Igneous Rocks for Dummy Geology Students

Intrusive rock classification

We've noticed recently that, in what can only be assumed to be a misguided attempt to freshen their content, the people at Leaf Group have been assigning a cadre of "experts" to rewrite some of their content. The problem, at least as far as we can tell, is that some members of the rewrite team seem woefully unqualified for the task. We say that because when we compare the current version to the original (via the Wayback Machine), it's not uncommon to find that the original was better written and more factual. Here's an example: rewrite team member Claire Gillespie tackled "Differences Between Extrusive and Intrusive Rocks" for Sciencing.com (it was rewritten in April, 2018). We'll compare it to the original, written by Victoria Martin.

First, though, let's ask our staff geologist. According to him, there's only one difference (to a layman, anyway), and it's pretty simple: crystal size. Intrusive rocks are phaneritic, meaning "large crystals," and extrusive rocks are aphanitic, meaning "small crystals." That's pretty much the size of it if you aren't a petrologist. But that's not the way eHow works: contributors to the site were required to pound out 300-500 words.

Let's compare some of the information the two freelancers shared in their introduction:

• "Geological processes form rocks -- such as when heat, water or pressure act on mineral compounds. (VM)" vs. "Rocks form when mineral compounds react with heat, water or pressure. (CG)" – Advantage: Martin, although slight. She at least mentions processes; both use the ludicrous construction "mineral compounds."
• "Rocks exposed to the intense heat within the earth liquify [sic] and cool to form a particular kind of rock geologists call igneous rocks  (VM)" vs. "Intense heat that liquefies inside the Earth makes hot molten material called magma. "(CG)" – Advantage: Martin. Gillespie's sentence doesn't even make sense: heat can liquefy?
• "Rocks have different features, such as their mineral composition, hardness and particle size, which give geologists clues about their origins. (VM)" vs. "Basalt is the most common extrusive rock while granite is a very common intrusive rock. (CG)" – Advantage: Martin. Victoria actually introduces a critical fact about rock identification, Claire merely throws factoids onto the page.

Once the introduction is out of the way, Gillespie needed to add another couple hundred words to meet the all-holy minimum word count. To do so, she told her readers,

"Extrusive igneous rocks come from lava, forming at the surface of the Earth and cooling quickly, meaning they form very small crystals. Intrusive igneous rocks come from magma, forming deep underground and taking longer to cool, meaning they form larger crystals."

We'll buy that, at least for a layman. After all, she left out both volcanic glass (obsidian and pumice), which has no crystals, and pegmatite, which can have house-sized crystals. The problem is that, since she's mentioned the only simple difference, she must now repeat herself:

• "...extrusive rocks form from lava at the surface of the Earth, whereas intrusive rocks form from magma underground..."
• "Extrusive rocks cool quickly because they are at the surface of the Earth. Intrusive rocks take a lot longer to cool because the temperature under the Earth's surface is a lot higher"
• "The most obvious difference between extrusive rocks and intrusive rocks is crystal size. Because extrusive rocks cool quickly, they only have time to form very small crystals such as basalt..."

All that padding in order to say the same thing three times! And while we're at it, Claire, basalt is not a crystal. Finally, Gillespie pads further with her take on igneous rock classification:

"You can break all igneous rocks into four main types, regardless of whether they are extrusive or intrusive rocks. They may be felsic, intermediate, mafic or ultramafic, depending on the ratio of light minerals to dark minerals. Felsic rocks, like rhyolite and granite, are high is [sic] silica, one of the most common elements on Earth. Intermediate rocks, like andesite/dacite and diorite/granodiorite, have a lower silica content and are darker than felsic rocks. Mafic rocks, like basalt and gabbro, have low silica content but contain iron and magnesium..."

Corrections: 1] silica is not an element, it is the oxide of silicon (Martin made a similar error); 2] "light to dark" is highly simplistic, since there are, of course light minerals that are relatively low in silica (e.g., peridot); 3] andesite/dacite is an extrusive/intrusive pair, diorite/granodiorite are both intrusive with different compositions, 4] duh: "mafic" means rich in Magnesium and Fe, aka iron. No, this rewrite did very little to improve upon Martin's original. In fact, from a factual standpoint, Martin's original text is superior, and it wasn't all that great to begin with. But Victoria's not around to take possession of the Dumbass of the Day award; that honor belongs solely to Gillespie. Guess that MA in English Lit went to waste...

While we're at it, Claire, what is a "porphyry"? and what does "hypabyssal" mean?

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