# What is limestone made of?

I know that it is calcium carbonate (CaCO3). I also know that these are the dead bodies of microorganisms.

My question is, where do they procure the building material? The limestone seems to cover the whole Planet with a multimeter-thick layer. How is this possible - after we assume the conservation of atoms? Where did the limestone molecules come from? How long can the process of sediment formation last? Is the building material coming from space or due to Universe Expansion (black matter)? Is it right that the Planet becomes thicker and thicker with every year if we assume conservation of matter?

• This question seems a bit 'trolly' but I'll answer it anyway. I don't think it qualifies as 'a common misconception'. Equally happy to support its deletion. Nov 22 '14 at 13:05
• @kwinkunks Do you mean common misconceptions do not deserve to be debunked? I agree, the question is not intriguing at all and you, serious people, must delete it. Stackexchange is a bad format to debunk and enlighten anyway. The public must stay in the dark. Nov 23 '14 at 12:53
• No, I said that your question about limestone coming from space and making the earth expand is not a common misconception. It's the sort of thing that some basic research would have cleared up. Nov 23 '14 at 13:17
• @kwinkunks The fact that Wikipedia does not mention the source of new layers of limestone does not mean that there is no misconception among ignorant people. It actually says something opposite. Referencing to articles that do not "clean up" anything at all (no cycle is mentioned, neither rock cycle nor carbon cycle, in particular) do not clean up anything at all. Therefore, in addition to removing my question, I suggest to shut down the Q&A sites altogether. If you can be satisfied by any article, which somehow contains the first matching keyword, you is a bot and do not need any answers. Nov 23 '14 at 15:31

The building material comes from minerals dissolved in the ocean, mainly $\mathrm{Ca}^{2+}$, $\mathrm{Mg}^{2+}$, and $\mathrm{HCO}_3^{–}$, but of course there are all sorts of biochemical details. Most limestone originated as the skeletons of micro- and macro-organisms, such as plankton and coral.

The minerals come in turn from the erosion of older rocks (among other places). For example, here are the famous white cliffs near Dover, UK, where massive amounts of calcium carbonate, mostly stable since the Cretaceous, are falling into the sea, where it will be re-used:

This attrition is part of the rock cycle and a very slow-moving part of the carbon cycle. These are well-studied and reasonably well-understood.

You seem to suggest there is a paradox, that there's just constant deposition and no erosion or dissolution. But there is no paradox. The earth is not becoming thicker and thicker, material is not coming from space (not in substantial quantities anyway), and there's no need to invoke any processes we don't understand sufficiently to explain our large-scale observations. We know this because those processes would have a measurable effect on all sorts of things: the earth's orbit, climate, plate tectonics, and so on. There would be, I suggest, a strong low-frequency trend in all sorts of signals (stable isotopes, organism growth rates, sedimentation rates, volcanism rates, and so on).

The image is by Flickr/Harvey Barrison, CC-BY-SA.

• Ok, the answer is basically almost all limestone was formed hundreds of millions of years ago and does not grow up since then. I would wonder then how is it possible that we find more ancient skeletons on deeper layers if they dissolve as you say and how can something dissoluble settle at all? Nov 23 '14 at 12:51
• As you said in your question, much limestone is the fossilized remains of marine organisms (I edited my answer to make this clearer). Limestone is being formed today all over the world, but especially in shallow, tropical seas, such as the Caribbean. As for solubility, it depends on saturation and redox potential. You seem to still be looking for a paradox, but there isn't one. I suggest you do some reading. Nov 23 '14 at 13:23
• It is intuitive to suggest that limestone dissolves in water, once you know that it survives hundreds of millions of years, since its formation. Yea, I know paradoxes are very intuitive, especially to non-experts in the field. Nov 23 '14 at 15:39