Answer: Plankton Fossils

Chalk is just some soft rock that rubs off easily on other rocks like slate, right? Nothing interesting to see, that is unless you break out the electron microscope and delve into the microtopology and history of it.

Peer close enough at a hunk of chalk and you’ll make an interesting discovery. Chalk is composed of millions of tiny, soft calcite fragments (calcium carbonate) all crushed together under pressure. These fragments are minute calcite plates shed by a type of plankton, coccolithophores, millions of years ago.

In areas of the ocean rich with these little organisms, trillions upon trillions of little shed plates would accumulate like snow on the bottom of the ocean and then, under the enormous pressure of the water, would become fused together. Movement of the tectonic plates slowly pushed these enormous deposits of chalk onto land where, eons after their creation, humans happily mine them.