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.
1) Naturally occurring
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.
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?
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.
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.
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 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.
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.
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.