Historical Geology

Minerals and rocks

In this introductory article I shall explain what this textbook contains and why I wrote it the way I did.

What is historical geology?

Geology can roughly be divided into physical geology, which studies the materials of the Earth and the processes operating in it, and historical geology, which aims at a reconstruction of the history of the Earth.

Historical geology requires some knowledge of physical geology for its elucidation. (Imagine, by way of analogy, forensic scientists diagnosing cause of death as a gunshot wound, which is a historical question. It would obviously be necessary for them to know something about the behavior of guns, which would be a physical question.) However, the aim of historical geology is to understand the past, and knowledge of physical geology is merely an adjunct to this aim.

We may also speak of applied geology: for example, finding and extracting oil would fall under this category. This depends on both physical and historical geology: when petroleum geologists extract oil, it is certainly their understanding of the physical nature of rocks that allows them to extract it; but when they locate oil, it is their understanding of historical geology that makes them able to find oil with a success rate better than that which would be achieved just by guessing.

That is one application of historical geology. Here is another example: suppose someone wants to build a structure such as a major dam or a nuclear power plant at a certain site, and it is discovered that a geological fault runs under the site. It would then be crucial to discover when last there was movement along the fault: if it was a hundred or even a thousand years ago, then the proposed location is dangerous; if it was ten million years ago then it is probably safe.

However, historical geology is by no means confined to facts about the past that are presently useful: it is what is called a "pure" science, in which knowledge is sought for the sake of knowledge itself, whether it turns out to be useful or is merely interesting.

Pre-requisites for reading this book

This is an introductory work, and there are no pre-requisites for reading it except perhaps that the readers should be able to remember the chemistry they learned in high-school; for those who cannot, or who have not yet attended high school, an article on chemistry for geologists is provided in Appendix C.

In writing this textbook, I have not assumed that the reader knows any physical geology, and so have introduced concepts from physical geology where necessary. I have tried not to introduce more physical geology than is necessary, although occasionally I may have let my enthusiasm get the better of me.

The purpose of this book

This book has a single purpose: to explain how it is even possible to reconstruct the history of the Earth from data available in the present. The emphasis of this book is therefore methodological: rather than explaining what is known, we shall look at how it is known.

While these issues are sometimes mentioned in introductory geology textbooks, none that I know of addresses such questions systematically: one textbook will explain how geologists originally determined that granite is an igneous rock, but devote only a cursory page to radiometric methods; another, conversely, will provide us with lots of interesting information about the isochron method, but take the igneous nature of granite as read. I, on the other hand, have tried to be thorough in explaining how we know what is known.

There are a number of reasons why I have taken this approach.

Firstly, because other textbooks do not, and it seemed to me that this was a gap that I could usefully fill.

Secondly, because it seems to me that this is where the interest in geology really lies. Geology is not a set of dead facts: it is a detective story in which the application of reason takes a thousand diverse clues and unifies them into a single narrative that makes sense of it all. It is not, for example, particularly interesting that the core of the Earth is made mostly of iron — at least, it would be more interesting if it was made of cotton candy — but the fact that we can find out what it's made of should inspire awe in any reader not too jaded to feel that emotion.

Thirdly, because the lay reader may feel an understandable skepticism when hearing an expert expound on what happened 100 million years ago. Skepticism, when it is honest, is an admirable attitude, and deserves an honest reply.

Finally, the average reader with only a basic scientific education will probably have ended up with a better grasp of scientific facts than of the scientific method. This is probably an inevitable consequence of the demands of a broad science curriculum, and in making this observation I intend no criticism of science teachers; nonetheless, it appears to be the case. It is my hope that for readers in that situation, this textbook will serve as a worked example of the scientific method. Geology is ideally suited for this purpose, since it requires clarity of thought but not (at least at the introductory level) advanced mathematics.

The introductory nature of this book

This is an introductory work: it has been my aim to keep it short. When I inform the reader that the geologists Chilingar and Wolf managed to write a book of 808 pages on the single topic of the compaction of coarse-grained sediments, you will appreciate how much more I have left out than I have put in — and will probably be grateful.

Its brevity, however, means that it will probably leave some questions of some readers unanswered. It is my hope, however, that it will at least give readers the concepts and vocabulary to ask intelligent questions, and to understand the answers when they find them.

Selection of material

The material in this textbook tends to reflect my own interests, in that it dwells more on what can be learned from sedimentary rocks and fossils than from igneous and metamorphic rocks, and more on the history of the Earth from the origin of complex life onwards than on the history of the early Earth. This emphasis is a matter of personal preference: other authors have other preferences, and have written other books.

Except for a brief summary of the geological column in the article devoted to that concept, I have not attempted to give an account of what the history of the Earth actually is. Instead, as explained above, I have concentrated on methodological questions. It is not difficult, after all, for someone with internet access to discover what happened in (for example) the Ordovician period; but accounts of what happened will typically not explain how such things can be known: it is this gap that I have attempted to fill.

