This weekend I created a version of the ubiquitous "rock cycle" diagram. It took me nearly 16 years on About.com to do so, because I've never had any use for one myself and geologists don't use it either. A Google Ngram search shows that the phrase "rock cycle" was unknown before 1900 and little used until the age of modern textbooks. Now I see there's a flood of cheap educational pamphlets using the term in their titles. Little good will come of this, I mutter darkly to myself.
But every textbook has a rock cycle diagram, useful or not, and many of them get something wrong. Have a look at a couple hundred of them on Google Images. The vast majority qualify as "chartjunk," Edward Tufte's memorable word for overelaborate, needlessly pictorial graphics. The most common outright error is in drawing an arrow directly from sedimentary rocks to igneous rocks. That does not happen: sedimentary rocks are metamorphosed first. But it makes for a nice, irresistibly symmetrical diagram. I have resisted easy symmetry in making my rock cycle diagram, and I hope you will benefit from its simple truth.