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In my (fantasy) world, a civilization sets off a superweapon that releases so much heat that it glasses an entire desert a little smaller than our Sahara. Is this feasible without killing an entire earth-sized planet's population or worse? What would the side effects be?

Edit: To clarify - The weapon backfires by accident. I'm imagining a massive (purely-thermal) explosion in the middle of said desert as the source.

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  • $\begingroup$ The entire desert? I'd have to run some numbers but my gut is telling me that'd be disastrous for life on Earth, if not necessarily apocalyptic. It'd definitely have widespread long-term ramifications for the environment depending on the method used to achieve this (I'm assuming thermal energy produced by a massive explosion going off of your description). $\endgroup$ Commented Dec 6, 2016 at 0:23
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    $\begingroup$ Well, melting your mini-Sahara to a depth of 4 inches within a radius of 1500km would take a significant fraction of a teraton of TNT (that's about 4.184e21 joules, and this would be around 1e21, so a few hundred gigatons). For the sake of comparison, the asteroid that killed the dinosaurs rated about 100 teratons, so you're not quite at planet-killing yet. Unfortunately, this is still way, way more energy than the entire nuclear stockpile, every volcanic eruption in recorded history, and the hypothetical eruption of Yellowstone put together (times ten). You tell me how bad that would be. $\endgroup$ Commented Dec 6, 2016 at 0:34
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    $\begingroup$ Z.Schroder -- consider moving your comment to an answer. $\endgroup$
    – SRM
    Commented Dec 6, 2016 at 5:07
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    $\begingroup$ I would note that magic gives you a great deal of leeway in having this occur. Think about the sort of magical/superweapon explosions often seen in the movies, for example: the weapon blast expands to a fixed radius, then collapses in on itself or dissipates. Your superweapon could open a portal to another dimension that sucks the energy back in before it can render the planet uninhabitable, for example. Magical “explosions” can really occur any way you like. $\endgroup$
    – Obie 2.0
    Commented Dec 6, 2016 at 5:10
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    $\begingroup$ Can some sort of GreyGoo scenario work here? Some kind of chemical explosion that converts nearby sand to glass and is self propagating till coastal surfaces? $\endgroup$
    – AEonAX
    Commented Dec 6, 2016 at 5:42

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How much sand are we dealing with?

The Sahara Desert is about 9,200,000 km$^2$. Lets say we want to glass everything down 10 meters. That is 92,000 km $^3$ of sand, or $9.2\times10^{13} \text{m}^3$. Lets just assume for the sake of argument that the whole thing is covered in sand (its not).

The density of sand is around 1500 kg/m$^3$ for a total of $1.4\times10^{17} \text{kg}$

How hard is it to melt sand?

This paper on the properties of silicon dioxide gives molar heat capacity varying from 44 J/K at 300K up to 81 J/K at the 1700 K melting point, and a molar enthalpy of fusion as 9395 J.

One mole of SiO$_2$ is about 60 g. Integrating from 300K (which is about desert temperatures) to 1700K and adding the enthalpy of fusion gives 108 kJ per mole, or 1800 kJ per kg.

Multiply required energy by mass and we get $2.4\times10^{23} \text{J}$

That is 240,000 Exajoules, or 59,369 Gigatons of TNT, if you prefer. Since not all energy would be directed into the ground/sand, you would expect to at least double the blast energy.

How big of a blast is that?

EDIT!!! Major math errors! My original edit forgot that I was using kJ and was low by a factor of 1000. Your blast delivers about 20% of a dino-killing asteroid just to the sand! If you assume that at least as much energy is delivered to the atmosphere in heat and shock waves, then you pretty much have an extinction level event on your hands. Good luck!

About the same energy as an asteroid impact of 1 km diameter (which happens every half million years or so). The last mega asteroid hit was Chicxulub which was probably around 240,000 Gigatons of energy released.

