Earth origins and tectonics

I like to look for media online that would be useful as demonstrations in class. It’s fairly hard to find good ones, and the best usually are just clips of longer programs such as Nova, Horizon or various National Geographic programs (as this one is). This clip does a fairly good job of explaining and visualizing how plate tectonics works. The video even includes a nod to how tectonic processes got started during the Hadean Eon (over 3.8 billion years ago), when Earth was still getting slammed together by impacts with other loose debris in the newborn Solar System.

My only real gripe with this production is how it depicts the mantle, as a “molten semi-solid” that “boils in extreme slow motion”. Actually, the mantle isn’t molten at all. The “semi-solid” bit is closer to fact, because the mantle is made up of rock that is solid but bendy. Under the heat and pressure of the deep mantle, rock acts like really slow taffy, but if you could examine a piece of it in your hand (and your hand was inside an Iron Man suit of heat-resistant armor), it would look like just a chunk of rock. Glowing red hot rock, but rock. Mantle rock is fluid… but fluid in the sense that glass is a fluid. It’s warpable, flowing and unstable, but it doesn’t flow fast enough to watch.

Lava does come from somewhere, but that somewhere is near the upper edge of the mantle, where tiny traces of ocean water dissolved in the rock make its melting point low enough to truly liquify. Volcanic lava doesn’t come from deep inside the mantle, usually… only the heat comes from that far down. Sometimes a blast of weirdly hot heat can rise from the lower mantle, from turbulence and current flow in glowing taffy-ish rock down there. When those rare blasts occur – forming hotspots at the surface, like Yellowstone, Iceland, and Hawaii – the infernal temperatures can melt deeper rocks than usual, and you can get real lava from way down inside the upper mantle. But that’s unusual.

They also shouldn’t use the word “boiling”, when talking about mantle rock. Boiling is turning a liquid into a vapor, and that doesn’t happen to rock in the mantle. The word they should have used was “convecting”, which solids and liquids and gases can ALL do, and which the mantle is doing in this case.

But overall… sorry, we do not float on a buried magma ocean. A lot of sources get this wrong in the “popular science” media. That’s too bad, because the reality is far weirder and more interesting.


~ by Planetologist on October 13, 2008.

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