Which came first, the chicken or the egg?

I'm either the biggest nerd ever, or just truly bored while shelving-I'm guessing it's all the above. This morning, I was just doing my thing, shelving AV materials at our library, and I stumbled across a real "that will get me thinking all day long unless I know the true answer" kind of thing. I hate that. "Volcano" DVDs (551.21) were ordered one decimal number less than "Earthquakes" (551.22). Ok, well, ordering movies and books through the Dewey Decimal System is not my thing, but I did spend a heck of a lot of money on useless courses I took, like Earth Systems, and Geology, all that never served me until now... as I wonder why the heck "Volcanoes" were ordered before "Earthquakes." In my puny understanding of plate tectonics and seismic activity, it was my understanding that most volcanoes are direct consequences of the activity from earthquakes. In the briefest of brief paragraphing, an earthquake happens, the plates shift, an opening allows magma to come upward through the earth from pressure, thus becoming a volcano of some sort. Which, my point being, begs the question: which came first, the volcano or the earthquake ("Earthquake, earthquake!"). So, why is it that the national organization system for classifying these "events" call that "Volcanoes" come first? See my problem?

I do realize that after reading this, you are thinking that "yes, Anna, you are a big nerd (and probably loosing it a bit)," but I do get bored while shelving, and the matter just bugged me. So, what did I do? I did what any pained-brain person would do at the library: ask the reference desk. Did she have the answer? No. Did the person she called through catalogue know? No. Did the Book of Dewey Decimal System-ing have the answer? No. Basically, we were all stumped. It sure did make us chortle at the fact that the ordering wasn't logical, and the fact that we were just thinking about the whole thing like it mattered at all whether or not we actually found out the answer, before we got back to our hum-drum day. I told them, if they ever did decide to change it, by mere chance, I would go down in history as that "weird knitting gal who was such a big, big nerd."

And how do you know that you are working WAY TOO MUCH in the library shelving? You'll know when you look at the clock by chance at 7:46 and think of knitting...while at the same time wondering why the heck knitting popped into your head in the first place, until it dawns on you that the 746's are knitting books section...and once again, you are such a nerd.