would lava crush a fireproof person?

[halfeye]

This started in another thread, but ended up being totally off topic in it:

http://www.giantitp.com/forums/showt...al-World-logic

Just being near molten rock can suffocate a person, ... Complete immersion would crush a person under the weight of the rock, irrespective of their heat resistance.

This is hogwash, if it was at a nice temperature (which is a whole third argument) the weight of liquid rock would be spread out all around you, and wouldn't do you much harm, people dive to 400 feet deep in water, the pressure there would be higher than at the surface of a pool of lava. The temperature would kill you quickly, the pressure wouldn't do much harm.

The density of lava is thrice that of water and its viscosity is between 100,000-1,100,000 times greater. To be clear, this means it would be virtually impossible to sink into the stuff and, if you were somehow fully submerged and were somehow not instantly mummified and then disintegrated by the heat, weight, and pressure, you'd rise to the surface. This means I should qualify my statement: lava pressure would escalate far more rapidly than water pressure and crush you far more quickly at a far lesser depth.

The viscosity depends on the chemisty and the temperature.

Spoiler: picture

Show

You're saying that's a million times more viscous than water? Wrong.

https://en.wikipedia.org/wiki/Lava

Yeah, it's dense and you'd float. You'd burn too, but we're ignoring that. It's denser than water, but less dense than mercury. If you were fireproof and dragged under by an asbestos or otherwise fireproof rope while wearing a fireproof aqualung, the pressure wouldn't kill you immediately.

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That's a 10 meter spray of lava. The smallest "drops" are still fairly big and chunky. If it was as runny as water, it would be breaking up into a fine mist at the edges. Do an image search for a spraying fire hose and see how the water breaks up into very fine droplets that are much, much finer than the lava in that picture.

Water has a viscosity of 0.00089 Pa*s (Pascal seconds). Lava has viscosities ranging from 100 to 1000 Pa*s. It can be objectively measured. The lava in your picture is at the low end of the range which would put it around 10,000 to 100,000 times as viscous as water.
...

...

I said it was between 100,000 and 1,000,000 times more viscous than water, which, as Xuc Xac mentions, is measurable and has been measured. Molten rock is still rock, not boiling kool-aid.


I already agreed with you on that. "This means I should qualify my statement: lava pressure would escalate far more rapidly than water pressure and crush you far more quickly [than water] at a far lesser depth." You're right that it wouldn't be immediate, but the pressure both starts higher and scales differently, and due to the viscosity, it'd be impossible to move voluntarily while submerged. Full submersion in lava would immobilize and crush a person (if they were somehow immune to heat, but we're granting that).

The "drops" of lava in the picture are typically flat and squarish, with water, droplets are more or less sphereoidal. Obviously, lava is not water. The lava is cooling and rising in viscosity as it leaves the source of the plume.



I suspect it's lower than that. However, as you imply it's a guess on my part. Glass has a higher viscosity than water, and lava, until it melts.



Rock being rock means it is dense, the viscosity depends on the temperature and pressure. Higher temperature (for a given chemical composition) means lower viscosity.



Syrup and molasses have significantly higher viscosities than water, but you can move in them. To crush a person would require a pressure gradient, which at the surface of a lava flow you won't have. Water kills a lot of people, but mostly not by crushing them. If the density of lava is only three times the density of water, and people can survive to 400ft in water, it follows that it would take 100ft plus depth of lava for the pressure to kill you. You'd burn, obviously, so this is definitely a "don't attempt this at home", but given a theoretic fireproof person, they'd have about 100ft to sort things out.

There's an exponent missing from your logic. Carry a gallon jug full of water on top of your head for several paces, then cover the same distance with a basalt rock of the same volume. Now imagine that difference in heft pushing on every square inch of your body.

Have you tried lifting a gallon jug? That's heavy.

You don't feel that weight when swimming. That's the difference I'm talking about, the water you swim in is just as heavy as the water you carry, but it doesn't press on you the way you'd perhaps think it would, because it pushes up as well as down.

Glass doesn't have a viscosity until you melt it. It is not a liquid. The ripples you sometimes see in sheets of old glass are a result of the casting process, and nothing else.

Oh, never mind. Don't guess. Go look it up.

So, is there nobody else who thinks that the depth at which you'd be crushed by lava is the depth at which you'd be crushed by water divided by the difference in density, or some reasonable function of the difference in density?

[Zeb]

If you could wiggle under it, probably. A neat set of questions about Lava were answered here: https://www.livescience.com/33622-si...cano-lava.html

But the bit we care about is:

First, lava is more than three times denser than water; because humans are made mostly of water, it's three times denser than us, too. The laws of physics therefore dictate that we will float on its surface, not sink. Secondly, lava's viscosity its resistance to flow is between 100,000 and 1.1 million times higher than that of water. That means a pit of lava is about as fluid as a jar of peanut butter or a vat of Crisco.

