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Author Topic:   Introduction To Geology
Dr Adequate
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Message 271 of 292 (694964)
03-31-2013 3:32 PM


Banded Iron Formations
This should really have gone back in the section on sediment, but as it also relates to climate I guess it can go here.

Banded iron formations

Introduction

In this article we shall look at what banded iron formations are and how they might have formed. The reader may find it useful to look back at the main article on siliceous ooze and chert.

BIFs defined

Banded iron formations, or BIFs are sedimentary rocks consisting of alternating bands iron-rich sediment (typically hematite (Fe2O3, and magnetite, Fe3O4) and iron-poor sediment, typically chert; the size of the bands ranges from less than a millimeter to more than a meter in thickness. The image below shows a fairly typical banded iron formation: the red bands are the iron oxides.

While BIFs have a wide geographical distribution, they are localized in time. They start to become common about 3.5 billion years ago), peak around 2.5 billion years ago, vanish about 1.8 billion years ago, make a small comeback around 1 billion years ago, and then essentially vanish from the geological record; none are being produced today.

BIFs and the rise of oxygen

With the exception of saline giants, it is usually very easy to explain the origin of sedimentary rocks, because we can see identical sediments being deposited in the present: chalk looks just like lithified calcareous ooze, tillite looks just like lithified glacial till, aeolian sandstone looks just like lithified desert sand, and so on.

In the case of BIFs, however, no BIFs are being formed in the present, nor even recently. It seems, then, as though in searching for a cause for BIFs we must be looking for an event which could only have happened at in the past.

Fortunately, one comes to mind. According to biologists, the first living organisms neither produced nor consumed oxygen. Indeed, they would not have been able even to tolerate oxygen: oxygen is a very reactive gas, and is toxic to organisms which are not adapted to its presence (for example the modern bacterium Clostridium botulinum, which can only survive in the near-total absence of oxygen).

Biologists are also agreed that in the absence of oxygen-producing organisms, the atmosphere would have been very poor in free oxygen (i.e. the molecule O2).

What does all this have to do with BIFs? Well, one of the interesting things about iron is that elemental iron (Fe) dissolves in water, whereas the various oxides of iron (as found in banded iron formations) precipitate out. The waters of the early Earth would certainly have had sources of iron, such as emissions from submarine volcanoes, and iron liberated from rocks by chemical weathering. It follows that when organisms arose that produced oxygen, iron dissolved in the oceans would combined with dissolved oxygen to form iron oxides which would then have precipitated out, producing the iron oxides that characterize BIFs.

The iron would, indeed, form an "oxygen sink"; only after the iron had been used up in this way would O2 have begun to constitute a large proportion of the atmosphere. The accumulation of oxygen in the atmosphere, which according to geological dating methods started about 2.4 billion years ago, is variously known as the Great Oxygenation Event (GOE), the oxygen catastrophe, and the oxygen crisis.

How do we know?

The scenario given above is plausible according to biologists; indeed, if they're right about the history of early life, we should expect to see this kind of geological evidence of a rise in oxygen as a result of the rise of oxygen-producing organisms. And biology aside, it is certainly chemically very plausible: iron is soluble in water and iron oxides are not, and this at least is something we can check by direct observation. It is also plausible in that it explains the localization in time of BIFs in terms of something that we would expect to happen only once.

But was there in fact a change from an oxygen-poor to an oxygen-rich atmosphere? Studies of minerals before, during, and after the GOE answer this question in the affirmative.

The fact that the production of free oxygen is indeed a plausible explanation for BIFs is a point in favor of this scenario; what is more, this explanation is borne out by the nature of the iron oxides in BIFs: they tend to be iron oxides with a low oxygen to iron ratio such as hematite (Fe2O3, and magnetite, Fe3O4) rather than, for example, goethite (FeO(OH)); which is what we would expect if they formed in conditions in which oxygen was still scarce.

However, we would be verging on circular reasoning if we explained BIFs by the advent of free oxygen, and if our evidence for this event consisted solely of BIFs. Fortunately, this is far from being the case: there are other indications of the GOE in the mineralogy of the early Earth.

For example, before, but not after, the date assigned to the GOE, the minerals uraninite (UO2) and siderite (FeCO3) can be found in river sediments (for information on how to identify such sediments, see the main article on rivers). The significance of this is that these minerals would not survive in waters containing dissolved O2, as all rivers do today; so the rivers that deposited them must have co-existed with an oxygen-poor atmosphere.

