Allan prefers blackboards to whiteboards because of the rich history of chalk. I had no idea where chalk comes from, so I looked it up. Turns out it’s a long, long story.
One hundred million years ago, tiny creatures called coccolithophores were hanging out at the surface of the ocean, soaking up some sun. Then they died, and their calcium carbonate-filled skeletons dropped to the sea floor. Then more died, piling on top of the others and eventually creating a layer of lime mud.
Over time, as the bottom layers of gunk were exposed to more and more pressure, and more and more heat, they turned into the soft, porous rock we know as chalk.
During this era of the Earth’s history, sea levels were incredibly high, which meant that there wasn’t much land around to drop other kinds of sediments into the floor. That’s why chalk is mostly white.
The skeletons of many other sea creatures, though—such as sea urchins, moss animals, and sponges—did get trapped in the chalk deposits. Less frequently, live animals—such as this stunning starfish—were also caught in the chalk, to the great delight of today’s fossil hunters.
Millions of years after the horizontal layer of chalk formed, continental movements gradually pushed it up into large mounds that we can still see today. For example, take a peek at “The Needles” chalk formation off the Isle of Wight, in England:
The coccolithophores, it seems, made it back to the water’s surface.