmosphere, oceans would also just boil away.
Think of a piano falling on you, and a sofa. Sofa compresses much better than a piano, so it’s likely to do less damage than a piano (you’ll experience much less pressure).
Air is far, _far_ less dense than water.
The greater the density of a substance, the more rapid the pressure changes are within that substance as one climbs or descends.
(_Specifically, this is because the substance with greater density will have greater mass for the same volume, and that greater mass means that gravity pushes it more._)
The density of dry air at sea level pressure and 0 C temperature is about 1.29 kg/m3. The density of water at 4 C temperature is about 998 kg/m3.
It only takes about ten metres of water to equal the entire weight of air in the atmosphere.
The deepest cave is only a bit over 7,000 feet deep. The Kola Superdeep Borehole (deepest hole we’ve ever made) wasn’t just shy of half of 25K feet. I think that deep Earth would compress most things.
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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