Earth's Center for Dummy Fifth-Graders

Earth's layers

It goes without saying that laypersons who write about elementary science often attempt to simplify the more technical concepts. Mostly, it's because they aren't familiar enough with the heavier stuff to do the simplification. Plus, a lot of them don't do the necessary research to correct their own misconceptions, which is quite probably where Fraser Cain¹ ran into problems with his UniverseToday.com post, "How Far Down is the Center of the Earth?"

Cain started with a homely little tale of a 3rd-grader trying to dig a hole all the way to the core, along the way emitting his first misinformation:

"Most of this journey would be through temperatures hot enough to melt rock, getting as high as 7,000 Kelvin at the center."

Every reference we can find, save one, says 6000 K (the odd one says 8000° C). That's just a quibble, of course, but we prefer that people get the simple stuff right before they tackle the hard stuff. Speaking of the hard stuff, Fraser got plenty of that wrong, too, beginning with the nonsensical illustration at the top of the post (we reproduced it above). Too bad he didn't 1) mention that the core isn't to scale, 2) explain why he was babbling about seismic waves, and 3) explain the dates near the North Pole. One could only wish...

Moving right along, here is some of Cain's other misinformation: "The first 35 km or so of digging would be through the outer crust of the Earth." – It depends: you can fully penetrate the crust within 10 km in the ocean basins, and wait for more than 50 km in some places on the continents "...the Earth’s mantle... [is] about 3,000 km of rock heated to such a high temperature that it’s a liquid." –  Only if you consider frozen molasses to be "liquid." Scientists call the mantle "plastic," which means that it is v-e-e-e-e-ery viscous,² and will bend rather than break when deformed. It ain't "liquid" like that orange stuff in Terminator movies.

• "Volcanoes are points on the Earth when magma from the mantle breaks through to the surface." – Ummm, no. Rift zones are lines on the Earth where the mantle squeezes up. Most volcanoes occur at subduction zones, and the magma there is melted crust that rises via buoyancy.
• "...the core of Earth... [is] comprised almost entirely of [sic] iron, with a little nickel, and some other trace metals. And it’s even hotter than the mantle above it. This is where temperatures get to 7,000 Kelvin. Assuming we could bore through the iron, and could withstand the heat, we could get down to the center of the Earth." – Gee: we wonder why this "educational" post neglected to mention something that is readily apparent in the accompanying image. Only the inner core is solid nickel-iron, the outer core is liquid. Maybe that's where Fraser got the notion that the mantle is liquid? Who knows...

So what if Cain got the radius of the planet right? He botched the temperature of the core and failed to mention that there are two different zones in the core, one of which is liquid. He also reiterated the tired blooper that the mantle is "liquid magma." How could we not name him our Dumbass of the Day?

¹ No, not the Kelsey Grammer character on "Frasier." This guy's actually the publisher of the site.
² The viscosity of the mantle is estimated to be on the order of 10²⁰ Pa-s, or 10²⁴ centipoise. For comparison, the viscosity of ketchup is on the order of 10⁶ centipose.

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