Geological Misconceptions – Ooey Gooey Lava

Published 2008-01-22 by Chris

The Core is so hilariously wrong in so many places, it’s unimaginable to think that no one in the script room thought to pass that trainwreck of a movie’s script to a scientist before they started filming (the movie actually had a geophysicist by the name of J. Marvin Herndon as a science advisor – although once you learn his background, you start to be less surprised, anyway I digress). Though I don’t want to point out all the problems inherent in the film, as I have things to do, I find it a useful jump-off point to outline two of the most annoying geological misconceptions I’ve come across.

These are the often seen but rarely thought about scenes in countless movies where someone sinks into lava and the idea that the Earth below the continental and oceanic crust is mostly liquid. Like a big pot of caramel being brought to the boil by a radioactive hotplate. Since there’s two topics, I’ll split this article into two sections, each tackling a single misconception.

That Sinking Feeling
You’ve seen river folk (Fig. 1) and terranauts (Fig. 2) befall the same fate. Tripping, falling and/or just plain sinking into lava. It’s not so much a silly misconception as it is applying common experience to an unknown situation with poorly thought-through variables. Lava is a liquid, after all, and when the producers of multi-million dollar movies or fantasy novel trilogies get to thinking about the last time they sat and washed themselves, likely in the bath all those months ago, it dawns on them that they pretty much tend to sink into the water, which is also a liquid.

Figure 1: Gollum sinks into the lava of Mt Doom trying to save the one ring. Notice the low viscosity of the lava. Screenshots are © Copyright 2003 New Line Cinema.

Figure 2: A member of “The Core” Terranaut team sinks, nay, splashes into lava, in a geode, thousands of Km beneath the crust. Sigh. Screenshots are © Copyright 2003 Paramount Pictures.

The above screen shots show lava, which is not water, and creatures sinking into that lava which are mainly water. This is a problem because the material they are sinking into is liquid rock with a density of at least twice that of water. Forget for a moment that the cause of death would be extremely rapid combustion, or that you probably couldn’t see the bodies sinking due to the smoke they’d generate. The problem with these scenes is that you’d float, very easily, on a body of liquid with a density of ≥2.5 g cm³. The Dead Sea has a density of ~1.25 g cm³ due to it’s extremely high salinity. Just to illustrate how easily you sink in a liquid this dense, here’s a picture of a dude floating on his back reading the newspaper in that body of water;

Now that’s how you relax at 300-odd metres below sea-level. From Wikipedia.

The human in that image has a density of ~1 g cm³, so therefore won’t sink into a liquid of higher density (1.25 g cm³). This, of course, is a very simple explanation, and completely disregards the fact that lava is hundreds of thousands of times more viscous than water – which would further reduce the ability of endoskeletal animal to sink. What is possible, however, is for lava to roll over the top of you, so don’t think you’re low density is going to save you should a lava flow come slithering your way. That’d just be highly dense on your part.

Fountains of Fun
Continuing on the theme of lava and it being liquid, let’s take a look at this impressive shot of a lava fountain from Hawai’i;

A 10 m fountain of delicious lava at Pahoeoe, Hawai’i. From Wikipedia.

You’ve seen lava flowing out of the ground, flowing from volcanoes, flowing here there and everywhere. So as someone who’s not studied the finer inner workings of the science of geology, you’ve likely extrapolated your observations and concluded that the mantle is liquid too. Well, you’d be wrong there. The mantle is solid, but has the unique ability to flow in the solid state.

The reason we know this is the same reason we have a pretty good idea of the structure of the Earth; seismology. Seismic waves come in two major forms when you’re studying deep structures, S-waves, which look like the letter S, and P-waves, which are bands of varying pressure. We know the density of the Earth, we know the speed at which S- and P-waves travel through materials of those densities and we can tell how far away an earthquake was by measuring the time between the P-waves arriving and the S-wave arriving. We can also measure how and where these waves refract over different layer boundaries (Fig. 5). We can do this because the mantle is predominantly solid, which is good, because S-waves can’t travel through liquids. This is the same reason we know we have a liquid outer core; S-waves from large earthquakes don’t make it through the core. This is also frustrating, because it means we can’t see the basic structure of the inner core – which we presume to be mostly solid iron and nickel.

Figure 5: Paths of the waves from a large seismic event refract and are measured at various locations around the Earth. From Wikipedia.

So next time you think “I wonder how great it’d be to swim down through the mantle in a flame retardant suit”, stop and think again. Because not only would you have a hard time sinking into the lava, you’d also hit a solid wall, as it were – the mantle.

The splash image used on the front page for this article was source from http://www.volcanos.ms/

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