How to Build a Lava Moat (with xkcd)

There are a lot of reasons for wanting a lava
moat around your house, but not that many how-to videos explaining how to build one. So we’re here to change that. It’s actually pretty easy to make lava,
at least in principle – the ingredients are just rocks and heat. Lots of heat. Most rocks melt at temperatures between 800
and 1200 celsius, which is hotter than your oven, so you’ll need a gas or charcoal powered
forge, or an electric furnace. It’s tempting to choose a kind of rock or
even metal or glass that melts at a lower temperature, but then it won’t glow as much:
the intensity and color of the glow depends on how hot the lava is, and you definitely
want a lava moat that glows (preferably a nice bright orange-yellow color). For this reason, we recommend only top quality
Keweenawan basalt obtained from the upper midwest. One of the challenges in maintaining a glowing
lava moat is that it’s literally radiating away its energy in the form of heat and light;
this means you can’t simply melt your lava, pour it into your moat, and call it a day. Your lava moat is going to need some kind
of built-in heating apparatus to make up for the losses: a ceramic crucible with high temperature
electric heating coils will work just fine. And you’ll definitely want a good layer
of insulation to keep heat from leaking into the ground – because you’re going to need
a lot of heat! At the temperatures we’re working with,
lava radiates roughly 100 kilowatts of heat per square meter (that’s equivalent to 1000
100-watt lightbulbs per square meter – though you can’t fit 1000 100-watt light bulbs
into a square meter, if that gives you any sense of how intense lava is). Anyway, electricity typically costs around
10 cents per kilowatt hour, so each square meter of lava moat is going to cost you around
$10 per hour. This means that a lava moat a meter wide and
enclosing an area roughly the size of a football field will cost $60,000 per day to keep running. If you don’t like the idea of being reliant
on somebody else to keep your lava moat home security system operational, you could instead
power it with solar panels, or build your own power plant. Each of these has its pros and cons: solar
power never needs fuel resupply shipments: the sun is going to keep burning for a while. However, every square meter of lava moat requires
2000 square meters of solar panels to keep it glowing day and night. We don’t need to explain the security risks
of putting your lava moat’s power source outside of the protection of the lava moat,
so if you want a lava moat a meter wide powered entirely by solar panels inside of the moat,
the math works out that the moat needs to surround an array of solar panels 8 kilometers
across. Which is clearly absurd. You can have a wider and yet more reasonable-sized
moat if you instead build your own commercial scale power plant inside it – coal and nuclear
power plants produce enough energy to heat a lava moat 10 meters wide and encircling
an area 500 meters across – enough space to fit the power plant and your house inside! The downside is that (unless you build your
moat on top of a coal or uranium deposit) you’ll need to bring in outside fuel, so
your lava moat won’t be entirely off-grid. Perhaps the best off-grid option for powering
your lava moat is simply to take inspiration from the source of lava itself: the earth’s
own internal heat. Assuming you don’t have access to an island
in a lava lake in an active volcano, the next best option is geothermal power: by choosing
the right location, a good geothermal power plant can heat a modest-sized lava moat that’s
perfect for protecting a single family home. I suppose you might also want to learn about
precisely how wide your lava moat should be to deter intruders, or how to cool your house
once it’s successfully encircled by lava, or how to deal with all the noxious fumes
given off by lava. And for that, you need to check out the full
lava moat instructions available in the book, “How To” by Randall Munroe, which this
video is based on (and supported by). How To is an absurd and entertaining self-help
guide full of ridiculous over-the-top advice about everything from how to dig a hole, to
how to be on time, to how to ski, to how to catch a drone. You can find a how-to guide for how to obtain
a copy of “How To” in the video description, and a big thanks to Randall and “How To”
for supporting and inspiring this “How To” video.

100 Replies to “How to Build a Lava Moat (with xkcd)”

  1. Because lava is so dense, I feel like there's a good chance the moat could easily be traversed by a floating plywood plank, assuming you can run quickly enough. Jury is still out on whether or not the plywood should be soaked in water first. If it isn't, it might burn really quickly. But if it is soaked, the Leidenfrost effect from the rapid vaporization (I know it's evaporation, but it's so fast that I feel like "vaporization" really gets the point across better) might make it unworkably slippery.

