Beware of Movies! The Interior of the Earth

The Beware of Movies! series is meant to point out some of the scientific inaccuracies of popular movies, specifically in points related to the geological sciences.

This blog post will point out the major inaccuracies portrayed in movies about the structure and composition of the Earth.

One might think that we have no way to know what the interior of the Earth is like. The Earth is over 6000 kilometers in diameter, and we’re doing good to drill a few kilometers into the crust.

Here’s the reality of the Earth’s structure. At first pass, the Earth is composed of the crust, the mantle, and the core. The mantle and the core are each divided into two parts, the upper and lower mantle, and the inner and outer core.

Structure of the Earth

The crust is the part that we live on. It is very, very thin compared to the rest of the Earth, going down only to about 7 to 70 km (depending on where you are). The base of the crust is marked by the Moho (which is short for the Mohorovičić discontinuity). Below that is the upper mantle down to a depth of about 660 km, followed by the lower mantle down to about 2900 km. It is a common misconception that the mantle, being very hot and under pressure, is molten. This is absolutely untrue. The mantle is solid rock, with one tiny exception in the upper mantle that we’ll get to in a moment.

Below the mantle is the core. The outer core, from 2900 to 5155 km in depth, is the only part of the interior of the Earth that is fully molten. It is composed of molten iron and nickel. It is here where the Earth’s magnetic field is generated. This is one of the few things that were correctly attributed in the movie “The Core.” From 5155 km to the true center of the Earth is the solid inner core, which is composed mostly of iron and nickel.

This simplistic, three-part division (crust, mantle, core) is the extent of the description of the interior of the Earth provided by the movie, “The Core.” Equally, “Journey to the Center of the Earth” (2008) is no better. Both are over-simplifications.

 

Before we move forward, it is probably worthwhile to explore how we can possibly know this much about the interior of our planet. As I already pointed out, we haven’t actually drilled completely through the crust. How can we possibly know the structure or the composition of the interior of the Earth.

The movie “The Core” actually begins to show us how this is done. Solid rock can transmit seismic waves. Every time there is an earthquake somewhere on Earth, it sets off waves that pass completely through the Earth and can be recorded at seismic stations throughout the globe. Some waves pass through the Earth, others can only move along the surface. Seismic stations record both types and seismologists can determine when and where the earthquakes happened based upon when the waves are recorded.

The way that waves pass through the Earth are affected by the types of rock. Some rocks speed up the waves, others slow them down. Waves bounce around inside the earth, too. So a seismic wave might bounce off of the core-mantle boundary. These properties, combined with multiple seismic stations, make it possible for us to know the composition of the Earth and the approximate position of any important boundaries within the Earth.

Furthermore, one type of seismic wave, shear waves, won’t pass through liquids. So if a seismograph recording an earthquake shows no shear waves, we know that the seismic waves went through a liquid. This is how we know that the mantle is solid rock and that the outer core is molten.

Beware of movies: In “The Core,” the terranauts find huge open cavities in the mantle. If such open spaces existed, they would have been evident from seismic studies. No seismic wave could pass through an empty space like that, and we would know about them. Similarly, in “Journey to the Center of the Earth” (2008), the main character discusses the existence of tubes that bypass the mantle and go straight to the core, which in the movie is hollow. Again, seismic waves would have shown such tubes and caverns.

Since earthquakes occur all around the Earth all the time, one needs only to have a global set of seismic monitoring stations, and we can learn all about the Earth’s structure. Such a global network exists. Because of that, we know that the Earth is actually far more complex than just crust, mantle, and core.

 

Many people are aware of the important concept in geology called “Plate Tectonics.” At a first pass, Plate Tectonics explains why it seems like South America and Africa would fit so well together, like puzzle pieces. That’s because they did once fit together and have since moved apart. The moving units on the Earth’s surface are called ‘plates.’ There’s a South American plate and an African plate. They were once together and have since moved apart.

Individual plates are not just pieces of crust moving around on top of the mantle (a common misconception). The plates are pieces of the “lithosphere,” which includes the crust plus the uppermost part of the mantle down to about 100 km. Crust plus the uppermost mantle (called the lithospheric mantle) equals the lithosphere. The crust and lithospheric mantle move as one big piece called a plate.

The structure of the Earth, including the lithosphere.

Below the lithosphere, is the asthenosphere, which goes down to about 400 km in depth – basically much of the rest of the upper mantle. It’s in the asthenosphere that flow occurs.

But wait! The mantle is solid rock! How can it flow?

Two things happen: 1) Just like wax is a solid that can flow, so can the mantle. Individual atoms within the minerals of the mantle can move causing very, very slow flow. 2) Because of temperature and pressure gradients, between 100 and 200 km in depth there is the tiniest bit of melting. The whole rock doesn’t melt, just a few of the minerals. This slows down shear waves, but doesn’t stop them completely, which is how we know this slight melting exists. We call it the low velocity zone, because it slows down the shear waves.

 

The various layers of the Earth have fairly specific compositions. The core is mostly nickel and iron, as previously mentioned. The rest of the Earth’s composition can be explained using Bowen’s reaction series. Read more about it here.

Bowen’s Reaction Series

The basic rocks of the universe are ultramafic rocks, so we would expect that the bulk of the Earth is composed of ultramafic rocks, which are primarily the mineral olivine. Ultramafic rocks are characterized by having high iron and magnesium and low silica.

The crust of the Earth has some mafic rocks and minerals, but most are intermediate to felsic in composition, meaning that there’s high silica, potassium, aluminum, and sodium. These compositions are attained when the mafic rocks of the mantle are melted and erupted, then cycled through the rock cycle multiple times. Every time a rock is re-melted, the most mafic parts of it tend to be last to melt, and might not erupt, resulting in rocks that become more and more felsic over geologic time.

Beware of movies: In “The Core,” the terranauts discover huge amethyst crystals in the upper mantle. Amethyst is a type of quartz, which is a felsic mineral. The mantle is ultramafic. What this means is that quartz would not be stable, it could not exist, in the mantle. This is a big mistake.

Beware of movies: Both “The Core” and “Journey to the Center of the Earth” make the mistake of thinking that diamonds would be abundant in the mantle. The reality is that there are diamonds in the upper mantle, where conditions of temperature and pressure are suitable to form diamonds. And, “Journey to the Center of the Earth” never really specifies where they are in the mantle – probably fairly high, so maybe that’s ok (although there’s an additional problem with muscovite in that movie). In “The Core,” however, they discover huge diamonds presumably in the lower mantle, at least fairly close to the core-mantle boundary. Conditions aren’t suitable for diamonds low in the mantle, lack of carbon notwithstanding. The giant diamonds are bogus.

 

This is the general understanding of the interior of the Earth, and how we know what we know. On this topic, movies are typically either pretty-darn-good or completely wrong. As always, extreme caution needs to be used when trying to apply the science of movies to real life.

*****Added January 15, 2013*****

I’ve discovered that I left some things out of this post. Please visit this post to learn more about what meteorites and magnetism have to do with our understanding of the Earth’s interior.