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I have a volcano spewing a lavaflow over the countryside. I have a small valley area protected by an advanced forcefield. It is an extended version of a space going ship's forcefields.

When lava flows on the surface it can form a 'crust' between the air and the 'cooler' surface lava while the internal lava stays hot and fluid. I'm having trouble determining if the lava can act in a similar manner when interacting with a forcefield. I have a feeling that the lava will flow fast and hot against the forcefield...which is not what I want.

I need to know when the forcefield is shutoff/collapses, if the previously heldback lava will continue flowing into the area or not, and how fast (relative to humans fleeing in panic) the lava would then flow.

  • What type of plot forcefield would allow the lava to solidify on sustained contact? What plot forcefields should be avoided to avoid hot runny lava dams?

The forcefield will be covering the top of a steep sided valley, so the lava (and heat) will end up only flowing on top of the valley (like a roof). The lava will eventually cover the entire shield but could be thicker on the edges if the shield is slightly curved. It would be more viscous slow moving lava and not originally very runny. Ie more the 10-20km/h lava than the 60km/h lava types.

The forcefield is currently the standard scifi stop radiation/asteroids etc type of forcefield. It stops matter (and air) but allows visuals so I therefore assume energy (as already mentioned by users). It is not a battle ship so only has minor energy deflecting shields (mostly just the normal radiation energy shielding, not designed for directed blasts of energy).

Good point on the baking temperature. Will definitely provide me with some story scenarios in itself.

  • I could possibly invest in two shield types. 1 forcefield to stop the actual lava that can be extended across the entire small valley and a 2nd much smaller energy force field to protect an isolated spot in the valley or on/near the ship from the heat (not necessarily both connected to the ship). This would require extra energy generators etc.
  • I could just have to use the shields as diversions to give people enough time to flee altogether (was hoping to trap them).

  • This revision does not negate any answers provided so far. Which is why I have not removed the originally phrased question. This is because the question is essentially the same, how does the lava act? I knew it was one of two options, fluid and crusty. I have just focused the direction of answers in my intended direction to how can I get the lava to act a certain way (which I now hope is not too story-based).
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closed as primarily opinion-based by L.Dutch, sphennings, Ash, Green, March Ho Sep 27 '17 at 13:44

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ How are we expected to answer considering that a force field is a fictional device? Your story, your rules. $\endgroup$ – L.Dutch Sep 27 '17 at 8:47
  • $\begingroup$ Define how your force field works. How it interacts with air? Does it allow heat transference? $\endgroup$ – Vylix Sep 27 '17 at 9:01
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    $\begingroup$ Does the lava surround the force field entirely at any point? I ask because if it does and you allow the lava to transfer heat through the shield then anyone inside is probably going to bake to death, regardless of whether the lava actually touches them. $\endgroup$ – Joe Bloggs Sep 27 '17 at 9:09
  • $\begingroup$ @L.Dutch. A forcefield as a plot device still needs to make logical sense (at least that is what I prefer). I'm asking for some help making my forcefield plot devise stand up to the rigours of readers disbelief. Should I tag this instead as reality check? $\endgroup$ – EveryBitHelps Sep 27 '17 at 10:29
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    $\begingroup$ And you still leave the original question on your post. Please consider removing it to avoid confusion (and prevention of reopening) $\endgroup$ – Vylix Sep 27 '17 at 15:11
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This all comes down to the mechanics of the forcefield. Does it allow the heat from the lava to pass through and be carried away by air convection on the inside? If it does then the lava will cool and solidify. If it doesn't then it will not. It really is that simple - and without knowing the behavior of the forcefield there is no way for us to guess.

Note that Lava in contact with the ground does cool as well (although less so), you tend to get cooling all around the stream with the center staying hotter.

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  • $\begingroup$ Thanks. This is pretty much the simple answer to my simple question. Do you know if it would be feasible to mix shields a little bit? So as to reduce the amount of heat entering the valley alittle but still allow the lava to cool due to heat transference. Obviously this would then increase the time needed to allow the lava to solidify... $\endgroup$ – EveryBitHelps Sep 27 '17 at 11:06
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    $\begingroup$ @EveryBitHelps that's entirely down to you. Force fields are common in fiction but utterly impossible to build in the real world. Should it be possible to build them we have no idea how to do it or what they would look like. In other words you can give them whatever behaviour you want. The key is to work out what that is and then be consistent. $\endgroup$ – Tim B Sep 27 '17 at 11:17
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Quite a few unknown variables, but since I cannot comment (low rep), I will take a stab at this with assumptions.