After the date assigned to the GOE, on the other hand, we see a great diversification of mineral types in the geological record, as after the GOE new minerals could then be produced from old ones by oxidization; and it is just such minerals that we find after the GOE. (For more details, see Sverjensky and Lee (2010) The Great Oxidation Event and Mineral Diversification, Elements, 6(1).)

So we have abundant data pointing to the rise of free oxygen in the atmosphere; and such a rise would explain, indeed necessitate, the extensive formation of iron oxide deposits such as are found in BIFs.

BIFs: some questions

It seems, then, as though we have a good explanation for BIFs. However, the reader should bear in mind that what I have sketched here is only a broad outline of a broad consensus. There is still controversy over details.

The reader may well already be puzzling over a couple of these details. Firstly, why is the sediment in the dark bands of BIFs so frequently chert? And why are BIFs banded at all --- why were the chert and iron oxides not deposited simultaneously as a mixture?

The chert may have been deposited by silica-forming organisms. It is true that we don't find in it the tests of diatoms and radiolarians, so it probably didn't have its origin as the sort of siliceous ooze produced by these organisms today; but one cannot rule out the possibility of other silica-producing organisms active in the pre-Cambrian and now extinct.

Alternatively, if there were no silica-producing organisms in the pre-Cambrian, then silica could just have built up in the seas until it reached saturation point and precipitated out by itself.

The rhythmic nature of deposition seems to suggest a cyclic variation in conditions. One candidate cause is a repeated cycle of ecological boom and bust. On biological grounds, it would be reasonable to suspect that the earliest oxygen-producing organisms could not tolerate high levels of oxygen. (This is not as paradoxical as it sounds: most organisms can't live in an environment full of their own waste products. Animals, for example, produce CO2 and would suffocate in an atmosphere dominated by it.)

This suggests the following scenario: oxygen producing organisms would grow and flourish until they had produced toxic levels of oxygen; the population would then collapse almost to nothing, surviving only in low-oxygen refuges; the oxygen would be removed from the atmosphere by combining with the iron in the water to produce the iron oxide bands of BIFs; in these low-oxygen conditions the oxygen-producing organisms could once more increase in number, and the cycle would begin again. This would plausibly account for the periodic precipitation of iron oxides. It is even conceivable that the oxygen-producing organisms were identical with the (hypothetical) silica-secreting organisms mentioned above.

But here we have gone into the realms of speculation and controversy. It is quite possible that these questions will remain controversial: since geologists labor under the handicap of not being able to watch BIFs form in the present, BIFs will never be understood with quite the same certainty as other sedimentary rocks.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


  
Dr Adequate
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(1)
Message 272 of 292 (695087)
04-02-2013 4:04 PM


As The Rabbit Said ...
And that's about it, really.

I need to go back and put some stuff in. For example, I need to write articles about faults and folds. While I was writing this, I thought during the section on plate tectonics that I should wait 'til the section on stratigraphy, and when I was doing stratigraphy I thought it should be in the section on plate tectonics, so I never really got around to it. So I'll go back and do that.

But anything from now on is out of order, and should have been posted earlier.

So, I'm pretty much finished. Can anyone point out to me something that I should have explained and haven't? Thanks.


  
Dr Adequate
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Message 273 of 292 (695308)
04-04-2013 1:58 PM


Folds
Ok, there now follows stuff that I should really have done early.

With faults and folds, when I was doing tectonics I thought I should do them in stratigraphy, and when I got to stratigraphy, I thought they belonged in plate tectonics. Having thought about it, they definitely fall under stratigraphy. So here's something about folds.

---

Folds

Introduction

In this article we shall look at folds and how to recognize them in the geological record. The reader will find it useful to have read the articles on orogeny, Steno's principles, and way-up structures before continuing.

Origin and appearance of folds

The appearance of folds shows that they are produced by the landscape being pushed sideways, as described in the article on orogeny, rather than being pushed up or pulled down from below.

This explains the normal appearance of folds (prior to erosion) as a set of parallel ridges where the landscape has been folded up (anticlines) alternating with troughs where it has been folded down (synclines). The photograph below shows an anticline on the left and a syncline on the right, seen from a side-on perspective.

Sufficient lateral motion will push the folds themselves over sideways, resulting in a recumbent fold, as shown in the article on orogeny.

Folds: how do we identify them?

When, as in the photograph above, a fold is intact and we can see it from the side, it is obviously a fold.

But now consider a sequence of events such as that shown in the block diagrams below: sediments are laid down in flat layers according to the the principle of original horizontality and the principle of original lateral continuity, lithified, folded, and then eroded.

