Read the previous post in the map building series: Where did all the mountains go?
I mentioned volcanos very briefly in the last post in this series in talking about where mountains are usually formed. Often, the volcanos appear where one plate is sliding under another, forcing the upper plate even up-er, and providing a weak spot for all that yummy magma and lava to come spewing out. Yay, fire and destruction!
But there is a second way for volcanos to appear, and since it isn't on a plate boundary, it's kind of a neat writerly world building trick that's almost as good as a deus ex machina for getting a volcano and/or string of islands wherever and whenever you want them.
Raise your hand if you've heard of Hawaii. Good. Now keep your hand up if you think you could point to it on a map. Keep your hand up if you think you could point to it on THIS map (you can find it if you click on the image to make it bigger).
Found it yet? Okay. Question. Is it on a plate boundary?
Hopefully, we agree on the location of Hawaii, and you've said no. Excellent. So, Hawaii is a chain of islands with both active(ish) and extinct volcanos - and it's in the middle of nowhere, not actually near a plate boundary. How does this happen?
One word: Hot spots.
Randomly, some places of the plate will be thinner than others, allowing the magma to break through to the surface even though there's no plate boundary in sight. This is called a hot spot. If the hot spot is under land (less likely, since the land plate is thicker than oceanic plate), you'll get a regular volcano; if it's underwater, you'll either get an underwater volcano, or if its strong enough, a volcanic island.
But here's the thing: the plates are moving, right? And some times, the hot spot isn't caused just by the thinner crust; it's also mysteriously caused by a literal 'hot spot' in the magma underneath. So when the plate moves on, rolling its way from one boundary to another, the hot spot stays behind - and a new volcano appears.
Rinse and repeat, and you get a lovely chain of volcanos/islands, each of which will become extinct as it moves away from the hot spot and a new volcano erupts behind it. So,
Lesson #2 in Map Building: You need to have a reason for where you put things on your map. But you can pretty much invent a reason for anything.
Doncha just love how rules in writing are made to be broken? :D