Readers who have read other textbooks on historical geology will notice one unusual omission. It is customary for such works to provide at least a brief account of the theory of evolution and the evidence for the fact of evolution. I have not done so. According to the plan of this book, with its emphasis on asking: "How do we know?" if I were to deal with evolution I would have to review evidence which is not only copious in quantity but also extraordinarily diverse in kind, more so than in any other branch of science. I estimate that to give even an outline sketch of the evidence would increase the length of this book by half again. A brief review would be too brief; an adequate review would be too long.

Even so, I would undertake this task if it was of critical importance, but it is not: all it would do for the reader is that, understanding evolution, the reader would understand why the principle of faunal succession ought to be true. But to practice geology, it is enough to know that it is true; and for the purposes of this book we need go no further than investigating the evidence that it is true, without an exceedingly lengthy digression explaining its underlying causes. In the end, in the hierarchy of ideas it is geology that supplies evidence for evolution, and not vice versa.

Arrangement of material

The material in this textbook is badly arranged. This is because the material in every geology textbook is necessarily badly arranged. The problem arises from the necessity of arranging it at all: of presenting it to the reader in a certain order. There is no right order because all the concepts in geology fit together to form a unified whole.

So with a few obvious exceptions it would be best if every article in this textbook, or any scientific textbook, came last in the book, so that the reader could read it in the light of all the other articles; but this is not possible. The reader does, however, have the option of reading through the whole book twice, and I recommend this course of action to any serious reader of this or any other similar textbook.

Vocabulary in this book

New terms will be marked by the use of boldface. This may not mark the absolute first use of a word, but the place in which it is first fully defined.

All words so introduced will also be defined in Appendix A.

Note on references

I have thought it unnecessary to provide references to facts which can be found in all or most introductory works on geology, and which may be considered common knowledge among geologists. I have, however, provided references to support specific pieces of data which are not so widely known, and for which a practicing geologist might wish to see a reference.

A bibliography is provided in Appendix B.

Note to potential editors

By contributing this book to the Wikibooks project, I am making it possible for others to edit it. It would be courteous to contact me before making any major changes or additions to this book. I would particularly ask that no-one should try to expand this book in a way that digresses from its stated purpose, since that would increase the length of the work without helping to achieve its aim.

Minor corrections of fact, grammar, spelling, etc, are of course welcome. Note however that where I have consistently used some stylistic feature this is deliberate. In particular I have purposely used only standard scientific units such as meters, grams, degrees Celsius, etc, without translating them into feet, ounces, degrees Fahrenheit, etc, as I feel that any reader who wishes to study geology should get used to the standard units, and the sooner the better.

Acknowledgements

I would like to thank the people, too numerous and usually too anonymous to mention, who have pointed out errors or infelicities in early versions of this text.

I should particularly like to thank Louis Kirstein for his patience and diligence in reviewing the entire work. This was undertaken in his personal capacity and so should not be construed as endorsement or approval by the distinguished body that employs him.

Any remaining errors or omissions are of course my own fault.

Tim Hardcastle

Las Vegas

April 2013

Minerals →

Historical Geology/Minerals and rocks

In this article we shall introduce some useful basic definitions: in particular, we will look at the definition of a mineral and see how it is different from the definition of a rock.

Minerals

A crystal is defined as a solid in which the constituent atoms are arranged in an orderly repeating pattern. The diagram to the right, for example, is a ball-and-stick model of the molecular structure of halite. The large red balls represent atoms of chlorine; the small green balls represent atoms of sodium; the sticks represent the bonds between atoms.

A mineral is a naturally occurring inorganic solid which is defined by a chemical formula and a particular crystal structure. For example ice is a mineral since it meets all the requirements of being a mineral, and coal is not a mineral since it is organic.

Take for example the mineral halite (more familiar to you as table salt). It has the formula NaCl, because it is formed from units consisting of one atom of sodium (Na) and one atom of chlorine (Cl), and it has a cubic structure as shown in the diagram above. The formula and structure define the mineral.

It is perfectly possible to have two different minerals with exactly the same formula but different crystal structures. One commonly used example of this are the minerals diamond and graphite. Both consist entirely of atoms of carbon, and so have exactly the same chemical formula, but graphite has a hexagonal crystal system and diamond has a tetrahedral crystal system; as a result their physical properties are very different. Two minerals having the same formula but different molecular structures are known as polymorphs.

Some minerals are what is known as solid solutions. Take for example the mineral olivine. This has the formula (Mg,Fe)₂Si O₄. The part of the formula which says (Mg,Fe) indicates that there are positions in the crystal structure each of which can be filled by an atom of either magnesium (Mg) or iron (Fe). What proportion of these positions is filled by iron and what proportion is magnesium will vary from sample to sample of olivine. It is convenient to treat these as varieties of the same mineral.