The first question here is what kind of an "extended" forcefield is this? I assume it was created for the purpose of diverting or holding back the lava. Since you mention spewing, I will assume it diverts. Depending on the forcefield's frequency, it might disallow the lava from pooling and cooling off. I think it's kinda up to you to define how the lava reacts to the forcefield since this is an unknown tech by today's standard so it can be adapted to do whatever you like.

It also depends on your narrative. If the forcefield stays up for months and the lava flows against it and it solidifies, it might be safe to turn off the forcefield then. If you want to destroy the town on purpose by accidentally turning off the forcefield, flowing lava would be more dramatic.

As for the people living in the town, consider Pompeii. Of course in this case the deaths of the people were so fast due to the pyroclastic effect. But in your case, the inhabitants of that place live with the lava problem daily and hopefully you have put a warning system in place. If that warning system goes off, then the speed of the released lava might not be an issue since lava doesn't move that fast. But I guess if it's downhill to the valley, it might gain some speed. Also, the distance of the forcefield plays a major role. If it's 10 KMs out of the city limits, again there's enough time to evacuate. Having buildings next to it, then they're probably a goner.

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The short answer is that the lava would form a crust against the force field, but the cooling time would be a bit less than the cooling time of a surface lava flow.

The reason I say this is because most force fields in science fiction allow the passage of energy but not matter. So a person behind a force field can’t get through it, but you can still see them behind the force field. So energy not matter can pass. If this is the case then the lava piling up against the force field will be able to radiate heat through the force field and cool down. Probably not as quickly as would have been expected on a lava flow as there would be no convection cooling through the force field, but still quite rapidly.

The lava could flow fast and hot against the force field if the force field does not allow energy to pass. But in this case the force field should appear as a black dome or other shape as no electromagnetic radiation such as light can pass through it.

If the power was shut off the lava might continue flowing but it would depend on the length of time the barrier had been in place the longer it is there the greater the cooling effect and the more likely the solidified lava is going to act as a barrier to further lava. Lava comes in a variety of viscosities depending on the rocks from which it was made. it can be treacle like or oil like or anywhere in between.

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  • $\begingroup$ "most force fields in science fiction allow the passage of energy but not matter" Depends on the energy. There are many SciFi universes which have energy-based weapons and force fields which can be used to defend against them (Like Star Trek with phasers vs. deflector shields). In many universes they don't block visual light, though, which is also a form of energy. $\endgroup$ – Philipp Sep 27 '17 at 9:13
  • $\begingroup$ In that case any answer is dependant on the nature of the force field and its properties. Almost anything is possible. $\endgroup$ – Slarty Sep 27 '17 at 9:24
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I have a feeling that the lava will flow fast and hot against the forcefield...

Then that is what happens.

You are talking about a fictional device with fictional properties. You have not specified how your forcefield works, what mechanism it uses (and you can't, because its fictional).

That means you can hand-wave it however you want.

If the force field does not conduct heat, then the lava flows hot and fluid. But if no energy passes the forcefield (no heat), then how would light pass through it? Ah, simply say that it does.

If the force field acts more like an actual, physical barrier, it will conduct heat. it will be hot on the outside of the forcefield (where no lava is), but this process that carries heat away will allow the lava to cool down.

I need to know if when the forcefield is shutoff/collapses, if the previously heldback lava will continue flowing into the area or not, and how fast (relative to humans fleeing in panic) the lava would then flow.

It flows at the speed of plot.

If you want to have situations in your world where people are threatened by failing force fields, make it flow. If you want to have a world in which force fields are used to mold lava into shape and then be safely turned off, then make it so.

It depends on the kind of world you want and the kind of stories you want to emerge in your world. Both are possible, and both lead to very different worlds and stories within those worlds.

As far as flow velocities are concerned, lava flows at different speeds depending on composition. Lava flows can range from 10km/h to 60km/h. it obviuosly flows faster if it was piled up against a high force field before (like a bursting dam).

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In traditional sci-fi, there are two types of force fields:

  • Deflector shields
  • Energy shields

The first, deflector shields, generally behave as the name implies: like a deflector. They don't necessarily stop the impact, but they absorb the vast majority of the energy associated with it. Examples would include the X-Wing shield generator, Star Trek deflectors, and Goa'uld personal energy shields.

