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| Spheroidal weathering in granite |
Ahhh, research: once we've completed formal schooling, it's the main way we learn and verify new knowledge. Of course, doing so works best when our sources are knowledgeable, right? That's why we at the Antisocial Network do our part to call bullshit on freelancer writers whose purpose is to make money, not to share knowledge. Their motto might be "Accuracy be damned!" while ours is "Accuracy rocks!" And speaking of rocks, that's our topic this week: we put the staff geologist to work slapping around freelancers who contribute to the stupidification of the internet by saying dumb stuff. So while we're on that subject, meet eHow.com's Claudia Newcorn, freelancer and owner of a marketing business. What Newcorn isn't, though, is scientifically literate; a deficit she proved by writing "How to Identify Granite Rocks" at GoneOutdoors.com.
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Newcorn's cascade of misinformation about granite starts out with:
"Granite rocks are part of the igneous rock class, formed by slowly cooling pockets of magma trapped beneath the earth's surface. "
On a broad level, that's a correct description of granite, although the phrase "granite rocks" is redundant since (by definition) granite is a rock. We wonder, though, why the scientifically illiterate types keep mentioning those "pockets" whenever they describe the subsurface; but we figure asking one of them would be a waste of time. Moving on, for Claudia granite identification is a three-step process: |
"1) Inspect the rock closely. Granite has large, tightly fitted crystalline grains. Classic granite has a 'salt and pepper' appearance, with grains of black and white."
First, "large... grains" is misleading: all that's necessary to identify an intrusive igneous rock is that the individual grains be distinguishable with the naked eye. Second, "tightly fitted" is likewise a misnomer, probably chosen because Newcorn was afraid that using the correct term -- "interlocking" -- would trip a plagiarism checker. And last, "black and white"? the dominant mineral of most granitic rocks is orthoclase (potassium feldspar), which is pink. Black and white granite is probably quartz and hornblende, with perhaps some albite (sodium plagioclase feldspar) -- a rather unusual composition.
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| Intrusive igneous rock classification |
"2) Look for quartz in the rock. Granite always consists of quartz and feldspar, which usually give granite a light, almost glittery color, ranging from almost translucent white (from the quartz) to a pale pink (from the feldspar). This lighter color is mixed with grains of other darker minerals, creating the “salt and pepper” look."
Yes, granite -- by definition -- contains at least 20% quartz by volume. So, too, do other rocks such as monzonite, granodiorite, quartz monzonite, tonalite... Quartz is one of the most ubiquitous minerals and can show up in all but the most basic igneous rocks, rocks such as anorthosite and gabbro. So it's not the presence of quartz that helps identify granite, it's the amount of quartz present. That's part of the definition, Claudia!
"Look at the fracture pattern. Because of its crystalline structure, granite naturally fractures in planes, resulting in a rough chunky shape. Gradual erosion by water and ice bevels this into smoothness, as characterized by the Granite Domes of Yosemite National Park"
Dumbass. Fracture patterns aren't controlled by "crystalline structure," they're controlled by stress fields. Besides, those aren't fractures in granite, they're joints. Joints are caused by shrinkage during cooling and by unloading as the overburden is eroded away. That "[g]radual erosion by water and ice" Newcorn cites describes the erosion of any rock, so it's of zero use for identifying granite. If she had known jack about granite, Claudia might have mentioned spheroidal weathering — but even that isn't diagnostic of granite, since it affects many different plutonic rocks.
No, the only two metrics for identifying granite are the size of the crystals and the rock's mineralogy. Claudia munged the first metric and fell flat on the second, which we declare is sufficient to be chosen today's Dumbass of the Day. Now stop it...
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SI - GEOLOGY
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