An 8 on the VEI index. La Garita Caldera was the most powerful volcano of the Cenezoic (and the biggest explosion of any sort since the Chicxulub impact) with an estimated energy release of 250 gigatons. This produced enough magma to fill Lake Michigan.

So your weapons is going to be around the scale of Garita Caldera. It was probably an order of magnitude more powerful than the Tambora eruption, which caused pitch black skies for over two days within 600km, and the 'year without summer'. So your explosion will be a worldwide event; though fortunately not as worldwide as a dino-killing asteroid.

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  • $\begingroup$ But if it's not just surface, but closer to half sphere, he is doomed, right? $\endgroup$
    – Mołot
    Commented Dec 6, 2016 at 7:12
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    $\begingroup$ @Mołot Edited for bad math. Now he's doomed anyways. $\endgroup$
    – kingledion
    Commented Dec 6, 2016 at 14:09
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    $\begingroup$ Why would OP want to glass everything down to 10 meters? That sounds really excessive. The top 4 inches (0.1 meters) at its worst would still be extremely impressive. That scales back your number by a factor of more than 100. And at the fringes of the desert the effect would not be a complete crust but rather pearls and pools of glass. That would make for a nice ramp-up of the mystery as the explorers move closer and closer to the hypocenter of the explosion. By comparison, the desert floor at the Trinity site glassed only about 2 cm down but that was more than enough to generate interest. $\endgroup$
    – MichaelK
    Commented Dec 6, 2016 at 15:06
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    $\begingroup$ @MichaelKarnerfors I don't know how far it would glass, down that is just a guess. Comparisons with Trinity might not be valid; he's looking for one shot to glass the entire Sahara desert. Without solving heat transfer equations, I estimate there is no way to do that without a lot of extra heating conducting downwards into more sand. Maybe 1 meter is more accurate, thus giving a factor of 10 reduction; but certainly more than 4 inches. In any case, still extinction even size, even reduced by 10. $\endgroup$
    – kingledion
    Commented Dec 6, 2016 at 15:46
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    $\begingroup$ @MichaelKarnerfors I originally had another answer proposing such magical solutions as you recommend. The OP commented that he intended for it to be a heat bomb, and edited the question accordingly, as you can see. So now my second answer is purely thermal. So while I don't necessarily disagree with your assessment of magic, that is not what the OP is asking for. $\endgroup$
    – kingledion
    Commented Dec 6, 2016 at 16:15
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As Kingledion pointed out in his answer, an explosion that can do that kind of thing is going to be pretty hard. Explosions are pretty inefficient at this kind of thing because they waste a lot of energy in the wrong direction on things that aren't purely thermal.

Luckily for you (but not for your desert) you are working with backfiring magic, and not a chemical or nuclear explosion.

You are going to need temperatures of around 1,760 degrees Celsius (3,200 degrees Fahrenheit) to melt sand and rock.
So say you have your evil mage. He decides to glass his enemies kingdom so he writes a very localized magma spell that will start and spread along the ground to cover all the borders of the kingdom. He calls forth all the dark energies, starts to spell, but gets confused and uses his own coordinates instead of his enemies as the starting point. And since he was exiled his "kingdom" is the desert waste. So the spell starts as a field that magically magnifies the ambient temperature ten fold, and it spreads out toward the borders of the desert, melting sand and stone and igniting brush as it goes.

The spell is designed to be purely thermal, and directed toward and through the ground.

Thankfully it's a pretty dry desert, and so there isn't much to burn, meaning no fire storm, but there is a huge thermal plume that goes up from the cooling sand which causes some big storms around the planet for the next weeks.

But since it wasn't an explosion there was no crater, no debries thrown up into the atmosphere, no shock wave or other things like that.

