Tectonics and Magma for Dummies

Mantle convection cell diagram

Though many college students poke fun at "rocks for jocks," a remarkable number of liberal arts majors take Geology 101 as their elective science course (physics and chemistry apparently being too "hard"). Sadly, a lot of them don't remember what they learned – enough of them that as the Antisocial Network staffers search for random scientific inaccuracy, one of the words they search on is "magma." It's cropped up again, this time at the newly-minted Sciencing.com, Leaf Group's¹ rebranding of science topics from eHow.com, where Angela Libal attempted to answer "How Does Pressure Affect Plate Tectonics."

It only takes one paragraph for Libal to run afoul of the facts as geologists know them:

"The Earth's surface is called the lithosphere, or 'rock ball.' It's made up of enormous plates of rock, floating on the semi-solid mantle beneath. These rock plates crash into, grind past, and sink underneath one another in a continual process called plate tectonics. The pressure that affects plate tectonics can come from above -- the weight of the plates -- or from below -- the force of magma."

Angela starts off correct, although the verbiage is rather flowery (which one might expect a fiction writer who at one time claimed she was studying "cryptozoology"). By the last sentence, however, she's gone off the rails and the last word – magma – is utter bull. Unlike what many eHowians, errr, Sciencingians, try to tell their readers; that "semi-solid mantle" is not a gigantic pool of liquid magma. Instead, it's solid rock that is under such extreme pressure from the overlying crust and subjected to so much heat from the earth's core that it acts like a very, very, very viscous liquid – thicker than cold peanut butter! The near-solid rock is termed "plastic," a scientific term that has nothing to do with Chinese toys, but means that it can deform without breaking. The plastic mantle flows very slowly in gigantic convection cells. These cells are what is believed to drive the motion of the crustal plates on the planet's surface.

So, to answer the OQ about pressure? The answer is straightforward: the confining pressure caused by the mass of the overlying lithosphere (crust and upper mantle) prevents the asthenosphere (lower mantle) from completely melting; thereby allowing the majestically slow movement of the convection cells.

Despite Libal's attempts to insert the word "pressure" into brief and often misinformed descriptions of convergent, divergent and transform margins; she never gets to the point. Instead, she talks around it because she doesn't understand either the question or the answer. That, folks, is precisely why Angela is picking up her second Dumbass of the Day award.

¹ Leaf Group is itself a rebrand; the company is the former Demand Media Studios

copyright © 2017-2022 scmrak

SI - GEOLOGY