Density for Dummy Physical Geology Students

Lithosphere density

Sometimes it might seem as though we are awfully hard on journalism majors who seem to be scientifically illiterate. Well, no, we're not: we think that one of the reasons we're in a lot of messes – global climate change, fresh water shortages, antibiotic resistant bacteria, pandemics – is because too many people are scientifically illiterate, and the journalists who should be helping them understand science aren't... because they're just as illiterate! Take, for instance, Taylor Echolls, a journalist who attempted to write some science for eHow in "Density & Temperature of the Lithosphere" (now at Sciencing.com).

That's the kind of topic that could be answered fairly simply, which may be why Echolls glommed onto the title at eHow.com. To some extent, Taylor managed to transcribe the answers from whatever source he plumbed for information, managing to find a definition for "lithosphere" and get across the notion that there are two kinds of lithosphere, oceanic and continental. He did botch the comparison, however:

"Continental lithosphere is the thickest in the world. Beneath the ocean the lithosphere is thinner, extending down only about 100 kilometers (60 miles)."

We'd have liked to have seen a thickness for that continental lithosphere ourselves; say, 150 km or 90 miles on average, maxing out at around 240 km (145-150 miles). That's not to mention that the comparison should really be about crust, since the thickness of the portion of the lithosphere that's upper mantle remains fairly constant (except at rifts). But what the heck, it's only facts...

Echolls truly blows it with his discussion of density, however:

"The density of the lithosphere varies depending on temperature, depth and age. At about 50 kilometers (30 miles) below the Earth's surface, density measurements reach 200,000 pounds per square inch (13,790 bars)."

Yup, the scientific illiteracy of a liberal arts major rears its ugly head: someone who has at least passing familiarity with the physical properties of matter would have known that density is not measured in "pounds per square inch." That's a unit of pressure: density is mass per volume, not mass per area!

Taylor went on to inform his readers that,

"Due to the pressure from the above crust and mantle, lithospheric density generally increases as both age of the surrounding rock and depth increase."

Depth, sure... but what's this business about age? When it comes to temperature, Echolls doesn't perform much better, explaining that,

"Temperature of the lithosphere can range from a crustal temperature of zero degrees Celsius (32 degrees Fahrenheit) to an upper mantle temperature of 500 degrees Celsius (932 degrees Fahrenheit)."

First, we love Taylor's precision: "932 degrees"! Second, why "zero degrees Celsius"? why not -40° in Antarctica? In reality the surface temperature is generally taken to be the year-round average ambient temperature. It varies from place to place and, more importantly, the geothermal gradient varies from place to place as well, depending on the heat flow from the mantle. But our Dumbass of the Day wasn't saying that...

No, there's no good answer even buried in there; not to mention a massive science booboo. Such is the wages of a liberal arts education... no wonder those in the know bemoan our citizens' STEM illiteracy!

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