Rock Classification for Dummies

Schematic of the rock cycle

Many years ago, our senior staff researcher (the big high muckety-muck in the research group) learned that politicians would much rather give speeches than write essays. The reason? It's easier to, shall we say, "hedge the facts" for listeners than for readers. That's the main reason why our staff has heretofore avoided taking on video content, even though it dominates the newly redesigned eHow.com. eHow also knows it's harder to spot utter bull in a video than in writing. Alas, the time has come; and so we're taking on our first video DotD, eHow's Jane Stammer, as she explains (sort of) the process of "Classifying Rocks."¹

Let's open this by reporting that nothing Jane says in her video is demonstrably wrong. Our awards committee discussed this for several minutes -- that's several times as long as Stammer's 59-second video (in reality, 51 seconds, given her 8-second introduction); in which she tackled the question she expanded to "How to Classify Rocks" in a mere 114 words.

Stammer spits out the requisite mini-definitions (taken here from eHow's transcript of her video) of igneous, metamorphic and sedimentary rocks. In doing so, however, she made some broad statements that our staff geologist found questionable, at best:

"Igneous rocks are formed... when big pools of magma crystallize and form minerals which later coalesce to form different rocks."

Stammer's overly simplistic description of the crystallization process raises the question, "How does this "coalescence" occur?" In fact, there's no coalescence at all: except for some rare cumulate deposits, minerals crystallize in place.

"Metamorphic rocks are formed at very high temperatures and pressures deep, deep under the ground and at these high temperatures and pressures, the rocks actually melt and recrystallize and metamorphose into new and different rocks."

While Jane gets the "high temperatures and pressures" part right, geologists will tell you that the rocks don't "melt and recrystallize" in metamorphism: if they crystallize from a melt, they're igneous rocks. During  metamorphism, the elements in the minerals' crystal lattices rearrange themselves to align with the pressure gradient -- that's metamorphism. Oh, and by the way, Jane? Don't forget contact metamorphism... "...sedimentary rocks are formed from the erosion and later deposition of preexisting rocks that are deposited in low lying areas and later compacted over time." That's (more or less) true of clastic sedimentary rocks, but Stammer entirely forgets biogenic rocks such as limestone and chemical sedimentary rocks such as gypsum and halite. Jane finishes by declaiming,

"And that is how to classify the three main types of rocks."

...to which we say, "No, that's not." Misstatements and omissions notwithstanding, Stammer failed to point out "how to classify" rocks: all she did was discuss the three classes. She made no mention of texture and mineralogy, the main characteristics used to classify rocks. We think someone who claims to be a "lab manager and research scientist" at the prestigious Colorado School of Mines should have done a much, much better job. That's why Jane is our first ever Dumbass of the Day, video edition!

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/video_4774965_classifying-rocks.html

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