Rock Classification for Dummies

A former classroom teacher of our acquaintance says that while she was teaching fifth grade, some of her students never seemed to attempt to go beyond the bare minimum of work required to finish an assignment. This especially distressed her when the assignment required that the students think for themselves. A case in point is the short essay: the best answers were when the kids synthesized information from multiple sources and came up with their own answers; the worst were the ones where the kids just regurgitated someone else's words. We've noticed that content-farm freelancers come in the same two flavors; and today's nominee is definitely from category number two. Meet Michael Mason and his rather strange OurPastimes.com post, "Difference Between Metamorphic and Sedimentary Rock."¹

The answer is actually quite simple: sedimentary rocks are those that are formed at the surface of the earth, whether by sedimentation and lithification of particles ("clasts"), chemical precipitation, or biochemical precipitation. Metamorphic rocks are formed at depth through the reorganization of the elements in existing rocks due to heat, pressure, and chemically-active fluids.

Mason managed to say all of that, except it took him 400 words instead of fifty. What he appears to have done was a fast copy-reword-paste job on an kid's encyclopedia entry; except that he had to change words to avoid being nailed for plagiarism. Had he known what he was talking about (he clearly didn't), Mike would not have said rubbish like,

• "Over millions of years, [metamorphic]  rocks have been changed by heat, pressure or by the permeation of gases or liquids." – "Permeation of "gases"? So much for rewording "fluid"; not to mention that "permeation" is a bit of oversell.
• "...metamorphic rocks are the oldest known rocks on the earth, some being 3.8 billion years old." – Strange: our references say the oldest rocks on earth are 4.28 billion years old... Oh, well, at least he's right about metamorphic.
• "Metamorphic rocks can have colored streaks." – Rocks don't have streaks, Mike; the minerals in those rocks have streaks.
• "Many mines and quarries are found in the zone where hot magma touches preexisting rocks to form what is called contact metamorphism, as new crystals and hence gemstones form in the contact layer." – Not really, Mike. Mineralized zones are more likely to be caused by hydrothermal alteration (see image), which is not the same thing as metamorphism; instead the mineral deposits are created by fluids following fractures, faults, and other planes of weakness in the host rock.

Much of Mason's post is given over to the copy-pasting lists of rock types with a smattering of information thrown in, verbiage such as,

"Sedimentary rocks started off as clay, sand, gravel, lime from skeletons of sea creatures, precipitates, or volcanic sediments such as ash and cinders. Common sedimentary rocks include coal, gypsum, halite or rock salt, amber, limestone, dolomite, sandstone, shale, and conglomerate, which is a sedimentary rock made from pebbles and boulders that are cemented together with calcium carbonate, iron oxide, silica or sometimes clay."

The grammar problem notwithstanding (ever heard of a semicolon, Mike? Mike's content editor?), such as list is pure regurgitation and of little use to someone wanting to know the difference between the two rock types instead of lists of names. For work of C-minus (a generous grade, we think) quality, Mason is the latest recipient of a spanking new Dumbass of the Day award.

¹ The original has been deleted by Leaf Group, but can still be accessed using the Wayback machine at archive.org. Its URL was   ehow.com/info_8510584_difference-between-metamorphic-sedimentary-rock.html

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