Silica, Crust, and Geology for Dummies

crust types

A not surprising number of the search queries scraped and "answered" an ye olde eHow.com appear to have been asked by middle-schoolers (or clueless college freshpersons). One of the giveaways is the misuse or misreading of technical terms. In a perfect world, the eHow contributors who tackled those questions would have a) known what was wrong and b) known the right answer. Not so, however, with the likes of Susan Revermann, who tried to tell some poor kid "Which Layer of the Earth's Crust Contains the Highest Concentration of Silica?" at Sciencing.com.

What Revermann should have said, according to our geologists, was "Don't you mean 'Which layer of the Earth'?" Because, after all, the crust is a layer of the Earth! That, however, is not the greatest problem Susan had, no indeed.
You see, Susan – perhaps because she knew nothing about geology – had to look up "silica" in the dictionary; and she found that it means "silicon dioxide," aka quartz, to a layman... which is wrong in the context of the question. Here's what she said:

"Silica is a mineral compound made of silicon and oxygen, SiO2, and is found on the Earth’s crust in three main crystalline varieties: quartz, tridymite and cristobalite."

While technically correct, Revermann's answer is contextually wrong. In this context, "silica" refers not to the mineral quartz but to silicate minerals in general. Silicate minerals are those that contain silicon and oxygen in their composition, about 90 percent of all minerals on Earth. Quartz is 100% silica, while other minerals have varying amounts of silicon and oxygen by weight. The minerals found in granitic rocks, i.e., continental crust, tend to have higher proportions of silica than the minerals found in basaltic rocks, i.e., oceanic crust.

In the grand old tradition of blind pigs and acorns, Revermann somehow managed to come up with an answer that, if you squint just right, is correct:

"Even though the oceanic layer [sic] is thinner, it is more dense and heavier than the continental layer. However, you will find more silica on [sic] the continental crust."

Leaving aside the questionable value of the factoid that oceanic crust is relatively dense, it's pretty clear that Revermann thinks that the presence of larger quantities of quartz "on" continental crust is why silica is more concentrated in the continents. Mmmmm, no.

Blind pig? Must be another name for Dumbass of the Day!

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