Dikes and Sills for Dummies - The Freelance Files MMLXXXVI

Igneous intrusions

Several freelancer types we've come across in our wanderings have decided – for whatever reason – that they should be their preferred content farm's go-to for a given discipline. Of all the websites we've surfed, and there have been more than a few, we're pretty sure that the site most likely to feature utterly unqualified people to attempt to explain science is the self-described WiseGEEK.com. Having jumped on the niche bandwagon, the site is slowly moving its allegedly wise and geeky content to AllThingsNature.org. That's where we found not-so-wise geek Andrew Kirmayer attempting to explain a geological term he didn't understand – again – in "What Is a Sheet Intrusion?"

That "again" is because this is the sixth time we've found Kirmayer expounding, at length, on earth science; and the sixth time he's gotten most of it wrong. Unless WiseGEEK deletes all his geology posts, this time probably won't be the last. 

For starters, let's look at a definition of a sheet intrusion. According to the folks at Wikipedia (who got it right), it's "a planar sheet of roughly the same thickness, that forms inside a pre-existing rock." Sheet intrusions are classified on the basis of their relationships to surrounding rocks: sheets that are injected more or less parallel to surrounding rock layers are "sills," while those that cut across structure are termed "dikes" ("dykes" to BritEng speakers). Most sheet intrusions are igneous in origin, although there are known examples of sedimentary sills and dikes.

Andy managed to include the words "igneous," "sill," and "dike" in his post; we'll give him that much. It's the rest of the words and how they form into factoids that gave our staff geologist the heebie-jeebies. Here are a few examples where Kirmayer's "creative writing" got him into factual trouble:

• "A rock formation that is created by the rising of magma in between the top layers of the Earth's crust is referred to as a sheet intrusion." – What's this "top layers of the Earth's crust" crap, Andy? Can't you just say "pre-existing rocks"?
• "The top layers can expand as a result, and the underlying material is sometimes exposed because of surface erosion. " – We think the first half might reference sheeted dikes injected into rifts (although his later fascination with laccoliths probably renders that assumption wrong) but we have no idea what the second clause might mean.
• "Rock sheets are usually somewhat flat in shape..." – No, Andy, they're always flat: it's part of the definition!
• "...a sheet intrusion... is generally oriented across, over, or under the landscape" – That certainly covers a lot of possibilities. In fact, since it's an intrusion, it is by definition "under the landscape" – WTF "across... the landscape" might mean, again, escapes us.
• "By the time [laccoliths] are discovered, the overlying layers have usually eroded away and it can be difficult to determine the original shape of the intrusion." – Difficult for a creative writing major, perhaps, but for a geologist? Not so much.

• "A sill is generally a kind of sheet intrusion that sits flat in between two other layers of rock, or even solidified lava. " – No, it's always a sheet that sits flat. Plus, on what planet is "solidified lava" not rock?
• "...geological processes can alter a relatively flat sheet intrusion into a more vertical position." – It this is Andy's attempt to explain dikes? 'Cause if it is, it's wrong: dikes are defined as cross-cutting, which does not mean "more vertical."

There are, of course more Kirmayerisms, but we got sick of reading through the many clumsy attempts our Dumbass of the Day made to sound authoritative. Trust us, he isn't – on this topic, for certain, and pretty likely on earth science in general.

SI - GEOLOGY