There are thousands of known minerals, most of which are of interest chiefly to collectors, and which we can largely ignore in an introductory text such as this. Of those that we shall mention, far and away the most important class are the silicate minerals, and the next article will be devoted to their physical and chemical properties.

Mineraloids

A mineraloid is a substance which is similar to a mineral in some respects but does not fulfill all the criteria necessary to be a mineral. There does not appear to be a complete consensus on what is a mineraloid and what is just a plain non-mineral, and the term is little-used.

One important sort of mineraloid is naturally occurring glass, such as obsidian or pumice. Glass by definition does not have a crystal structure, but is a confused mess at the molecular level. Such a structure, or rather lack of structure, is described as being amorphous.

Rocks

A rock is an aggregate of one or more minerals or mineraloids. For example granite consists mainly of the minerals quartz, mica and feldspar. In granite the crystals of the different minerals are actually visible to the naked eye, giving it its characteristic speckled appearance, as in the picture to the right.

A rock can consist of a single mineral: for example the semi-precious stone known to dealers in gemstones as "rock crystal" consists entirely of quartz.

← Introduction · Silicate minerals →

{{:Historical Geology/Silicate minerals 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 that a rock is an aggregate of one or more minerals or mineraloids.

Igneous, sedimentary, and metamorphic rocks

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 three types of rock can be converted into each other by geological processes. Metamorphism can turn igneous and sedimentary rocks metamorphic; erosion can turn igneous and metamorphic rocks into sediment which can then become sedimentary rock; and sedimentary and metamorphic rocks can be melted down into molten rock which can then cool to form igneous rock.

The relation between the various kind of rocks can be summarized by a diagram of the rock cycle. One representation of it is shown to the right.

How do we know?

At this point we are beginning to touch on the main theme of this textbook. 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 this history is inferred. This question will be answered in separate articles on igneous, sedimentary, and metamorphic rocks.

In the meantime, let us observe 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 is not just because this course is about historical geology: it would be the same if it was an introduction to how to find oil. Geology is so intrinsically a historical science that 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.

← Silicate minerals · Igneous rocks →

In this article we shall look at igneous rocks: what they are and how they are classified.

Intrusive and extrusive igneous rocks

Igneous rocks are rocks formed by the cooling and solidification of molten rock. They fall into two main categories:

Intrusive rocks are those which are caused by the cooling of molten rock underground. Subterranean molten rock is known as magma.

Extrusive rocks are those formed from molten rock on the surface, which is known as lava.

Igneous rocks can be further identified and classified by their texture and their chemistry, as will be described in the following two sections of this article.

Texture

It is a universal law that fast crystallization makes small crystals and slow crystallization makes large crystals. This is because crystallization is a kinetic process: for a molecule to join onto a crystal it must bump into it and then align with it.

The thermal properties of rock are such that magma cooling underground will cool slowly as compared to lava cooling above ground. Hence, by looking at the texture of the rock, we can find out how it cooled: an intrusive rock will be coarse-grained; an extrusive rock will be fine-grained.

Sometimes lava is ejected from a volcano with such force that it goes shooting high up into the air, causing it to solidify so quickly that it doesn't have time for crystals to form at all, making an amorphous solid known as a glass. The glass in windows is an artificial glass produced by the rapid cooling of molten silica; examples of natural glasses are obsidian and pumice.

Occasionally magma will begin to cool below the surface and then be ejected on to the surface; in this case it will have a porphyritic texture, with a few larger crystals (phenocrysts) embedded in a finer-grained ground mass.

Chemistry

The simplest way to classify the chemistry of igneous rocks is by the amount of silica they contain.

An igneous rock with a high silica content is said to be felsic, and an igneous rock which is low in silica is said to be mafic. You will recall that these are the same terms used for high-silica and low-silica minerals; and in fact it is the case that felsic rocks will contain felsic minerals and mafic rocks will contain mafic minerals.

Classifying rocks by their silica content is convenient because typically the chemistry of igneous rocks lies on a continuum such that if you know the proportion of silica in an igneous rock, you can say what minerals it contains. The rules for doing so can be represented by the diagram to the right. Note that this applies only to igneous rocks, and not to sedimentary or metamorphic rocks.

To read the diagram, look along the bottom of the graph for the silica content of the rock: then a line drawn directly upwards from that point cuts through the minerals it will contain in their relative proportions. So, for example, if we tell you that a certain rock contains 50% silica, then you can see from the chart that it contains about 5% olivine, 75% pyroxene, and the remaining 20% will be calcium-rich plagioclase feldspar.

This diagram divides the rock types into fairly coarse divisions. It is possible to make finer distinctions: we could, for example, have put granodiorite between granite and diorite, as a rock type having a silica content lying between granite and diorite; or we could have placed dunnite to the right of peridotite, to denote those rocks which consist of pure olivine. The divisions we have proposed are, however, sufficient for our present purposes. It is more important that the reader realizes that whatever divisions we impose on the diagram, they are arbitrary: there is a continuum between felsic and ultramafic rocks.

