November 13, 2023 | Jamie Hayes

Chalk Is Made Of Millions Of Skeletons


A rock is a rock is a rock. But what about when that rock is chalk? Is chalk rock a rock? Let’s talk.

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Shell Is Other Plankton

If you look at chalk under a microscope, you’ll find something surprising: Your boring old hunk of rock is actually made up of millions of delicate, shell-like structures. They look almost…alive. 

And that’s because they once were.

Coccolithophores are microscopic, single-celled organisms that still form a part of the essential phytoplankton in today’s oceans. Like many organisms, they produce calcium carbonate—in this case, to surround themselves with plates called coccoliths, forming a mesmerizing shell called a coccosphere.

The Chalk Era

Even though they’re only 5-100 micrometers across, coccolithophores make up for their size in numbers. They’re still an essential part of the carbon cycle in today’s oceans, but they were far more common during the Cretaceous Period—the time of the dinosaurs.

In fact, so much chalk piled up during the Cretaceous that it gave the epoch its name: Creta means “chalk” in Latin.

When the meteor that killed the dinosaurs struck the Earth, it also wiped out 90% of coccolithophore species. Their bodies sunk to the bottom of the ocean, where they piled up, eventually forming today’s chalk deposits such as at the iconic White Cliffs of Dover.

So the next time someone calls you up to the chalkboard, remember: With each stroke, you’re leaving behind thousands and thousands of little skeletons. Spooky.


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