As a result of the erosion, the shape of the fold will be destroyed. Even then, if we could see the rocks from the side it would not be hard to deduce that this was an eroded fold. But geologists are not usually so lucky, and instead have to view the landscape from the top.

Nonetheless, it would not be hard to deduce what happened. Looking at the rocks, a geologist could see that the formation labeled A is the same kind of rock as a, B is the same as b, and so on. Applying the principles of original horizontality and lateral continuity, the only possible explanation would be that the rocks were folded and then eroded. The symmetric pattern of bands of different rock types across the landscape are by themselves an excellent indication of an eroded fold.

Even if the types of sediment didn't vary during deposition, there would be plenty of other evidence. Looking at the individual beds in the rocks, we would find them sloping up to the right on one side of the anticline, and up to the left on the other. Again, the principles of original horizontality and continuity would tell us that at one point the layers would have been flat and met up in the middle.

Similarly we could look at the way-up structures in the rocks: on one side of the anticline they would indicate that the original up direction had been tilted to the left, and on the other side to the right.

One interesting sign of folding is that clasts and fossils within the rocks can be stretched and deformed as the rock gradually folds. Amusingly, before this process was well-understood, early paleontologists would identify the same species as two different species according to how the specimens were stretched: a fossil fish stretched one way would appear long and thin, whereas another originally identical fish stretched the other way would look short and wide.

Indications such as these allow us to identify folds even if they have been eroded, and even if we can't see them from the side.

Edited by Dr Adequate, : No reason given.


Replies to this message:
 Message 274 by JonF, posted 04-05-2013 10:48 AM Dr Adequate has responded

  
JonF
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Message 274 of 292 (695443)
04-05-2013 10:48 AM
Reply to: Message 273 by Dr Adequate
04-04-2013 1:58 PM


Re: Folds
I've seen some claims that folds in unconsolidated sediments can be distinguished from folds in lithified sediments by the existence of many radial cracks. Any comment?
This message is a reply to:
 Message 273 by Dr Adequate, posted 04-04-2013 1:58 PM Dr Adequate has responded

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Dr Adequate
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Posts: 15472
Joined: 07-20-2006
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Message 275 of 292 (695482)
04-05-2013 4:23 PM
Reply to: Message 274 by JonF
04-05-2013 10:48 AM


Re: Folds
I've seen some claims that folds in unconsolidated sediments can be distinguished from folds in lithified sediments by the existence of many radial cracks. Any comment?

Radial cracks would be a sign of post-lithification folding, but their absence is not necessarily a sign of pre-lithification folding. A rock can be folded without cracking, it depends on the composition of the rock, the rate of folding, and most important, how ductile the rock is, which would be a function of how deeply it's buried when it's folded. (Look at the photograph of the limestone columns in the article on physical properties of rocks.)

Another sign you can look for is evidence of the stretching I mentioned in the article. Obviously pre-lithification folding isn't going to stretch the clasts.

It should be noted that a lot of pre-lithification folding isn't folding in the sense we're talking about, i.e. caused by lateral compression of the landscape. Look at the two photographs of soft-sediment deformation below, for example.

The folded strata clearly weren't folded by tectonic compression, since if they were the strata under them would also be folded.

I don't know if we can always certainly distinguish between pre-lithification and post-lithification folding by tectonic compression --- they'd look much the same because they'd be pretty much the same. Especially as there's no sharp dividing line between unlithified and lithified sediment.

Edited by Dr Adequate, : No reason given.


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 Message 274 by JonF, posted 04-05-2013 10:48 AM JonF has not yet responded

  
Dr Adequate
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Member Rating: 3.9


Message 276 of 292 (695557)
04-07-2013 2:12 PM


Faults
Faults

Introduction

In this article we shall look at what faults are, what causes them, and how we can recognize them. It will be helpful if the reader is familiar with the articles on the physical properties of rocks, on terranes, and on the principle of faunal succession.

Causes and appearance of faults

A geological fault is a planar fracture in a volume of rock caused by motion of one side with respect to the other. Motion along faults, and indeed the faults themselves, are caused by tectonic events; by the stretching or compression of the crust. This causes the rock to fracture when it is near enough to the surface to be brittle rather than ductile (as explained in the article on the physical properties of rocks).

Faults are classified according to the nature of the motion producing them. Dip-slip faults are those which involve vertical as well as horizontal motion; these can be classified as normal faults, where the landscape is being pulled apart, and reverse faults, where one part of it is being pushed over another. These are more easily illustrated than described, and are depicted in the block diagram below. The reader should note that there is nothing particularly normal or common about "normal" faults: the name is just a name. In dip-slip faults the rock lying above the fault is known as the hanging wall and that below the fault as the foot wall.