Also, as we look along the continuum from felsic to ultramafic, the rocks are progressively denser; they have a higher melting point; and they have a less viscous flow when molten. This is the same progression as we see as we pass from felsic to ultramafic minerals, and is a natural consequence of the fact that felsic rocks consist of felsic minerals and mafic rocks of mafic minerals.

We should perhaps add a note on the presence of komatiite (extrusive ultramafic rock) in our diagram, as some textbooks omit it entirely from such diagrams. Komatiite is never observed forming today: as ultramafic magma rises from the hot interior of the Earth to its cool surface, it will fall below its melting point before it gets near to the surface, forming peridotite, komatiite's intrusive counterpart. Consequently komatiite is found only in rocks dated to over 2.5 billion years ago, consistent with geologists' belief that the Earth was hotter at that time.

Igneous structures

The diagram to the right shows some of the structures formed by igneous rocks. The black represents igneous rock; the other colors represent sedimentary rocks.

As this is a cutaway diagram, it may be slightly misleading. The reader should bear in mind that a fissure is a crack in the surface; we have shown it end-on. Similarly, the lava flow which emerges from a fissure will be a sheet of lava; and a dike is not a spike of rock, but a vertical or near-vertical sheet of rock. And a sill, again, is a horizontal sheet of rock.

That last statement needs a little qualification. In the diagram, we have shown the layers of rock lying flat, except around the lacolith (item (6) on the diagram) and so we have shown the sills as horizontal structures. However, layers of rock can be folded by tectonic activity. When a sill intrudes into rocks like this, it intrudes between the layers of rock (this is the definition of a sill) and so will itself be contorted.

We shall have more to say about igneous structures when we consider stratigraphy and cross-cutting relationships, but for now this brief introduction is sufficient.

How do we know?

How do we know that igneous rocks are igneous? Like everything else in geology, this had to be proved at some point: indeed, there was once a body of thought known as "Neptunism" which asserted (amongst other things) that granite was sedimentary.

In the case of extrusive rocks, the answer is obvious: we can see basalt (for example) forming when lava flows cool: so it certainly can form as an extrusive rock. But it could not also form as an intrusive igneous rock, because under such circumstances, being thermally insulated, it could not cool quickly enough to produce a fine-grained structure, and the physics of the situation would dictate the formation of gabbro instead.

Since we can actually watch the formation of basalt, we can make further deductions about it. When basalt cools underwater (as observed by divers), it forms the distinctive shapes known as pillow basalt, which is not the case when it is observed forming on dry land. This criterion allows us to distinguish between basalt formed on land and on the sea floor; a deduction confirmed by the association of pillow basalt with marine sedimentary rocks.

But what about intrusive rocks? Take granite, for example, since it is the commonest intrusive igneous rock. If we are absolutely right about how it forms, we should never see it forming. So how do we know how it forms?

As a matter of fact, the fact that we never see it forming is one of the predictions of the theory that it is an intrusive igneous rock, and so tends to confirm the theory. We do not see granite or granite-like sediment forming by surface processes; what else can we conclude but that it is formed underground?

In the second place, as we have observed, granite has the same chemical composition as rhyolite, differing from it only in its texture. Now, as we know that larger crystals form when cooling is slower, and as the thermal properties of rock as opposed to air or water will lead to slower cooling underground, we must conclude that granite is exactly what we should expect to see if the magma that forms rhyolite when extruded onto the surface was to cool below the surface instead.

A close look at its texture through a microscope confirms the igneous nature of its formation. The picture to the right is a photomicrograph of granite. Note how the crystals, however bizarre their shape, fit together perfectly. We may compare this with the texture of sedimentary rocks such as sandstone, which are clearly made of non-interlocking particles cemented together.

Then we may consider the structures formed by intrusive rocks. It is difficult to see how something such as a dike, which, as explained above, is a vertical or near-vertical sheet of rock, could form by any process except the intrusion of magma into a crack in pre-existing rocks.

Finally, we may note that the rocks into which granite intrudes are typically changed in ways we would expect if they had been subjected to great heat; for example, when granite intrudes through a layer of limestone, the limestone immediately adjacent to the granite will be turned to marble. This suggests that the granite was itself once at a high temperature and has subsequently cooled, consistent with the theory that it is an intrusive igneous rock.

For these reasons, we may conclude that granite is an intrusive igneous rock; similar remarks might be made about the other rocks classified as igneous intrusive.

Note on vocabulary

Igneous rocks are sometimes called primary rocks, extrusive rocks are sometimes called volcanic rocks, and intrusive rocks are sometimes called plutonic rocks. We shall not use these terms in this text, and mention this only for the benefit of those readers who wish to pursue a course of further reading.

