Its a terrain generation glitch. Near as I can tell, it happens when Minecraft tries to generates structures like dungeons or abandoned mineshafts in Superflat. There's no normal terrain to place them in, so Minecraft places normal terrain in those chunks.
On an individual basis this happens fairly rarely (has only happened once for me), but its common enough that I see posts asking about this on the forum at least once a week.
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It must be great to be a newbie at minecraft still. BTW I'm assuming your using a snapshot? Because with the latest snapshot these chunk errors are bugs, so I have heard. Also grats on a successful image post.(Relatively successful...)