Thursday, January 28, 2010

More incorrect TV science

Watched a few minutes of Naked Science on National Geographic Channel tonight.  The show was titled Ice Age Meltdown.  They really should pay a geologist a couple of bucks to help edit these shows - there were a couple of inaccurate statements.

First some background.  Most everyone knows that the outer layer of the Earth is called the crust and it lies over the mantle.  Most people are also aware of plate tectonics, the fact that the outer layer of the Earth is split into rigid plates which move relative to each other over time.

Here's where some misunderstandings occur.  Many people knowing this much assume that the tectonic plates are composed of the Earth's crust.  It's actually a bit more complicated.  Below is a nice image I stole from a geologist's website via a Google image search.

There are two basic types of crust.  Oceanic crust, which is 5-7 km thick, is basaltic (essentially solidified lava).  Continental crust, however, is much thicker (averaging 35 km but thicker under mountains) and granitic in composition (mostly metamorphosed).  Below the crust is a layer of the mantle called the rigid upper mantle.  It's solid rock like the crust although it does have a different composition - it's called peridotite and is mostly composed of the mineral olivine.  Anyway, the crust plus the rigid upper mantle (which extends down to about 100 km below the surface) is called the lithosphere (from the Greek word lithos which means "stone").  The tectonic plates are made of the lithosphere.  Below the lithosphere is a softer layer of the mantle called the asthenosphere (asthenes is Greek for "weak").  The asthenosphere is not liquid, it's a soft solid that flows - think of Silly Putty.

Now how does this relate to a show called Ice Age Meltdown?   Well, during the last ice age (the Earth has had a number of "ice ages" in its history so you shouldn't say "during THE ice age), glaciers extended southward to their most recent maximum extent around 18-20 thousand years ago.  Thousands of feet of glacial ice is heavy.  Here is the mid-Hudson Valley of NY, it may have been a mile thick.  That depresses the crust - actually it depresses the entire lithospheric plate causing it to "float" lower in the mantle ("float" is in quotes because the mantle is not liquid).  This process by which the lithosphere is depressed when there's more weight on it and rises up when the weight is removed is termed isostasy.  It's one reason for local sea level change when glaciers grow and retreat.

So, on this "science" show, on the National Geographic Channel which should know better, they were discussing the crust and mantle of the Earth in relation to isostasy.  The crust rises and falls as it floats on the mantle.  OK, a bit of a simplification.  But then they mentioned, several times, how the mantle was magma and molten.  It's not liquid magma.  You can partially melt it to form magma under some conditions, like by lowering the pressure by thinning the lithosphere, but it's solid (you would never call Silly Putty liquid, would you?).

Is it too much to expect freshman geology accuracy from a supposed science show?  Students in my introductory geology class know better.

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