The rocks that we have described as fine-grained and coarse-grained are also known by the terms aphanitic and phaneritic respectively. These terms are rather commonly used by geologists, but I shall stick to the more self-explanatory terms.

Finally, just as silicate minerals are sometimes referred to (erroneously) as "acidic", "basic" and "ultrabasic" rather than felsic, mafic, and ultramafic, the same is true of igneous rocks; as in the case of minerals, I do not intend to use these terms, as they are obsolete and misleading.

← Rocks · Sedimentary rocks →

This article is a brief introduction to the various kinds of sedimentary rocks. Further information about the sources of sediment, its transport, and its deposition, will be covered in further articles; indeed, in much of the rest of this textbook.

Types of sedimentary rocks

Sedimentary rocks can be divided into three main classes:

Clastic sedimentary rocks are formed from sediments created by rocks being broken down into small particles (clasts).

Chemical sedimentary rocks are formed from sediments created by dissolved chemicals being precipitated out of the water they're dissolved in.

Biochemical sedimentary rocks are formed from sediments consisting of dead organisms, or parts of dead organisms.

In some schemes of classification, biochemical sediments are treated as a sub-class of chemical sediments, but this leaves one with the awkward question of what to call chemical sediments which aren't biochemical. For this reason I shall treat them as two non-overlapping classes.

Before we review the main types of sedimentary rocks, it is worth mentioning the process by which they turn into rock: this is known as lithification. In some cases, such as shale, mere compaction, along with the resulting loss of water, is sufficient. Coarser sediments, such as sandstone, are both compacted and cemented, as can be seen under a microscope. The cements are minerals precipitated out of the water in which they are dissolved: silica and calcium carbonate are the commonest forms of cement, with iron oxides and hydroxides coming a distant third.

In the sections below we shall list the main types of sedimentary rock.

Clastic sedimentary rocks

As defined above, clastic sedimentary rocks are formed from broken pieces ("clasts") of pre-existing rocks.

Gravel is defined as clasts with diameter 2mm or more. We should note that when geologists speak of rounded clasts, they do not necessarily mean that they are round like a ball, but merely that the sharp corners and edges have been worn off them by erosion. Conglomerates are rocks formed mainly from rounded gravel which has been compacted and cemented together.

Sediment: rounded gravel. Rock: conglomerate.

Breccia is like conglomerate except that the gravel is angular: that is, it has not been rounded. This reflects a different history, since gravel that has been transported any appreciable distance by water, or which has been rolled about by waves on a beach, will quickly have its corners and edges worn away.

Sediment: angular gravel. Rock: breccia.

Sand is defined as clasts less than 2mm and more than 1/16mm in diameter. Sandstone is sand that has been cemented together.

Sediment: sand. Rock: sandstone

Most sandstone is quartz sandstone; that is, it consists of grains of quartz. This is because the process known as chemical weathering dissolves many rock-forming minerals, or, in the case of feldspar minerals, converts them to clay, leaving behind only the quartz from the original rock. We shall look at the process of chemical weathering in a subsequent article.

Arkose sandstone is sandstone with an appreciable proportion of feldspar minerals in it. This reflects a somewhat different history to quartz sandstone, in that it must have been formed when mechanical weathering (the physical process of breaking rock into clasts) has predominated over the chemical weathering that would otherwise have converted the feldspar minerals to clay minerals.

Greywacke is sandstone that, in addition to quartz and feldspar, also contains sand-sized fragments of igneous or metamorphic rocks. Similar remarks apply to greywacke as to arkose sandstone.

Silt is defined as clasts between 1/16mm and 1/256mm in diameter.

The term "clay" is a little ambiguous. In the classification of sediments, it is defined as particles less than 1/256mm in diameter. However, in mineralogy, clay is a class of minerals (technically, hydrous aluminosilicates). In practice, this need cause no confusion, because what is clay by size will be overwhelmingly clay by composition.

Mudrocks can then be divided into siltstone (formed from silt sediments); mudstone (from sediments that are a mixture of silt and clay); and claystone (from clay sediments).

Sediment: silt and/or clay. Rock: mudrock.

Most mudstone and claystone is bedded. When it is, it is referred to as shale.

Sediment: bedded mud or clay. Rock: shale.

Chemical sedimentary rocks

Halite, also known as rock salt, is an evaporite, formed by the evaporation of salt water.

It can be formed by complete evaporation of salt water, as seen, for example, in desert salt flats. However, complete evaporation is not necessary; it is sufficient that enough water should evaporate that the remaining water can't hold all of the salt in solution; so halite can also form in shallow seas or salt lakes in a hot environment.

Sediment: salt. Rock: halite.

Calcium sulfate is another substance to be found dissolved in salt water, and gypsum, like halite, usually forms as an evaporite under pretty much the same circumstances.

