Explain xkcd: It's 'cause you're dumb.
In structural geology, subduction is the mechanism by which one tectonic plate disappears under another. This process usually creates a mountain range on the second tectonic plate as water entrained in the subducting plate rises into the second plate and provokes volcanism, often resulting in a volcanic arc.
In this comic, Beret Guy is very happy because he has just received his subduction license, which may be a play with business term production license. His roommate Cueball very reasonably asks him: Your what? But instead of answering him, Beret Guy begins to move towards him in their small room. It turns out that the license has literally enabled him to initiate subduction. As he moves towards Cueball, he slowly sinks under the floorboards of the room, and in this process he creates a small mountain range on the floor. In the end, much to Cueball's consternation, these mountains turn his desk and chair over. Cueball actually falls out of the frame in the final panel, where Beret Guy is already halfway down beneath the floor. This would not be possible in real life. 
The title text plays on the double meaning of the word "normal", which Cueball means in the sense of "like most people, not strange," but which Beret Guy interprets in the geological sense. While subduction occurs when two plates crash into each other, a normal fault occurs when two plates are moving away from each other. Here, "normal" is used in the sense of "perpendicular," as the result of a normal fault is often that part of the crust moves vertically downward, forming a graben.
A similarly atypical license was mentioned previously in 410: Math Paper. Puns on geological terms (including types of faults) were previously made in 1082: Geology.
This comic was featured in a page of Thing Explainer as part of the explanation of the Big flat rocks we live on. Only the last three panels where used, probably because the words in the first panel were way too uncommon for the book - see more details here.
Subduction was again mentioned in 1829: Geochronology.
- [Beret Guy is looking at some mail he has received while Cueball is at his computer desk at the other side of the room.]
- Beret Guy: Sweet! I finally got my subduction license!
- Cueball: Your what?
- [Beret Guy starts sinking into the ground, causing it to ripple.]
- Cueball: ...What are you doing?
- [Beret Guy sinks further, forming a miniature mountain range in front of him. Cueball is frantically trying to keep his computer steady as his desk tilts.]
- Cueball: Stop it! Stop it!
- [Beret Guy is waist-deep, and snow caps have formed on the mountains. Cueball is falling backwards from his desk, and the monitor unplugs itself from his computer.]
- Cueball: Augh!
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I'm assuming "subduction license" is being comically reinterpreted here from some other meaning. What is a subduction license, normally speaking? Jevicci (talk) 15:20, 30 June 2014 (UTC)
Um, you're making it too easy to make me normal and rub away very fast 18.104.22.168 (talk) (please sign your comments with ~~~~)
I'm thinking the closest real term to "subduction license" is probably "Subversion License" - Subversion being a popular source code repository system. (Edit: Created a new account) KieferSkunk (talk) 21:02, 30 June 2014 (UTC)
- Nah, that's not it ... there's got to be some pun on license, or perhaps a term that sounds like -uction license. 22.214.171.124 23:14, 30 June 2014 (UTC)
- Could be the seduction license he should have applied for instead. He wishes to seduce his roommate, and has applied for a license for this. However he misunderstood the word and has applied for the other license, and has also read about it on Wikipedia ;-) Kynde (talk) 13:31, 1 July 2014 (UTC)
- Given Beret Guy's past and future expressions of unusual or impossible power, it's probably best to assume that it is literally a license that allows him to perform subduction. That it. -Pennpenn 126.96.36.199 05:59, 5 January 2016 (UTC)
Google supplies http://www.cafepress.ca/+subduction+license-plate-frames which offers "Subduction License Plate Frames", which I believe is an accidental verbal conjunction. I believe it's in the context of the web site selling a range of images and designs printed on various objects. In this case the object is a license plate frame, and the image is a diagram of subduction. Since the centre is cut out of the image in order to display the license plate, the combination is pretty useless. But, here it is, a subduction license plate frame, in which to place your subduction license. So why is that funny? Well, maybe it was the web site's special offer of the day, or, it was a Googlewhack. But now, a few days later, the Internet is awash with people asking "Why is 'subduction license' funny?" [email protected] 188.8.131.52 08:45, 5 July 2014 (UTC)
My second theory: by the time he sobered up and realised it wasn't funny after all, there wasn't time to draw a new comic. Although I could draw one of these in five minutes (lettering takes longer), so that doesn't work. Maybe he's ill, badly ill. [email protected] 184.108.40.206 08:45, 5 July 2014 (UTC)
My third theory: did someone make earthquakes illegal? Or make them legal with a license? It's in the news that scientists are satisfied with the evidence that licensed fracking is causing earthquakes in Oklahoma, but it seems to be pretty easy to get a license or permit to do fracking. But the news story appeared after this comic was published - if you get your geology news from regular newspapers. And obviously the question had been asked earlier. So, the comic may be based on that. [email protected] 220.127.116.11 08:45, 5 July 2014 (UTC)
Alternate explanation for the title text: A fault is a break between two blocks of the lithosphere (or the crust if you want to be more vernacular). The two blocks move in one of three ways: laterally side-by-side (making it a transform fault), away from each other (a normal fault) or toward each other (a reverse fault, which is the kind involved in subduction). If Beret Guy were normal, he'd have to be moving away from Cueball. Fewmet (talk) 15:15, 1 July 2014 (UTC)
Also, "AUGH!" reminds me of Peanuts: File:https://fenetreovale.files.wordpress.com/2015/01/charliebrownlucy-rugby.png 18.104.22.168 19:02, 14 February 2017 (UTC)Mandel
Why does the mini mountain range have snow on the peaks? 22.214.171.124 15:45, 26 April 2017 (UTC)