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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.

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.

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.

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.

Beware of Movies! will be a series of blog posts discussing important concepts in geology while making reference to scientific errors in movies and TV regarding geological concepts. These posts go in concert with a lecture series I’m preparing with the same idea. It seemed fun.

This is the first post in the Beware of Movies! series.

A common misconception about geology is that it is a science ONLY about minerals and rocks. Well, and oil. But that’s it. Geology is a tiny bit more complex than that. However, it does include rocks (and minerals) as an important fundamental basis. A person can’t say much in any field of the earth sciences without first knowing quite a bit about minerals.

MINERAL:

1) Naturally occurring

2) Inorganic

3) Solid

4) Specific crystalline structure

5) Specific chemical composition

Any compound that fits the above definition is a mineral. Ice (frozen water) is a mineral because it satisfies these criteria.  Bismuth hopper crystals do not satisfy these criteria because they’re synthetics, so, while quite lovely, they do not constitute a mineral.

CHEMISTRY:

When I start to talk about chemistry, my Introductory Geology students always groan. But I’m always able to assure them that what matters here is pretty basic and that it’ll be over soon. Chemistry matters because minerals have a specific chemical composition. This means that the individual atoms that go into a mineral are specified. The atoms are specific elements, like carbon and oxygen, and certain numbers of each of them combine (bond) to form the minerals. I will spare you the details of how the atoms bond. What’s important is that atoms have size. They’re like little spheres of differing sizes depending on the elements. When you try to stack the elements together, they stack in very specific ways. That is what causes the crystalline structure of minerals.

The stacking of the elements forming a mineral also results in all the recognizable properties of minerals that we use to identify them, including:

Color – what color is it?

Luster – how would we describe the shininess of the mineral? Metallic, earthy, glassy.

Specific gravity – how dense is it?

Crystal form – what shape are the crystals?

Cleavage and fracture – how do the crystals break? Do they break along specific planes or along random surfaces?

Hardness – is the mineral soft, like talcum powder, or hard like diamonds?

MINERAL GROUPS:

Minerals are grouped by their basic chemistry:

Oxides – have oxygen (O) like Fe₂O₃, hematite (rust).

Sulfides – have sulfur (S) like PbS, galena.

Sulfates – have sulfate (SO₄) like CaSO₄ H₂O, gypsum.

Carbonates – have carbonate (CO₃) like CaCO₃, calcite (chalk).

Native elements – gold, silver, carbon like diamonds!

Silicates – have silica (The silica tetrahedron – SiO₄) like SiO₂, quartz

Beware of movies: In Armageddon (1998), our heroes are forced to drill into an asteroid to implant a nuclear device. Where they wind up setting down, so claim the characters, is composed of “iron ferrite.” That’s not any mineral I’ve heard of. It’s a redundant name, actually, because the ‘ferrite’ part, like ‘ferric’ or ‘ferrous,’ refers to iron. So iron ferrite is an iron-bearing iron rock.

SILICATES:

This is the most important group of minerals on Earth, in that they constitute most of the Earth. Therefore, oxygen and silicon are the most abundant elements on Earth. Life is carbon-based, but we’re just a think veneer on a very, very large sphere. But if silicon is so common, why are there not silicon-based life forms?

Silicate minerals are categorized by how the silica tetrahedra relate to each other in the mineral. They can be isolated, or bonded to one another by sharing the one or all of the oxygens on the tips of the tetrahedra, to form chains, sheets, or complex networks. While this sounds like there’d be lots of silicate minerals, it turns out that there are relatively few that a person needs to know to be able to identify most minerals in ordinary rocks. We’ll get to that in a moment.

ROCK:

A rock is an amalgamation of individual mineral grains. Often there are several minerals in a rock, like granite that contains quartz, biotite, and two types of feldspars. But a rock can be composed of grains of all one mineral. The best example of this is a nice clean sandstone which can be made of only quartz grains.

Beware of movies: In Journey to the Center of the Earth (2008), Muscovite is called a “thin rock formation.” Muscovite is a mineral, not a rock. It does make nice thin sheets (because of its cleavage), but it’s not a rock.

BOWEN’S REACTION SERIES:

Bowen’s reaction series describes the stability of silicate minerals under different temperature regimes. Minerals at the top of the reaction series, form under conditions of extreme heat, but are not stable at the lower temperatures at the Earth’s surface. Minerals at the bottom of the series are more stable at low temperatures (like those on the Earth’s surface).

Understanding Bowen’s reaction series will help anyone identify minerals in a rock. See, the minerals that go together in a rock aren’t just randomly selected. Certain minerals occur together. Some minerals are never found together in a rock. The minerals that can occur together are those that are stable at the same temperature ranges. So Olivine and Quartz will never occur in the same rock, for example.

The minerals that are stable at high temperature are the mafic minerals, which are high in magnesium, iron,and calcium, and low in silica. The minerals that are stable at low temperatures (and are common on the Earth’s surface) are called felsic minerals, and are high in silica and hight in sodium, potassium, and aluminum. This doesn’t matter now, but will matter later when we think about volcanoes.

Beware of Movies: The movie “The Core” seemed to struggle a bit with Bowen’s reaction series. The mantle of the Earth (the layer below the crust, where we’re living), is composed of ultramafic rocks. This means, high iron, high magnesium, and low silica. However, in the movie, the intrepid ‘terranauts’ find themselves in a large empty cavity in the upper mantle that is full of amethyst (quartz) crystals. Quartz is felsic. It would never be stable in the mantle. This is a massive mistake.

ROCK GROUPS:

All rocks on Earth can be divided into three groups (and some might fit into more than one).

Igneous rocks: Those rocks that formed from the cooling of molten rock (magma). In these the crystals that make up the rock form according to Bowen’s reaction series (as well as the composition of the magma itself).

Sedimentary rocks: Rocks that formed from the deposition of bits and pieces of other rocks that have been broken down and (probably) transported elsewhere. Alternatively, sedimentary rocks can form from the precipitation of crystals directly out of water, rather like hard water deposits.

Metamorphic rocks: If pre-exisiting rocks of any kind are subjected to great heat and/or pressure, the minerals present, and their relationships to one another may change. This results in metamorphic rocks.

Beware of movies: Bowen’s reaction series helps explain what silicate minerals might go together. There are similar limitations on what other minerals might occur together by what rock type they will be found in. In Journey to the Center of the Earth (2008), they find rubies, emeralds, and diamonds together in a lava tube. None of these would occur together. Emeralds are usually found in felsic igneous rocks. Rubies are found in metamorphic rocks, and diamonds form deep in the mantle, far away from either of the places where emeralds or rubies might form.

ROCK CYCLE:

Any of the three types of rock (igneous, metamorphic, or sedimentary) can be changed into any other type of rock in what is called the rock cycle. Existing rocks can be melted and cool again to make new igneous rocks. Existing rocks can be broken down, transported, and redeposited into sedimentary rocks. And any existing rock can be exposed to high heat and pressure to form a metamorphic rock. The end result is that there are very few tremendously old rocks on Earth, because most have been recycled.

These are some of the basic concepts necessary to understand further topics in geology. Without this basis, it would be impossible to begin to interpret the Earth’s history from its rock record.