Sediment: hydrated calcium sulfate. Rock: gypsum.

Dissolved silica can precipitate out of the water in which it's dissolved to form chert. Note, however, that chert is more usually formed as a biochemical sedimentary rock.

Sediment: silica. Rock: chert.

Calcium carbonate, like silica, can precipitate out of water to form limestone. Sometimes it forms tiny spheres called ooids, which form around grains of sand or fragments of shell, which are then cemented together by the further precipitation of calcium carbonate; such limestone is known as oolitic limestone.

Sediment: calcium carbonate. Rock: limestone.

Most limestone, however, is biochemical sedimentary rock, formed from shells or coral.

Biochemical sedimentary rocks

Most limestone is formed from tiny hard parts of marine creatures which build their shells out of calcium carbonate; these settle on the sea floor to form calcareous ooze. Chalk is an example of such a rock: the tiny fossils that compose it can be clearly seen and identified under a microscope. The hard parts of coral reefs are sometimes preserved intact, giving us reef limestone.

Sediment: shells of calcium carbonate. Rock: limestone.

While calcium carbonate is the most popular substance to make shells out of, some organisms such as diatoms and radiolarians build their shells out of silica; these settle on the sea floor to form siliceous ooze which, when compacted and cemented, forms chert.

Sediment: shells of silica. Rock: chert.

Peat is plant material laid down in oxygen-poor conditions, so that it doesn't entirely decompose. Pressure, and the higher temperatures which come with deep burial, can then convert it into coal.

Sediment: peat. Rock: coal.

Modes of deposition

In the sections above we have principally divided sedimentary rocks by their composition. We can also classify them by their modes of deposition: for example, aeolian (deposited by the wind), or fluvial (deposited by rivers) and so forth. We shall have a lot more to say about this in subsequent articles.

Bedding

Sedimentary rock often exhibits bedding: that is, the rock has distinct layers in it and is fissile: that is, it splits more easily at the divisions (bedding planes) between the layers. In cross-bedded rocks, the layers are not flat but lie at an angle to the horizontal, as a result of the original sediment being formed into dunes or ripples by the action of wind or water.

The picture to the right shows a particularly large-scale example of cross-bedding in sandstone.

How do we know?

How do we recognize sedimentary rocks as sedimentary? How do we recognize the sediments that compose them and the manner of their deposition?

Such questions will be answered later in this course one type of sediment at a time. At present we shall content ourselves with sketching out a general answer.

In the first place, the rocks look just like we would expect if sediments became lithified; for example sandstone looks like it's made of sand: everything about the size, the composition, and the erosion of the grains of which it's composed is in agreement with the idea that what we're looking at is grains of sand cemented together.

Secondly, we can drill down and take cores of sediments, and we can see, as depth increases, how (for example) sloppy muddy ooze on the surface grades into hard mudstone with no sharp dividing line between them; similarly we can see calcareous ooze grade into solid limestone.

Then again, all types of sedimentary rocks can contain fossils (including, as we have remarked, those rocks which consist of fossils, such as chalk). This is consistent with the processes of burial of organic remains in sediment which we can see going on today.

Trace fossils are also a strong argument: when we find, in shale, the recognizable fossil footprints of land animals, it is hard not to conclude that what we are looking at once lay on the surface and was soft enough to take impressions such as we can see being made in mud today.

These considerations also allow us to figure out where the sediments were deposited: on land, in fresh-water, or in the sea.

The sedimentary structures within the rocks, such as bedding and cross-bedding, can be seen today in oozes forming on the sea floor; in sand-dunes; in ripples caused by tidal action, and so forth. Again, consideration of these structures will allow us to make the deduction, not merely that the rock is sedimentary, but also about the method of its deposition; if, for example, we find sedimentary structures such as can only be formed by tidal action, we are forced to infer that we are looking at lithified nearshore sediments.

Also, we may observe the topographic patterns of deposition. For example, when we see sedimentary rocks which because of their structures and fossils we identify as terrestrial (i.e. associated with the land) divided from sedimentary rocks which because of their structures and fossils we associate with the sea by a long thin strip of sedimentary rocks which because of their structures and fossils we associate with the nearshore, then this observation confirms our identification of the rocks as being terrestrial, marine, and nearshore rocks. If, on the other hand, we found alternating bands of marine and nearshore rocks, this would tend to falsify our theories. The fact that the topography of sediments is always consistent with our theories is therefore a point in favor of their correctness.

So the conclusion that sedimentary rocks are, indeed, sedimentary in origin, is a safe one; and we are certainly not without clues as to the manner of their deposition.

Note on vocabulary

Conglomerates and breccias are sometimes called rudaceous rocks; sandstones are sometimes called arenaceous rocks or arenites; and mudrocks are sometimes called argillaceous rocks.

The rocks which we have called clastic are sometimes called detrital.