Traditionally, these shields stand up well to high-impulse events: high velocity impacts, plasma bursts, laser fire, fists, etc. Their weaknesses are that they do not stand up well to sustained/continuous fire and they often have little to no effect on low-energy impacts; allowing, say, a person to reach through them harmlessly. I don't think this is the type of shield you would be looking for.

The second, energy shields, act like solid walls with the projector inertially locked to the shield itself. Physical objects, laser blasts, people, fists, and flowing lava behave as if they have run into a solid wall on impact with the shield, while the projecting object absorbs a portion of the energy expressed as impulse. Whether certain energies, like heat, teleportation beams, visible light, and radio signals, can pass through them is entirely up to the story in question. Examples would include the Hoth Defense Shield, City Destroyer shields, and Asgard energy shields.

Generally, energy shields are assumed to operate by absorbing the energy of the incoming object and dispersing it across the entire shields surface, dissipating the energy.

I think that it is pretty safe to assume that a properly powered energy shield that can absorb the energy from the lava at the same rate that the lava can radiate it will hold against the onslaught. Further, as the energy is absorbed by the shield it will be removed from the lava itself (conservation of energy) resulting in a rapid cooling of the portion of the lava in contact with the shield.

For a related concept, check out this clip from Stargate Atlantis where they use a defensive energy shield to contain a multi-megaton explosion.

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  • $\begingroup$ Who knew there were 2 species of deflector shield? Besides enpaul, I mean. + $\endgroup$ – Willk Sep 27 '17 at 13:16
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Simple answer, if you give it long enough the Lava will crystallise, but slower than you think because crystallisation is Exothermic so Lava heats as it cools. Depending on the exact properties of the force field crystallisation could be rapid, if the field dissipates thermal energy or allows a reasonably natural dissipation of thermal energy to occur then the "setting rate" will be similar to, or faster than, what you see with air contact. On the other hand if the field allows the Lava to hold it's heat then time to crystallisation could be years, worse if the Lava somehow gets heated by the field itself.

How stable the crystallised formation is after the field is removed will depend on how high up the field the Lava got, overhangs probably aren't stable and ʻaʻā takes a long time to settle down into stable forms even on flat ground.

Post-Edit:
Okay since we're talking about a dome of rock over the valley, when you turn the field off the valley gets crushed by falling rock, maybe not immediately but it does happen in the end, because "nature abhors a vacuum" and while not strictly speaking a vacuum the valley would constitute a void that will be filled. People under all that rock may be able to tunnel out before the world crashes in on them, if they find themselves under a solid and above all thick layer of solidified Pāhoehoe type Basalt, as several metres of solid Basalt will hold together for quite a while under purely gravitational pressure. To that end if the occupants can manipulate the field shape they'll want to form it in a Catenary Dome for maximum structural strength of the final formation.

Force field characteristics: You'll want a force field that blocks matter and heat or everyone will cook while the flows are cooling and you need to provide alternative air circulation or everything in the valley will suffocate. Gas exchange between the Lava and the valley/cave atmosphere would be a very bad thing too, the gases released by the lava will be Carbon Dioxide, Carbon Monoxide, and various Nitrogenous and Sulfurous Oxides none of which are healthy, and Water Vapour which will mix with them to make strong acids.

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  • $\begingroup$ Nice post edit additions. Had thought about gases and breathable air but hadn't yet connected all that with acid! Hmmm, the evil in me can have some fun with that! $\endgroup$ – EveryBitHelps Sep 27 '17 at 12:40
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    $\begingroup$ @EveryBitHelps Yeah have a look at this for just how acid things could conceivably get. $\endgroup$ – Ash Sep 27 '17 at 12:44
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The force-field will need to be both a thermal superconductor and energy-absorbing field. This is apart from its usual properties of acting as an impenetrable barrier. This will rapidly draw heat out of the lava causing it to solidify.

Considering the force-field will allow visual information to pass through it, it can be concluded that this force-field will permit a window to allow only sufficient light through to see through it but radiant heat that exceeds safe levels of thermal radiation will be absorbed.

In this case, when the force-field is turned off the solidified lava will remain in place. Of course, this will depend on the mechanical properties of solidified lava itself.

In summary, thermal superconductive and energy-absorbing force-fields will solidify lava in reasonable timescales

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