Often the closest analogy without heat would be drying cement or concrete.

[crayzz]

Depends on the depth. If you managed to get underneath it still able to breath and could survive the temperature, the pressure would be about 3 times the pressure of being underwater i.e. the equivalent of being about 3 times deeper in water. A quick google shows that the record for a deep dive is around 300 meters, so 100 meters of lava seems like a good limit on whats theoretically feasible.

[Yora]

It's still stone. Just like sand or gravel. Except more denser, because there are no air-filled gaps between the particles.

And it really doesn't take a lot of sand to make a person immobile.

[Rogar Demonblud]

So you won't float if you're under it, because it will hold you down just as the concrete will. Or an air bubble in a jar of peanut butter, if we're continuing that analogy.

Also, if you're submerged in the lava you will get encysted as the lava next to you is cooled due to your body absorbing the heat thanks to convection. Not sure if the 'fireproof' power would cover that.

Most importantly, if you're buried in lava you can't breathe, so surviving anything else is moot as you have suffocated.

[crayzz]

It's still stone. Just like sand or gravel. Except more denser, because there are no air-filled gaps between the particles.

And it really doesn't take a lot of sand to make a person immobile.

Immobilized and crushed are different things, though. You wouldn't be able to move, but the pressure felt by your body wouldn't be vastly more than the pressure you feel underwater.

Also, if you're submerged in the lava you will get encysted as the lava next to you is cooled due to your body absorbing the heat thanks to convection. Not sure if the 'fireproof' power would cover that.

I would probably rule that it does. If we're talking about "fireproof" in the fantastical sense, it makes sense to say that fireproof means the heat isn't transferred into your body.

[The Jack]

Most importantly, if you're buried in lava you can't breathe, so surviving anything else is moot as you have suffocated.

What could be breathable in lava?

Someone should map out how the biology of a lava-native creature could work, using as little magic as possible. So many fantasy games have creatures that live in or near lava. do your best to justify magmic life.

[Lord Torath]

What could be breathable in lava?

Someone should map out how the biology of a lava-native creature could work, using as little magic as possible. So many fantasy games have creatures that live in or near lava. do your best to justify magmic life.

I think we'd have to assume a heat-proof snorkel or SCUBA tank (also heat-proof). With a viscosity 100,000 times higher than water, no-one's going to be cycling this stuff in and out of their lungs.

[Rogar Demonblud]

Actually, any creature living in lava isn't going to be breathing in a way we recognize. Probably also a silicate based lifeform as well. Maybe even straight up Series VII.

[Chronos]

Yes, the crush depth in lava would be equal to the crush depth in water divided by the specific gravity of the lava. The world record free dive in water is over 200 m, so if lava has a density about three times that of water, that'd correspond to 70 m or so in lava. Which is pretty deep.

The viscosity would certainly slow the person ascending to the surface, but I'm not sure by how much. And if you started off on the surface, it'd be very difficult to dive under, as both the buoyancy and the viscosity would be opposing you.

[Knaight]

Viscosity is pretty irrelevant here - pressure/area is a function of density, gravity, and height of the fluid column. So, assuming you're supported by the lava from below as well as have it pressing down from above you can handle a fair amount of pressure, and the viable depth is going to be the viable depth of water divided by (water density)/(lava density).

In the context of actually being able to move, viscosity is suddenly critical, and a million times difference isn't a terrible figure (though it does vary). Water has a very low viscosity, and low viscosity lava is still high viscosity compared to most substances (though there are some tars that blow them out of the water).

[Aeson]

Regarding 'crushing,' it should be remembered that no real animal is an incompressible solid of uniform density and material composition. There are parts of your body that can be crushed by sufficiently high pressures - your diaphram, lungs, esophagus, intestines, oral cavity, ear canals, nasal passages, etc - if they are not filled by something that balances the pressure out, and if you're working over a sufficiently large pressure range you can also run into problems related to the fact that the incompressibility of liquids and solids is a model rather than a literal truth.

Additionally, pressure by itself is not the only issue; lava/magma is sufficiently dense that the pressure differential over your body might be problematic in an Earth-standard gravitational field - assuming a density of 3,000 kg/m³, an upright ~2m body immersed in lava/magma would experience a pressure differential of about half an atmosphere, which if I am not mistaken would be similar to experiencing a constant acceleration of about 3 gravities. This is insufficient to cause major bleeding through those parts of the body under less pressure or pop your eyes out of their sockets or anything like that, but it could certainly create circulatory problems leading to redout/blackout or heart failure after prolonged exposure, and it might be enough to rupture capillaries or cause damage to other relatively fragile parts of the body.

