It makes all the dirt glow. Or, actually, it makes only one speck glow. See, the thing is that it's actually up to you, the DM. Let me explain...
So, there's a thing you have to know about D&D 4e: the designers didn't care about this stuff. They wrote a game that rigorously defined some parts of the powers rules (mostly what effect they have in combat), and then left the details of how the powers actually work in the game world's reality either vague or undefined. That was a deliberate design decision on their part: they wanted to get away from rules that had different interpretations during combat, but wanted the roleplaying, out-of-combat creative stuff to be almost completely un-defined by the rules.
This was a big change from how D&D had been written until 4e. Your intuition that the rules should cover this somewhere is reasonable and common, especially given how specific the game's rules are otherwise, and since most games are written like that. It will take a bit of a switch of perspective to start looking at the rules differently, but it's actually useful, the way the game splits the authority between itself and the DM.
So, the result of all this is that the game doesn't tell you how this works. It suggests a little bit in the description of the power, but the "fluff" description and the Effect description actually contradict each other – the fluff says it's "a bright light" and the Effect says that the target itself (presumably the whole target) sheds the light – again, because the designers didn't actually care about how the spell "really" works in the game world and didn't bother nailing this down.
Consequently, how this works is entirely up to you. That's a great freedom and a great burden: the game won't really help you out with deciding things like this, but on the plus side, it won't tell you that how you think it should work is wrong, either. Based on what you wrote in the question, it sounds like you personally like the "it all glows" interpretation. So, you being DM, and the game not actually caring about the answer, your interpretation is right in your game.