Pumice for the Dummy Geology Student

What would geology week be without a whole lot of misinformation from the keyboards of familiar folks? We're well aware that we could probably mine that mother lode of dumbassery called eHow.com for decades, but you have to go afield every once in a while. That doesn't mean that we can't check up on some of our favorite serial dumbasses, though, and we happened to catch one of them dipping her toes into geology for this week. She's longtime contributor Joan Whetzel, caught holding forth on "What Is the Difference Between Pumice and Scoria?¹" at Sciencing.com.

Pumice

Joanie, as usual, makes a pretty good mess out of her explanation. She starts off on the right track, telling us that pumice and scoria have vesicular texture, and that "vesicles are holes created by gas bubbles..." So far, so good; though she might want to mention that a lot of said gas is actually water vapor. No harm, no foul though. Now, here's what Whetzel says about pumice:

"Pumice contains small, frothy gas holes, is light in color and, due to its viscosity, will float. It was produced by lava containing a large amount of volatiles (water and gas), large amounts of silica, and very little iron and magnesium."

Interesting, and at least partially correct, except that Joan apparently doesn't know the difference between viscosity and density. It would have helped, however, if Joan were to mention a critical fact about pumice: pumice is volcanic glass and, as such, has no crystalline structure. That's one KEY difference between pumice and scoria. Loss of points, Whetzel! So what does Joan then say about scoria? Hold onto your hats, folks:

"Scoria ranges in color from red to black, is heavy due to heavy metal content, and extremely porous with a spongy appearance caused by large gas pockets. The heavy metals cause scoria to sink in water."

She got the color right. The rest she got wrong, especially the totally bull misinformation about how the presence of heavy metals (a term not generally applied to iron or magnesium, for whatever that's worth) makes scoria too dense to float.  Besides the unmitigated rubbish about heavy metals, Whetzel fails to actually mention that compared to the rhyolitic (felsic or acidic) composition of pumice, scoria has a basaltic (mafic or basic) composition, though that might be what she's trying to say with the crap about heavy metals. The other main difference is that scoria is rarely glassy, usually having a crystalline groundmass. The reason it doesn't float when pumice does? well, it ain't "heavy metals" - scoria has fewer and often smaller vesicles, so it's less porous and therefore its bulk density is higher. Too dense? doesn't float. Ain't nothing to do with heavy metals, Joanie...

For making a complete mess out of her assignment, explaining the difference between pumice and scoria, the Antisocial Network hereby awards Joan Whetzel yet another Dumbass of the Day for her collection. The woman just makes this too easy...

¹ Leaf Group assigned this one to their rewrite team. Unfortunately, the new version wasn't much of an improvement... if at all.

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