A thrust fault can be defined as a reverse fault in which the angle of the fault is more than 45 from the vertical. An interesting point to notice is that in a reverse fault, and especially in a thrust fault, older rocks end up directly above younger rocks, presenting geologists with an interesting stratigraphic puzzle; we shall return to this point later.

In a strike-slip fault, the blocks on either side of the fault move laterally but not vertically with respect to one another in a direction parallel to the fault: the San Andreas Fault is the most notorious example of this variety of fault. The transform faults discussed in the article on sea-floor spreading are a special case of strike-slip faults.

Strike-slip faults can be categorized as right (or dextral) and left (or sinstral) according to their direction of motion: in the case of a left fault, for example, anyone standing on one side and looking at the other when it was moving would see the other side moving to the left. The strike-slip fault depicted in the block diagram is a left fault.

An oblique fault combines elements of a dip-slip fault and a strike-slip fault.

In these block diagrams I have portrayed faults in which an originally continuous piece of landscape has been disturbed by a fault. However, as discussed in the article on terranes, faults will also arise when two previously disjoint land masses are forced together by plate motions. For more information about such faults, see the main article on terranes.

We shall now turn to the question of how we would go about identifying inactive faults: that is, faults where the two sides of the fault are no longing moving relative to each other.

Fault rocks

When a fault is still active then it is easy to spot it. The San Andreas Fault, for example, is hardly inconspicuous, and the motion along it is measurable.

In inactive faults, where motion has ceased, we will still, perhaps, be able to discern a crack in the rocks, perhaps accompanied by a sudden discontinuity of rock types: but perhaps the crack is just a crack, and the discontinuity is just an unconformity. What we would like is evidence that there was once motion along the suspected fault plane. There are a number of clues that point us in this direction.

In the block diagrams above, the fault itself is just shown as a straight line with no width. In reality, this is not the case. The two sides of the fault do not fit neatly together and slide smoothly past one another; instead they grind off fragments, often large ones, and crush and mill them, producing fault rocks such as fault breccia; the milling process will continue until the breccia no longer impedes motion along the fault, producing finer material filling the gaps between the coarser clasts. Fault gouge is a similar rock but with finer clasts. In the photograph below, courtesy of the U.S. Forest Service, fault gouge can clearly be seen interrupting the horizontal strata on either side of it.

Fault breccia and fault gouge can be found in active faults. For example, when engineers constructed the aqueduct between Owens Lake and Los Angeles, they were forced to tunnel through the San Andreas fault, and found themselves tunneling through a thick sheet of fault breccia and gouge. So when we find something that looks like this in the geological record, but not associated with any present motion, then it is reasonable to conclude that this too was produced by motion along a fault; not just because of its similarity to rocks associated with modern faults, but because the nature of the fault rocks themselves unmistakeably indicate processes of fracturing, crushing, and grinding. When we also note that these rocks form a narrow sheet sandwiched between other rocks, and that the clasts of the breccia match in composition the rocks that they're sandwiched between, the conclusion becomes irresistible.

Another rock characteristic of faults is mylonite. This is formed at depths where deformation of rocks is more ductile. The effect of this on the fabric of the rock is to crush its component minerals and draw them out in streaks, producing a wood-like grain as shown in the photograph below, with the grain being parallel to the direction of motion of the fault.

Although mylonite is formed at depth, we can still see it in active fault zones because it is exhumed: that is, it is brought out of the Earth by the rising side of the fault if its motion has a dip-slip component. For example, the Alpine Fault in New Zealand is a dextral-reverse fault with the hanging wall rising at a rate of 6-9 mm/yr: the grain of the exhumed mylonite on the hanging wall is consistent with exhumation by the motion of the fault.

So when we find mylonite in the geological record in the shape of a fault and sandwiched between stratigraphic evidence for faulting (as will be discussed below) we can take this as an indication that we're looking at a fault, even if it is no longer active.

Slickensides are smoothed and striated surfaces produced by the friction between the two sides of the fault, or rather between the two sides of the fault and the breccia between them. These are not dissimilar to the smoothed and striated surfaces left by glaciers. However, slickensides often take on a much higher polish than rocks smoothed by glaciers, which is why they are sometimes called fault mirrors.

Faults and stratigraphy

In the case of a dip-slip or oblique fault, if the rocks are stratified and if we are able to look at the fault side-on, we are able to see discontinuities in the originally continuous layers at the fault plane, as illustrated in the block diagrams above. Sedimentary strata, lava flows, and volcanic sills will not join up across the fault. If we are lucky, we will be able to see how the strata originally joined up, and so figure out the extent and direction of motion; but if the motion has been sufficiently great and erosion has been sufficiently severe, or if the rocks do not have distinctive strata, or if we are not privileged to have a side view of the fault, this will not be possible.