    But if you construct your moat with a temperature gradient with uneven distribution of the heat generator's output, you might be able to get a convection current going that would make any floating bridge much less reliable.

  2. Having a hot, glowing sea of lava is on my bucket list of real "Common Real world Problems". I'll start with "How to make coffee" for now and will improve on that. Thanks for the recommendation 🙂

  3. dang, that geothermal as final option for 100Kw power input, just blast the smile of nuclear power supporter away
    sorry Chernobyl fans, …the planet just support itself in matter of heat production since it was formed 4.543 billion years ago

  4. if you willing to radiate all things around you with nuclear waste, or stubbornly pour trillion dollars to make a mini sun on our planet …just to power your electronics, why don;t you try to dig way even deeper into the Earth's core, to find out whether your accusation about Earth will lose its internal heat due to geothermal power plant in just several years is true or not, at least people will know that even when we did nothing to it, the Earth will still internally freeze for around a thousand years in the future

  5. oooooooh when you said football you drew a circle with one half a soccer field and the other half an american football field. Sneaky, clever, absolutely brilliant. Also, randall's "what if" book is, in my opinion, much more entertaining to read.

  6. This is definitely in his book How To on how to keep out intruders.

    Edit: I hadn’t finished watching the video, and I was SO CLOSE!

  7. Who even has 100 watt incandescent light bulbs anymore? They've been banned in Australia for over a decade, did the USA not also do this?

  8. Another option for power is the new SLIMM or VSLIMM liquid sodium nuclear reactors. If you bury them underground, you could just build your house on top (with the necessary cooling from the book) and power the entire moat with only one reactor, probably having enough juice left over to power your home.

  9. I hate to say it, but the melting point of plain quartz is about 1000-1100 degrees Celsius (or 1000 units kelvin for that matter) higher than is shown in the video. Also (and I could be wrong about this one as I have not personally melted it) Olivine seems to melt at between 1200 C and 1900 C (which is impressive given that sprinkling a little Mg in quartz will raise the melting point 200-300 degrees). Actually I am more assuming that the melting point of an average piece of Olivine is between 1200 and 1900 because I'm guessing the melting point varies roughly linearly as you move linearly between the extremes of the valid compositions for Olivine. (bit of a run on sentence there). Pyroxene seems like rock soup with everything thrown in so I have no Idea what its melting point is.

  10. You overlooked the most viable option though: Place the solar panels in space and beam the power down sith a super high power laser. Thanks to mirrors there won't be any downtimes at night as well 🙂

  11. I can see some problems with the city over your lava moat. It might be a good idea to pull permits before you start work. It would be really annoying to be almost finished with your lava moat and have the city shut you down because you don't have a permit. You will probably need to be ready for some fights with the planning department. They may not be as enthused about a lava moat in their city as you are.

  12. Hey dummy! Just have two nuclear power plants! A fusion one and a fission one. Put the fusions waste into the fission one. Let the fission one run then put the fission waste into the fusion one. I don't see what the big deal is. And honestly this seems like a pretty obvious solution. I don't understand how none of you scientists haven't solved the global power crisis. What do you people do all day?

  13. build your moat around a nuclear powerplant and you can dispose of the waste into the moat as well giving you some nice radioactive benefits as well

  14. The lava mote seems like a bit of a stretch for me. Could you please do a video on how to make a gun that can shoot lava an intruders? This seems much more like something I can afford.

  15. What if you use the 300 Watt Rating panels versus the typical pathetic 100 Watt rating panels? Does that shrink it down a bit? I also have 1000 Watt Rating panels at my disposal too. I know a group of special people who make 1000 Watt Rating panels…. So… Does that change anything??

  16. I imagine wind power might be a decent option for part of the power you would need here, as with the constant heat radiating off the moat, there would be constant air flow. I wouldn’t think it would produce enough power though

  17. So I was a little interested in the book when I saw that it showed that UPS is the fastest internet connection but now I really want it because it also discusses how to build a lava moat

  18. Ty @minutephysics my prof asked us to build a lava moat for uni and i couldnt find any resources online until i stumbled on this video. Now i have a question, the lava leaked a bit and is currently spreading through the labs in the basement. what is a quick and ez fix we could try so we dont fail the course. much love from germany

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