As usual, I shall employ a consistent vocabulary in this text; these terms have only been supplied for the benefit of the reader who wishes to pursue a course of further reading.

← Igneous rocks · Metamorphic rocks →

Metamorphic rocks are those in which a pre-existing rock (the parent rock) has been altered chemically or texturally by heat and/or pressure. In this article we shall look at metamorphic processes and their effects on the resulting rocks.

Types of metamorphism

There are two main types of metamorphism; contact metamorphism and regional metamorphism.

In contact metamorphism, the heat of magma intruding through rocks causes metamorphism to the surrounding rocks, leaving an aureole of metamorphic rocks around the igneous rocks formed from the magma.

Regional metamorphism is caused by tectonic events involving both heat and pressure, and will affect large, elongated regions of rock.

Besides these broad classifications, we can also class metamorphic rocks according to their grade of metamorphism; the temperature to which they have been exposed. As we shall see, this determines the chemical changes that metamorphic rocks undergo. Grades of metamorphism range from very low (below 300°C) to low (300°C - 500°C) to medium (500°C - 600°C) to high (600°C and upwards).

Chemical changes

Metamorphism often causes chemical changes to the minerals of which the affected rocks are composed. For example, at low temperatures clay minerals will be converted into chlorite; at higher temperatures the chlorite will itself be transformed into other minerals. Hence, if we find a rock with chlorite in it, we know that it has undergone low-grade metamorphosis. Minerals such as this, which reveal the grade of metamorphosis, are known as index minerals.

Geologists can correlate index minerals with grades of metamorphism because it is simple enough to repeat the processes of metamorphism in the laboratory; that is, they can take a piece of non-metamorphic rock, subject it to various regimes of temperature and pressure, and see which characteristic minerals form at which temperatures. So in shale, for example, we see a sequence (as temperature increases) from unaltered shale to rocks containing chlorite; then biotite; then garnet; then staurolite; then kyanite; and finally silmanite. The image to the right shows some particularly large garnets embedded in metamorphic rock.

We see this same sequence arranged spatially as we approach the center of an area of metamorphism: from unaltered shale through the "chlorite zone", the "biotite zone", the "garnet zone" the "staurolite zone", the "kyanite zone", and the "silmanite zone". We may not get all the way up to silmanite, that depends how intense the metamorphism was at the center of metamorphism.

The sequence, of course, depends on the parent rock; the sequence given above is specific to shale, and we would see a different sequence if we were looking at (for example) mafic igneous rocks.

Metasomatism

In the section above on chemical changes we dealt with the case where the parent rock reacts with itself as a result of heat or pressure acting on the rock. But in the case of contact metamorphism, we can also see metasomatism taking place: the parent rock mixes and/or reacts with the intrusive igneous rock and the hot fluids associated with its eruption.

The picture to the right shows a particularly attractive example of skarn, a very distinctive product of metasomatism.

Textural changes

Besides chemical changes, rocks that undergo metamorphosis suffer textural changes, such a recrystallization, foliation, and lineation.

In recrystallization, the original texture of the rock is lost as the minerals, under the effect of high temperatures, reform as a collection of interlocking crystals of similar size. The effect of this is seen most dramatically in sedimentary rocks.

So, for example, quartz sandstone loses its sedimentary structure of cemented grains to become quartzite, with a smooth texture consisting of interlocking crystals. As a result, except at very low grades of metamorphism, any bedding of the rock will be destroyed, as will any fossils that the rock contains. Similar textural changes produce marble from limestone, and hornfels from mudrock.

When rocks are metamorphosed by pressure as well as heat, they undergo foliation, in which sheet silicates, if they are present in the rock, rearrange themselves so that the sheets are at right angles to the direction of pressure. The picture to the right shows a view of foliation under a microscope.

Lineation is a similar phenomenon affecting silicates with the structure of a chain or double chain; the direction of the chain ends up, again, at right angles to the direction of pressure.

Not every metamorphic rock will display foliation or lineation: some rocks simply don't contain any sheet or chain silicates: an example would be limestone, which metamorphoses to marble. Also some metamorphic rocks are formed by heat without any significant pressure, as is usually the case with contact metamorphism; so, for example, mudrocks, which will form foliated slate or schist under pressure, will produce non-foliated hornfels without pressure.

Foliation comes in several varieties:

Slatey foliation is caused by the alignment of sheet silicates such as clay minerals and chlorite (which is produced by chemical changes to clay minerals). It results in rocks which cleave easily into thin layers.

Schistosity is caused by sheet silicates such as biotite and muscovite. Not only do they align, but they tend to separate out from the non-sheet silicates such as quartz, producing a rock that breaks easily into thicker leaves than those found in slatey rocks.

Finally, we come to gneiss. At high grades of metamorphism, sheet silicates tend to break down, and dark-colored chain silicates such as hornblende and pyroxene begin to appear. These are separated out into dark bands, again at right-angles to the direction of pressure, giving gneiss a distinctive streaky appearance, as shown in the photograph.

