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Andrew Alden

The Rock Cycle Diagram: My Take

By November 18, 2012

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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.

Comments

November 18, 2012 at 11:16 pm
(1) Jocelyn says:

I am glad you took the time to make this rock cycle diagram. I realized I have always been looking at the circled rock cycle diagram during my high school and undergrad years now! The way you put it makes perfect sense. This should be published in textbooks.

November 18, 2012 at 11:35 pm
(2) Karen says:

I love it, it’s so straightforward. But of course, if you were going to put it in a textbook, it would have to be “illustrated”.

November 19, 2012 at 8:56 pm
(3) Howard says:

I can’t help thinking we need a few more arrows and maybe another intermediate node (like your magma and sediment nodes) that would highlight biology. A student examining your diagram (or most of those you’ve tried to improve upon) could be excused for wondering where biogenic rocks would fit into the diagram: e.g. limestone and coal. The diagrams seem to imply the origin of sediment by the direct comminution of igneous, metamorphic and sedimentary rocks, which hides the contribution of living organisms that create limestone and coal by pulling chemicals out of the sea and air.

One can argue, of course, that the chemicals in the sea and air are largely–but not entirely–the products of rock weathering. But considering how much limestone (and dolostone) and coal there is (whole mountains made of these rocks), it seems a shame to hide it all within an arrow labelled “weathering”.

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