I should like to give an introductory course on geology.
It will consist of a series of articles presented in what I think is the right order for teaching geology. I shall present them perhaps at weekly intervals, except for the first few which will merely clarify definitions and which we can get through quickly. (...)
The objective of this course is to show how it is possible to reconstruct the past history of the Earth from our present observation of the rocks.
It will differ from other textbooks in that it will place a strong emphasis on asking and answering the question: "How do we know?" Most textbooks report certain aspects of geological knowledge simply as things that are known: for example, that granite is an igneous rock, or that sandstone with certain properties is aeolian; or that the Earth's core is iron; but without addressing, or at least without systematically addressing, the question of how these things are known in such a way as to satisfy the doubts of the skeptical or the inquisitiveness of the curious.
As a result, the average geology textbook does fairly poor service to the skeptical, or to those who wish to debate and convince the skeptical. It also, in my view, does a disservice to the science of geology itself: for the story of geology is in effect the world's longest-running detective story, and it is more interesting if geology is presented as such than as a collection of facts handed down from on high.
Finding the right order in which to structure a course in geology is perhaps the most perplexing decision facing its author. No solution is ideal, because (with the exception of the definition of basic terms, which clearly should come first) it would be best if every topic could be discussed last, so that the reader can come to it with the rest of the course as context. As this is impossible, some sort of compromise has to be made.
The contents of the course will be as follows:
(1) Rocks and minerals: in which I explain what is a mineral, what is a rock, what are sedimentary, igneous, and metamorphic rocks.
(2) Surface processes: including weathering and erosion, rivers, glaciers, nearshore processes, marine sediments, etc; with a systematic look at all the different types of sediment and their corresponding sedimentary rocks --- peat and coal; glacial till and tillite; deserts and aeolian sandstone; coccoliths and chalk; etc, etc, etc.
(3) Plate tectonics: in which we describe how it is known that plates move, what is know of the mechanisms, and what effects this has in terms of faulting, folding, orogeny, ophiolites, terranes, etc.
(4) Stratigraphy: a discussion of actualism, of Steno's principles, of way-up marks, of cross-cutting relationships, of the geological column, of index fossils, and so forth.
(5) Absolute dating: those dating methods other than the relative methods of stratigraphy. This will include a look at some of the methods of more doubtful value, such as racemization.
(6) [This is added by edit in November] I shall have to add a section, hopefully a short one, about paleoclimatology. This is something of an afterthought, but having written much of the rest of the course and repeatedly said that such-and-such a thing would be put off until I talk about paleoclimatology, I don't see how I can not talk about it. So be it.
At that point I shall have done what I set out to do, in that the reader will then have a grasp of the principles of historical geology. However, it may be that the readership will have further questions. In particular, the reader may want to see some historical geology actually done, or in other words to see some case studies. It may be possible to continue the discussion along these lines.
Note on sources
It will not be necessary to give references for notions which are the common property of geologists, such as the definition of a mineral or the fact that granite is felsic. However, I shall provide references to the more abstruse or particular facts to which I allude.
Thanks are due to Pressie for volunteering to review the material. Any remaining errors are, of course, mine. As a great man once said: "We all have our little faults".
Minerals and rocks: definitions
I think we should provide the "official" geological explanations of the words crystal and mineral.
Crystal: A homogenous, solid body of chemical element, compound, or isomorphous mixture, having a regularly repeating atomic arrangement that may be outwardly expressed by plane phases.
Isomorphous mixture: Two or more crystalline substances to have similar chemical composition, axial ratios, and crystal forms, and to crystallize in the same crystal class. Such substances form an isomorphous series.
Mineral: (a) a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties. Those who include the requirement of crystalline form in the definition would consider an amorphous compound such as opal to be a mineraloid.
(b) Any naturally formed inorganic material, i.e. member of the mineral kingdom as opposed to the plant and animal kingdoms.
From Bates, R.L. and Jackson, J.A. (Editors), 1980. Glossary of Geology, Second Edition. American Geological Institute. Falls Church, Virginia.
This makes organic material such as amber a mineraloid, not a mineral. Any organic material, such as coal, therefore is not a mineral (although used as such colloquially).
By a silicate tetrahedron we shall mean an atom of silicon bonded with four equally spaced atoms of oxygen forming the four corners of a pyramid having a triangular base.
Each tetrahedron can share each one of its oxygen atoms with one other tetrahedron, so that two tetrahedra can join together corner-to-corner (but not edge-to-edge or face to face). Hence each tetrahedron can be linked with up to four other tetrahedra, one for each corner of the tetrahedron; or three, two, one, or none. This gives us a wide variety of structures that can be built out of tetrahedra: three-dimensional lattices; two dimensional sheets, chains, double chains, rings, et cetera; so silicate minerals can be classified according to their silicate structure as lattice silicates, sheet silicates, chain silicates, and so forth. The diagram below shows some of the possible structures.
Key: (a) an isolated tetrahedron; (b) a pair; (c) a six-member ring; (d) a chain; (e) a double chain; (f) a sheet.
Note that the chain and double chain can be extended indefinitely in one direction and the sheet in two directions.
