Chalk It Up

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.Read more at...NOVA, December 2010.