Ask GeoMan...

Can you please tell me the difference between chalk and limestone? If they have the same chemical formula why are they called two different things?

Limestone is a general name for a vide variety of sedimentary rocks which are composed primarily of calcium carbonate. Calcium carbonate comes from many sources, most of which have a biological origin. Things like bones and teeth and shells are generally made of this amazing stuff, and when the critter dies, the calcium carbonate is returned to the earth for recycling into new and wondrous forms.

There are many types of creatures on earth, and quite a few of them (at least by the sheer number of individuals) are composed of a single living cell. There are quite a few different types of these primitive life forms. Many live in the ocean, and some extract calcium carbonate from the seawater to make a protective shell. One cell may not seems like much, but hey, it's all they've got and worth taking care of. But sooner or later the creature does one of two things: reproduce or die. Either way, the discarded shell settles to the seafloor, where untold bazillions of them pile up. Over the course of geologic time, the layers of calcium carbonate are lithified and converted into rock.

"Chalk" is a variety of "limestone" which is composed primarily of the shells of single-celled, calcium carbonate secreting creatures. Think about this next time you use chalk - that white stuff all over your hands is actually countless individual shells, each microscopic in size and the former home of a living being.

Click here for more information on primary sources of calcium carbonate.

Hope this helps.


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