A silicate mineral is a mineral containing silicate structures. Note that with the exception of quartz and its polymorphs, a silicate mineral will not consist entirely of such structures. Atoms of other elements must necessarily be involved so that the rings, chains, sheets or whatever form part of a three-dimensional crystal.
Most silicate structures can be described either by a descriptive English name such as "sheet silicate" or by a name which describes the same thing in Greek such as "phyllosilicate". Where a plain English term exists, I shall employ it in this text. The table below shows how the structures relate to the English and Greek names and gives examples of minerals important to this course.
|Structure ||English name ||Greek ||Important examples|
|Three-dimensional ||Lattice silicates ||Tectosilicates ||Quartz; feldspars|
|Parallel sheets ||Sheet silicates ||Phyllosilicates ||Micas; clays; serpentine; chlorite|
|Single or double chains ||Chain silicates ||Inosilicates ||Pyroxenes; amphiboles|
|Three, four, or six-membered rings ||Ring silicates ||Cyclosilicates |
|Pairs || ||Sorosilicates ||Epidotes|
|Isolated tetrahedra || ||Nesosilicates or orthosilicates ||Olivine|
Because tetrahedra link together by sharing the oxygen atoms at their corners, the structure formed by the tetrahedra is reflected in the chemical formula of a silicate. For example, quartz consists of nothing but tetrahedra linked together in a three-dimensional lattice. This means that every tetrahedron is linked to another at all four corners; which means that every oxygen atom is shared by two silicon atoms; which means that quartz has the formula SiO2. Similarly, if you look at the formula for zircon (ZrSiO4) you can see that it must be a neosilicate.
In some silicates, the structure is based not just on silicate tetrahedra but also on tetrahedra with a central atom of aluminum rather than silicon. Silicates which incorporate these aluminum-based tetrahedra as well as silicate tetrahedra are known as alumnosilicates.
Aluminum-based tetrahedra have a different charge from silicate tetrahedra. This means that you cannot have an aluminosilicate which differs from an ordinary silicate only by the substiution of atoms of aluminum for some of the atoms of silicon; there must necessarily be other differences. For example, it is chemically impossible to build a lattice just out of these two kinds of tetrahedra analogous to quartz; other atoms are required to balance the charge of the chemical formula. Hence lattice aluminosilicates such as feldspars do not have the formula (Si,Al)O2, which is impossible, but have formulas such as KAlSi3O8 and CaAl2Si2O8.
Felsic and mafic silicates
Silicate minerals which are high in silicon are called felsic minerals; the opposite of felsic is mafic; minerals which are very mafic are known, sensibly enough, as ultramafic. Note that this term only applies to minerals which are silicate minerals and so contain some silicate tetrahedra; no-one would call calcium carbonate (for example) an ultramafic mineral.
In some texts, particularly older texts and British texts, you may see the words acidic, basic, and ultrabasic used instead of felsic, mafic, and ultramafic. These terms refer to an obsolete hypothesis of mineral formation, and I shall not use them; I mention them only for the benefit of those readers who might come across them in further reading.
Some generalizations might be made about the differences between felsic and mafic minerals: as we progress from felsic to mafic the minerals are more dense (because the atoms in them which aren't oxygen or silicon are heavier elements such as magnesium or iron); they have higher melting points; and when they do melt they are less viscous (that is, they flow more easily).
Rocks: igneous, sedimentary and metamorphic
In this article we shall look at the most significant way in which geologists classify rocks. The reader should recall from the article on minerals and rocks that a rock is an aggregate of one or more minerals or mineraloids.
Igneous, sedimentary, and metamorphic
There are all sorts of ways that we might classify rocks. We might, for example, divide them up according to chemistry: and indeed the distinction between silicates and carbonates is a useful one. We might also classify rocks according to whether they contain felsic or mafic minerals, and as we shall see this is a good way to classify certain rocks. But the most fundamental way in which geologists classify rocks is to label them as igneous, sedimentary, or metamorphic.
Igneous rocks are rocks formed by the cooling and setting of molten rock.
Sedimentary rocks are formed by sediment (for example, sand or mud) turning into rock (such as sandstone or mudrock).
Metamorphic rocks are formed when rocks are subjected to heat, to pressure, to chemical reactions, or to any combination of these three, in such a way as to change the properties of the original rock in some way.
The rock cycle
The relation between the various kind of rocks can be summarized by a diagram of the rock cycle. Here is one picture of it which I find more or less satisfactory; I may at some point come back to this and prepare my own.
How do we know?
At this point we are touching on the main theme of this course. For to classify rocks as igneous, sedimentary, or metamorphic is implicitly to classify them not by their directly observable properties such as color or density or chemical composition --- but by their history as inferred from their present-day properties. The reader will, therefore, want to know: how do we know? This question will be answered in separate articles on igneous, sedimentary, and metamorphic rocks.
In the meantime, let us point out how intrinsically historical geology is. After we've got past the most basic of considerations such as defining a mineral and defining a rock, we are plunged into historical considerations. And this in not just because this course is about historical geology: it would be the same if it was an introduction to how to find oil. There is no geology that is not historical, and if we tried to do without historical inferences we might as well classify rocks by how pretty they are for all the good it would do us.