Also, it should be mentioned that many chemical properties are pressure-dependent, which, even if not relevant to the question of 'crushing' a body immersed in lava/magma, could be very inconvenient for an animal that wants to move any great distance vertically through a high-density fluid column in a gravitational field. Whether or not you're fireproof, you probably wouldn't want your blood - or any other part of your body - to undergo a phase change, for example.

[NichG]

Pressure doesn't work differently just because it's lava. If there's a crush depth for water, the crush depth for lava should be the point at which the pressure is the same - which is proportional according to the ratio of the density of lava to the density of water. That holds regardless of how detailed a description you use as to how 'crushing' would happen - chemically, phase changes, etc - and isn't a function of the viscosity of the medium.

Now, if there were some reason for part of that compression to be translated into shear, then you might be in trouble. That's where viscosity as well as density fluctuations might matter. If you were caught in a diverging lava flow, for example, it might be easier for the left and right sides of your body to be separated and carried along with their respective flows than for the lava flow to accomodate you. Similarly, just as it wouldn't be great to be the solitary structural element holding back the water pressure behind even something like a swimming pool (imagine a suspended pool with a vertical drainage tube and a person blocking the tube), it would be three times worse to hold back a swimming pool filled with lava - so if you end up plugging the flow downhill somewhere, then that pressure can turn into shear and rip you apart.

[Knaight]

Now, if there were some reason for part of that compression to be translated into shear, then you might be in trouble. That's where viscosity as well as density fluctuations might matter. If you were caught in a diverging lava flow, for example, it might be easier for the left and right sides of your body to be separated and carried along with their respective flows than for the lava flow to accomodate you.

It doesn't even need to be diverging. If the fluid is moving it's generally moving within some constraints or other. (a channel, a pipe, moving over non-infinite depths, etc.). At the boundary itself there's generally no motion (the no slip boundary condition), which means there's a velocity gradient with respect to position. That could easily lead to a pressure build up behind something submerged due to the forces involved, where viscosity is far more relevant.

[OracleofWuffing]

I think we'd have to assume a heat-proof snorkel or SCUBA tank (also heat-proof).

The SCUBA Tank got me wondering about how much Decompression Sickness would start playing a role on this. I mean, assuming somebody wants to get out of the lava.

I'm getting conflicting information from the Internet about how deep you need to go before your rate of ascension in water becomes a very significant concern, but they're generally agreeing that the warning spot is well below even a tenth of the 300 meter drive. Taking what Google gave me as an example, a water diver wouldn't want to ascend faster than 9 meters per minute. I'm taking the assumption that the lava is three times as dense as water from upthread, and thinking that means a lava diver's speed limit for ascending is 3 meters per minute. That's before you factor in breaks at certain intervals, so if I'm doing the math right at "way past bedtime," it's probably going to take over three hours to... Safely? Ascend out of 50 meters of lava, assuming the lava's in a state that allows movement. Better bring a heat-proof drill, too, in case the surface hardens.

Disclaimer: I live nowhere close to an ocean or a volcano and am vastly out of my leagues. Pun intended.

[Jay R]

1. Lava is just rock. Yes, enough rock will crush someone. The depth at which you would be crushed by lava is the same depth at which you would be crushed by rocks.

The crush depth of water is pretty irrelevant, because we are pretty much made of water. We aren't made of rock.

2. Does your fireproof person need to breathe? No air will reach him under lava, for the same reason that no air will reach him underwater.

[NichG]

The source of pressure doesn't really matter - it's the amount. When someone is crushed by rocks, the pressure experienced is significantly higher than you'd get if you melted those rocks down and submerged the person in them, because in the molten rock case that weight is distributed uniformly over their surface area whereas someone for buried in solid rocks there are small contact surfaces at the points/edges/rough areas which have to either support the entire weight of the rock, or permit the rock to sink into whatever is below it until the rock's weight is supported - namely, the person.

[factotum]

The viscosity would certainly slow the person ascending to the surface, but I'm not sure by how much. And if you started off on the surface, it'd be very difficult to dive under, as both the buoyancy and the viscosity would be opposing you.

The UK "science" show "Brainiac" actually had someone try to swim in a pool filled with a non-Newtonian fluid, whose viscosity depends on how fast the shear force acting on it is, and he was exhausted after a dozen yards or so. Lava, which is always really really viscous, is going to be worse than that.

[Fat Rooster]

It certainly wouldn't crush them any more than deep water would, but they would almost certainly float anyway unless the lava was cold enough that it would barely qualify. A person is about 1/3rd the density or lava, meaning that a 50kg person would have a buoyancy of ~100kg. A strong swimmer can manage a thrust of about 50% of body weight at peak, It is not a case of swimming, it is like more being dragged by a jetski. An air bubble would only have 50% more than that, and we see bubbles rising to the top of lava.

