Clastic Rocks for Dummies

Conglomerate

Every once in a while we dig up the staff geologist and force him to peruse the gobbledygook some of our DotD freelancers have managed to get published in their continuing efforts to stupidify the internet. Most of the time he's had to disabuse readers of the notion that Earth's tectonic plates float on an ocean of magma or that hydrocarbons collect in pools, pockets, and layers. The folks at WiseGEEK.com, however, often manage to screw up very specific scientific questions... questions like "What Are Clastic Rocks?" as supposedly answered by Mary McMahon for WiseGEEK niche AlltheScience.com.

McMahon gave a layman's answer, probably plucked from Wikipedia, in her introduction:

"Clastic rocks are rocks which are composed of small fragments of other rocks."

Actually, that's not the real definition. Clastic rocks are those rocks composed of fragments of rocks or biological structures that have been transported from their place of origin. Both the fragmental nature of the particles ("clasts") and their transportation are inherent to the definition. But, we suppose, Mary's definition is "close enough for WiseGEEK."
Mary has a few other problems in her post, mostly related to her unfamiliarity with sedimentology, geology, and science in general. For instance, in attempting to describe sandstone, Mary makes a common mistake:

"...sandstone [is] a type of rock made from sand-sized fragments of other rocks. Despite the name, sandstone is not necessarily made from sand, although it often contains at least some sand, depending on where and how it formed."

Oh, dear: Mary actually thinks "sand" is a... well, we aren't really sure what Mary thinks it is; probably synonymous with quartz. No, Mary, "sand" is a size designation, not a mineral description! Mary also gets confused about sedimentary rocks that, according to McMahon,

"...form when the deposition of sediments over time creates an area of high pressure..." We sure hope there's some cementation going on, too, Mary. Or check out her explanation of pyroclastic rocks: "Igneous rocks, which form as a result of volcanic activity [sic], may take the form of clastic rocks. Tuff, for example, is an igneous rock which is actually comprised of a number of fragments [bolding ours], making it both igneous and clastic in nature. Numerous other types of igneous rocks can include a mixture of rock fragments..." Besides her inane notion of "a number of fragments," Mary glosses over that last "mixture" business, by which we assume she's attempting to refer to volcanic breccia, ignimbrites, and inclusions in plutonic bodies. Duh.

We aren't really sure where McMahon came up with the notion that,

"Metamorphic rocks may also be clastic in nature, if they contain fragments of other rocks."

Since "clastic" implies transportation and there's no transportation involved in metamorphism, we are left to guess that Mary ran across the term "metaconglomerate" somewhere and decided it means a conglomerate formed by metamorphosis instead of a metamorphosed conglomerate. Again, Duh...
An accomplished writer must, of course, tie it all together with a whiz-bang ending (so, for that matter, must a freelancer). Mary's close is definitely a tour de farce:

"The formation of clastic rocks starts with weathering, in which existing rock is broken down into fragments. These weathered fragments are brought back together again by a variety of processes, such as deposition of mineral salts, pressure, volcanic activity, and so forth."

Clearly, McMahon does not understand the processes involved at all; especially if she somehow thinks that the grains in the tuff she mentioned above are weathered and "broken down into fragments"... but can you expect more of a three-time Dumbass of the Day? Probably not.

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