Limestone, marble and sea-shells

Alcyon and limestone cliffs

Alcyon and limestone cliffs

© Participe Futur

On its way from the Mediterranean Sea to the Spitzberg, the sailing boat "Alcyon" sailed along impressive limestone cliffs, on the south coast of England. We often see pictures of these magnificent limestone white cliffs, which can be seen on the English or the French coasts, but also in Australia and in many other places. Have you ever wondered what these stones were made of?

Is this limestone, or not?

"Name given to an important and very common rock, essentially made of calcium carbonate", says the dictionary...
Well this doesn't help us very much to know if a rock is really a limestone or not.

Limestone, with its white-yellowish aspect, is a very common rock: you can find it nearly everywhere. But this is not a good criterion either: a specialist would tell you that there are many other rocks with a white-yellowish aspect. And he would give you an endless list of technical names: dolomite, quartzite, gypsum, anhydrite... So what can help us make the difference?

Well, first of all, limestone is not very hard: it can be scratched with a knife. But its main characteristic is to react strongly with hydrochloric acid. One drop of this acid (even diluted) on limestone creates an immediate reaction: suddenly, many small air bubbles form, just like when you open a coke that had been shaked before. And now you know why most geologists always carry a small bottle of diluted hydrochloric acid in their pocket!

This experience also works with vinegar, even if the result is not as obvious. Vinegar, just like hydrochloric acid, dissolves limestone. As a matter of fact, numerous liquids have this same effect on limestone. This is why, on some home cleaning products, you will find this notice: "not to be used on marble or limestone materials" (we will explain further what marbles are).

What is the origin of limestone?

Limestone forms in the water and can have two different origins:

  • Chemical origin: limestone can settle by natural chemical reaction. This is what happens, for example, in your kettle, if your tap water is calcareous. When limestone settles in our houses, we change its name and call it "fur", in England, or "sediments" in America.
  • Biochemical origin: most kinds of limestone are part of this category. This kind of limestone is made of many particles that have been formed by living organisms (animals or plants). The most obvious example of these particles is sea-shells, which have all been secreted (or "built") by an animal. There are sea-shells of all sizes. Therefore, in the coarse limestones, you can find oyster or scallop shells.

Whereas chalk is made of one the tiniest limestone particle we know: "coccoliths" are elements of the outer envelop (or "armour") of microscopic algae called "coccolithophores".

This photograph has been taken with a scanning electron microscope (SEM). You can see the round and notched particles (the coccoliths) that are the main components of chalk.

The coccolithophores still exist nowadays and are part of what we call plankton, which is the basic food of many animals, such as whales.

On a white sand beach... with the dinosaurs!

The marine plants and animals that secrete limestone shells or structures prefer to live in warm waters. It's the case, for example, of coral which is a great limestone reef constructor.

This means that, when these particles were formed, warm waters bordered all the coasts where you find limestone today... It was the same kind of climate as the one that you can find in the Bahamas nowadays.

If you ever get to go there, just take a look at the sand with a magnifying glass: nearly all the sand grains are in fact tiny sea-shells!

But let's come back to the formation of our limestone: we were saying that, on the coasts where we find limestone nowadays, the climate was warm and that many algae and animals were secreting enormous quantities of limestone particles. After the algae's or the animals' death, the particles settled at the bottom of the water. Huge layers of "sea-shells" and other small limestone elements accumulated in this way. Then, they were buried under layers of other sediments, and slowly got compacted and solidified. This is how the limestone of the cliffs formed.

However, one should remember that we are talking of gigantic time scales and that the positions and outline of the continents and oceans have changed very much during these million years. The limestone that we see, very often settled in seas or oceanic basins that no longer exist (because of the movement of tectonic plates; for more explanations, see the article "what does the bottom of the Mediterranean See look like?").

The limestone that you can see in the region of Paris, in all the north of France, in Belgium and in England is very similar because they were all formed at the same time, when the dinosaurs lived, and in the same oceanic basin. On the other hand, limestone of the Alps is quite different because it settled in different conditions and in another oceanic region.

There are no fossils in marble!

Limestone settles and solidifies slowly, which explains that you can often find beautiful and very well preserved fossils in it.

Sometimes, it may happen that limestone layers get buried far too deeply and undergo very high pressures and temperatures. Limestone that has been "cooked" in this way (the precise term is "metamorphised"), transforms into marble. No fossil remains after such a treatment.

This is an easy way to make the difference between a limestone and a marble (even if both react when they are in contact with hydrochloric acid): just look if there are any shells in the rock or not.

But then... how come there are shells in the walls of the Parthenon, in Athens ? Because it is only partially built with marble and that the rest of the building is built with limestone!
In fact, there is a great confusion between marble and limestone: the problem is that, in the construction field, a polished limestone can be called a "marble"!

So next time you'll be walking in a vast building entrance, with a nice marble polished floor... look if you can see shells. If you do, then this is no marble: this is limestone!

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