Size also plays a factor. Absolute viscosity is less important than Reynolds number, and bigger scale means smaller Reynolds number. Water has a viscosity orders of magnitude lower, but we see bubbles rising in water that are best measured in micrometers.

[Traab]

I think the main question has been answered, yes, if you are buried under a deep enough pile of lava, you will be crushed by the pressure. Fireproof doesnt mean immune to being squashed like a tin can in a hydraulic press. I did like the part about shearing effects though. If the lava you are buried in is flowing, could it exert enough force to tear you apart? Again this is working under the assumption of being buried as I doubt boogie boarding your fireproof self down a lava flow would have any effect on your fireproof self aside from road rash. Yes you would eventually float to the surface, but im just curious about the forces involved in that.

[tomandtish]

The UK "science" show "Brainiac" actually had someone try to swim in a pool filled with a non-Newtonian fluid, whose viscosity depends on how fast the shear force acting on it is, and he was exhausted after a dozen yards or so. Lava, which is always really really viscous, is going to be worse than that.

Mythbusters did the same thing with syrup (also non-newtonian).

[Bohandas]

It's still stone. Just like sand or gravel. Except more denser, because there are no air-filled gaps between the particles.

Doesn't extreme heat generally make things less dense? Given space to do so, things expand when they heat up.

[NichG]

Doesn't extreme heat generally make things less dense? Given space to do so, things expand when they heat up.

Gravel, sand, and other granular materials have 40-60% void space. So melting them would be expected to roughly double the density. Thermal expansion of liquids and solids on the other hand is usually less than a percent. Basalt is something like 0.1% per thousand degrees. Mercury, which has a high coefficient, is 6% per thousand degrees.

[Knaight]

Gravel, sand, and other granular materials have 40-60% void space. So melting them would be expected to roughly double the density. Thermal expansion of liquids and solids on the other hand is usually less than a percent. Basalt is something like 0.1% per thousand degrees. Mercury, which has a high coefficient, is 6% per thousand degrees.

The void space dominates by far, but it is worth noting that thermal expansion is a temperature dependant property here - to the point where you actually generally see less and less of it as you add more heat (though it's far from a smooth decreasing curve).

[deuterio12]

Bit late but

Doesn't extreme heat generally make things less dense? Given space to do so, things expand when they heat up.

Heat is basically just atoms/molecules vibrating (with absolute zero temperature being zero vibration, that's why you can't go below it, it would take being more still than perfectly still), so the hotter you go the more you're vibrating so the particles tend to spread out. How much depends in the material composition since some have more room to shake.* Hot air floats up because it's less dense since its molecules are vibrating more.

Usually this also means the same substance will be less dense in liquid form because kinda by definition the connections between a liquid's particles are a lot looser than those of a solid and so spread out a lot more.

There's also some exceptions where the solid's structure is actually pretty spacious and results in being less dense than the liquid version.-water in particular. Ice floats because it's less dense than water. This also results in the bizzarre effect that as water cools down it becomes more dense, until it's almost at 0 degrees and then suddenly starts becoming less dense and that's why ice forms at the surface of water.

*Fun fact, lasers can be used to cool a gas by hitting it from different directions so the atoms are basically "caged" and so vibrate less.

[hajo]

1. Lava is just rock. Yes, enough rock will crush someone.

And it doesn't need much of that - there have been accidents on playgrounds,
where children have been buried, and died, under a few cm of sand.
Well, not 'crushed', but immobilized and suffocated.

Also, no need for sand/gravel/concrete/rock or high temperatures to make it extra dangerous:
air-filled water (aka snow) as in an avalance, gets several people every year.

[Imbalance]

Since I got dragged into this, halfeye, what have we learned about being crushed by lava?

[halfeye]

Since I got dragged into this, halfeye, what have we learned about being crushed by lava?

In real life, the heat would get you pretty quickly, but everyone agreed about that anyway.

I still think that if you were heatproof and had a heatproof aqualung, which would take magic, then you wouldn't be crushed by a depth much less than 100ft, some people seem to agree, some people seem to disagree.

[Lord Torath]

I think I can agree with not being crushed until around 30m. I'll add that if you were covered in more than about a meter of lava (and had the afore-mentioned magical heat-proofing and heat-proof aqualung) you'd be unable to free yourself. With only half-a-meter or so on top of you, the difference in density just might (slowly) float you to the top.

[Imbalance]

Does it help to clarify that death by crushing doesn't necessarily mean eyes popping, bones ground to dust, and flattened into a pancake?