How do we know?

How can we recognize rocks as metamorphic?

First of all, as we have observed, we can reproduce metamorphic processes in the laboratory. Marble, for example, is what we get if we heat limestone; quartzite is what we get if we heat quartz sandstone; schist is what we get if we heat mudrock and apply pressure. It would seem downright perverse to maintain that metamorphic rocks should have been produced by other processes not as yet discovered. (Note that the textures of metamorphic rocks exclude the possibility that they are sedimentary rocks, and their chemical composition usually excludes the possibility that they are igneous rocks.)

We can also look at the patterns we find in the rocks. I shall give some examples of the kind of predictions we can make from the theory of metamorphism; the reader will doubtless be able to think of other examples.

To take a simple example: when we look at an aureole of marble, we should expect to find it embedded in an outer ring of limestone, and not of (for example) sandstone, which would go with an aureole of quartzite.

Then again, according to our notion that metamorphic rocks are indeed produced by metamorphosis, we should not (and we do not) find, looking horizontally at sequences of rocks, alternating bands of unaltered rocks and high grade metamorphic rocks. Instead, as we have noted above, we find concentric zones of rocks with high-grade metamorphic rocks at the center of metamorphism, progressing to lower and lower grades of metamorphism until we reach unaltered rocks.

If we find a foliated rock like schist, then according to our interpretation of schist as produced by temperature and pressure, we should find other evidence of the pressure; we should expect to find the beds of rock buckled and deformed. And this is indeed what we see.

If, on the other hand, we find hornfels, which, laboratory experiments show, requires temperature without significant pressure (otherwise it would be foliated) then we expect to find (and do) that it forms an aureole around igneous rock, with progressively lower grade metamorphism in concentric zones around the igneous rock.

Other patterns are discernible: for example, we would not expect to find schist overlying limestone, because the events that created the schist would also have turned the limestone into marble.

In summary, the chemical composition and texture of the rocks that we have classed as metamorphic, together with their arrangement and relation to other rocks in the geological record, is just what we should expect to see if they are indeed produced by metamorphosis.

← Sedimentary rocks · Mechanical weathering and erosion →

Erosion and deposition

2.1 Mechanical weathering and erosion 2.2 Chemical weathering 2.3 Glaciers 2.4 Deserts 2.5 Volcanic ash 2.6 Soils and paleosols 2.7 Rivers 2.8 Deltas 2.9 Lakes 2.10 Peat and coal 2.11 Nearshore sediments 2.12 Marine sediments 2.13 Turbidites 2.14 Reefs 2.15 Ooids and oolite 2.16 Calcareous ooze 2.17 Siliceous ooze 2.18 Pelagic clay 2.19 Deposition rates 2.20 Glacial marine sediment 2.21 Saline giants 2.22 Banded iron formations

3. Plate tectonics 3.1 Physical properties of rocks 3.2 Seismic waves 3.3 The structure of the Earth 3.4 Geomagnetic reversals 3.5 Plate tectonics: overview 3.6 Continental drift 3.7 Sea floor spreading 3.8 Subduction 3.9 Hotspots 3.10 Terranes 3.11 Ophiolites 3.12 Orogeny

4. Stratigraphy 4.1 Actualism 4.2 Steno's principles 4.3 Way-up structures 4.4 Fossils 4.5 The principle of faunal succession 4.6 Index fossils 4.7 The geological column 4.8 Unconformities 4.9 Faults 4.10 Folds 4.11 Walther's principle 4.12 Cross-cutting relationships 4.13 Igneous rocks and stratigraphy

5. Absolute dating 5.1 Concepts in absolute dating 5.2 Erosion, deposition, and time 5.3 Dendrochronology 5.4 Varves 5.5 Amino acid dating 5.6 Radioactive decay 5.7 K-Ar dating 5.8 Ar-Ar dating 5.9 Rb-Sr dating 5.10 Other isochron methods 5.11 U-Pb, Pb-Pb, and fission track dating 5.12 Radiocarbon dating 5.13 Cosmogenic surface dating 5.14 U-Th, U-Pa, and Ra-Pb dating 5.15 Paleomagnetic dating 5.16 Sclerochronology 5.17 Tidal rhythmites and dating 5.18 Fossils and absolute dating 5.19 Absolute dating: an overview

6. Paleoclimatology 6.1 Paleoclimatology: introduction 6.2 Sediments and climate 6.3 Paleocurrents 6.4 Biogeography and climate 6.5 Leaf shape and temperature 6.6 Dendroclimatology 6.7 Scleroclimatology 6.8 Uk'37 6.9 TEX86 6.10 Ice cores 6.11 Milankovitch cycles 6.12 Climate models 6.13 Ice ages 6.14 Sea level variations

Appendices A Glossary and index B Bibliography C Chemistry for geologists