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What's the deal with coal? It's classified as an organic sedimentary rock, but rocks are combinations of minerals, and minerals are inorganic. Coal is made of decomposed plants, which are organic. How do I make this make sense to 6th graders?

--- Eva P. - South Middle School


This is a great question, and touches on a couple important aspects of the geological sciences. Probably the first and most important relates to the 1st Law of GeoFantasy, which tells us that geology isn't like physics or chemistry, where, if you know the initial variables, in most cases the outcome can be accurately predicted. In any study of the earth, only rarely does anything always work the same every time. "Exceptions to the rule" or "shades of gray" or whatever you want to call them, these inevitable variations can either drive you crazy, or add spice to your life (depending upon your point of view).

So yes, coal is classified as an organic sedimentary rock, and yes, rocks are supposed to be made of minerals and minerals can't be organic. All true, but… then there's coal. It looks like a rock and feels like a rock and is found in layers between other types of rocks – it sure sounds like a rock to me. And so what if it breaks one little rule? Some rules are more important than others (like standing quietly in the lunch line vs. playing with guns), and this one may fall into the lunch line category. And anyway, if coal is not a rock, what should we call it?

Thanks for the question, and keep encouraging your students to love and respect the earth – no matter where they go, there it is (and it's the only one they've got)!


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