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    $\begingroup$ Alternately the evil wizard got the spell right, but his enemy knew it was coming and so used a "I am rubber you are glue" spell that reflected it back, saving the kingdom and making him a hero. $\endgroup$
    – AndyD273
    Commented Dec 6, 2016 at 11:36
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    $\begingroup$ Alternately alternately the spell was aimed at the physical location of his enemy, but his enemy has tracked him down to his hideout in the desert. So just as he finishes the spell he hears a voice behind him "Morgo, what are you doing?" And the evil mage thinks "oh no..." as the temperature starts to rise. $\endgroup$
    – AndyD273
    Commented Dec 6, 2016 at 11:41
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I think kingledion's answer shows what would happen if this were an explosion with the glassing of the desert as a side effect. But you still may be able to make it work with less disastrous consequences by tweaking the weapon itself - so that the glassing is the primary consequence, not a side effect of an explosion or other destructive effect. Sort of having your weapon go off with a great pouf, instead of a big boom.

The idea I had was to focus in on a chemical-type magical weapon, instead of an explosive type one, where the "going off accidentally" spread a great, low lying cloud of particles (hence the great pouf instead of big boom) which slowly settled down and converted the sand into glass - basically, an area-effect weapon to begin with, designed to spread its effects far and wide with few extra effects outside that area. It could work chemically with "add compound x" to turn sand into glass, or thermally if it produces a lot, a lot of heat to melt the sand but doesn't "waste" effort with anything else, or magically with some sort of transmutation (turn-it-to-glass) or petrification spell (turn-it-to-stone) which for some reason works out as glass when starting from sand. You would get a great area with a thin layer (few inches?) of glass over the desert, centered on the weapon's location. It would either be spreading out equally from the center, or slanted with prevailing winds, with irregularities from geography and the eddies from local breezes.

Alternatively, you could have some kind of catalyst-reaction spell, designed to turn one thing into another (again, specifically to glass, or else to stone, or to something else which just ends up as glass when added to sand). It could be designed to convert an area, or better yet a complete conversion of a "single object" only nobody thought of what might happen if it touched the ground, or else natural variances should have formed boundaries (just what was actually hit, this rock or that stump or the other pocket of soil) but for some reason the sand was similar enough in composition to act like a "single thing" for the spell, and so the spell just kept going, pulling (too much) energy from its caster, or feeding itself by the energy it took to make the change, until it ran out of steam after having eaten the whole area. In this case, I would expect the glass layer to be much deeper at the center where the reaction was spreading downwards as well as outwards.

I would expect the minimum aftereffects to be storms, local and global and short and long term shifts in weather and winds from the alteration of the landscape, extreme flooding when it rains, things like that... because the air currents, heat absorption, and living ecosystem will all be altered by the substitution of the desert to the glass plain.

Additionally, I would expect some aftereffects specific to the method of the change - like using just heat to glass over the dessert would cause violent winds (difference in air temp and pressure), warming the local area (sahara-size means "local" is probably the whole continent and some nearby ocean) a lot and potentially leading to fires in nearby areas, more storms from the abrupt shifts in the winds, in evaporation and condensation, from heat retention, and a host of other things. If the change is chemical-analogous instead of just thermal, then I would expect the prevailing winds to play a huge role in side effects - lesser amounts might travel very far beyond what areas get heavily doused for the glass desert effect, and what a light dusting would do to different areas (ecosystems, foliage, animals) will have its own aftereffects (not to mention any actual aftereffects the weapon would have had if used correctly). If the change was catalytic, then I would guess there were aftereffects with tetonic motion (since it would be spreading downwards as well), and the way the ground shifts and flexes will change from a area made of loose sandy soil to a stiff brittle glass disk.

But despite these side effects, using a method more targeted to your glassing than an explosion should let you avoid some of the earthquakes and tsunamis caused by shockwaves, or cooling from where suspended soot and dust in the atmosphere shade the sun's light and warmth out, or any number of other consequences that might make this a planet-destroying incident. It would still be plenty serious, with violent winds and storms, and slowly altering weather patterns for decades (which is kinda the point of such a big disaster), but it should be survivable.

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