Divergent Margins for Tectonic Dummies

Rift cross-section showing sedimentation

Your grandmother may remember the long-ago quiz show called "Name that Tune," where contestants tried to identify a popular song from just the opening notes. Well, every once in a while one of the Antisocial Network staffers who patrol the internet can spot a DotD candidates in just one word. That's the case today, compliments of eHowian Rebecca C. Jernigan and her Sciencing.com post "Type of Rock Found in Divergent Boundaries." As always, any grammatical and logical errors in the title belong to the OQ: eHow.com only changed (some) spelling errors...

We asked our staff geologist this question, and he had a long, drawn-out answer. Basically, however, new oceanic crust is constantly being created within the rift at the center of a divergent boundary, and then sedimentary rocks are layered over them. Much of the real answer depends on where the rifting takes place: within a continent or within an ocean basin. In other words, the answer is... complicated.

Jernigan, however, turns the answer into something simple – simplistic, in fact. According to Rebecca, there is only one kind of rock found at a divergent boundary:

"Divergent plate boundaries, caused by the shifting of the Earth's tectonic plates, create igneous rocks as the plates move. The rocks are formed by cooling magma, and their specific type depends on the minerals available in the area."

We'll buy the "cooling magma" bit, but the rubbish about "their specific type"? Naaah: the composition of oceanic crust isn't dependent on "the minerals available in the area"! Jernigan's gotten rock type and composition confused. But more on that later...
Since Rebecca was required to pump out 300 or so words, she had to pad her answer with sections entitled "What Are Divergent Boundaries?" (her answer is simplistic, but close to correct) and "Type of Rock" (mostly padding about what igneous rocks are). It's when she got to Jernigan's section titled "Specific Rocks" that milk came out of our staffer's nose:

"Most rocks formed at divergent boundaries are categorized as malefic [sic] igneous rocks..."

Yup, that's right: Jernigan thinks that the rocks formed in a rift zone are evil! Rebecca also thinks that peridotite is common in divergent margins, which is only true if you sample the lower oceanic crust. No matter what she said after that "malefic" bull (or before it, for that matter), though, Jernigan's inability to transcribe the simple five-letter word "mafic" is all we needed to name her our Dumbass of the Day.

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