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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
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”


How to Build a Lava Moat (with xkcd)

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林宜悉 發佈於 2020 年 3 月 28 日
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