In the case of strike-slip faults there is no vertical movement, and so not such a pronounced disturbance of lateral continuity. However, strike-slip faults disrupt linear features such as dikes and riverbeds.

When two large pieces of landscape are pulled past one another, there will inevitably be frictional resistance, and this can pull the material of the rocks backwards (i.e. in the opposite direction to their directions of motion) at the top of the foot wall and the bottom of the hanging wall, distorting the structure of the rock and producing drag folds. The phenomenon is illustrated in the block diagrams below.

In a reverse fault, particularly a thrust fault, there will be places where older fossils (according to their usual arrangement in the fossil record) appear above younger fossils, in apparent, though not real, violation of the principle of faunal succession. If this was the only sign indicating a reverse fault, then perhaps we might suspect that we weren't looking at a reverse fault, but at an actual violation of the principle. But of course we can look for the other signs of a thrust fault, as listed above: slickensides, drag folds, fault breccia, etc; and we can also check that when we subject the rocks to absolute dating, the sequence of ages suggested by the fossils is confirmed by the dates, and that we really are looking at older fossils pushed by a thrust fault over younger fossils.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


  
Dr Adequate
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Message 277 of 292 (695735)
04-09-2013 1:16 AM


Lakes
So, obviously this should have gone with the other depositional environments. There's also a note at the end about how to tell freshwater from marine aquatic life, which I felt should go in one of the articles.

So that's it except that I've done an appendix on chemistry for geologists, which I would ask everyone to please read carefully when I post it, because ... how shall I put this ... there are things I'm better at than I am at chemistry.

---

Lakes

Introduction

In this article we shall look at the characteristics of lakes and how we can recognize former lakes in the geological record. The reader will find it useful to be familiar with the article on deltas before reading further.

Lakes

A lake is an inland body of water fed by rivers, streams, or sometimes by seepage of groundwater, as in deflation lakes in deserts. There is no universally agreed distinction between a lake and a pond, so you may as well think of a pond as being a small lake.

A large lake may be influenced by the tide, but even a large lake is small compared to the oceans, and the tides are correspondingly smaller; the highest tides in the Great Lakes of North America, for example, cause variations in water level no greater than five centimeters.

Most lakes are composed of fresh water; a salt-water lake can arise when water doesn't flow out of the lake (in which case it is known as a terminal lake) but is instead removed by the water evaporating, leaving behind the minerals that were dissolved in it: examples include the Great Salt Lake of Utah and the Caspian Sea.

The word lacustrine means "related to lakes", and so the sediments associated with lakes are known as lacustrine sediments.

Lacustrine sediments: how do we know?

If enough sedimentary rocks are exposed, then we can recognize a former lake by its shape: we'll find terrestrial sediments and fossils surrounding aquatic sediments and fossils, typically with coarser sediments nearer the shore and finer sediments nearer the center. If only a part of the former lake and its shore are exposed, the sediments can still provide us with ample clues.

Since lakes are not tidal, or only barely tidal, the shoreline sediments will not show the same tidal effects that we shall discuss in the article on nearshore sediments.

As discussed in the article on deltas, the foreset beds of a lacustrine delta slope at a different angle to those of a marine data, because the former are produced by fresh water flowing into fresh water. If we can find a delta with steeply sloping foreset beds, we know that the river was flowing into a freshwater lake.

Usually water will flow through a lake, in by one or more streams or rivers and out through others, but since a lake is so much broader than a river, the rate of flow in the lake itself will be small, and the sedimentary structures we associate with the flow of rivers will be small or absent. Varves, on the other hand (as mentioned in the article on glaciers) will often form in the relatively still waters of a lake.

When we look at the fossils in the sediment, aquatic fossils will usually be present; often so will some terrestrial fossils washed into a lake: leaves, for example can easily find their way into a lake, and are often preserved in the lake-bottom mud.

What's more, if, as usual, the lake is a freshwater lake, the fossils will not just be aquatic fossils, but freshwater aquatic fossils; when we find these, we're either looking at a lake or a river, and if the sedimentary evidence rules out a river, then we're looking at a lake.

Note on identifying freshwater organisms

The reader may wonder how we can identify freshwater organisms in the fossil record.

If they are recent organisms, then we can just recognize them. But what about extinct freshwater organisms? Well, if they are recently extinct, they will overlap Looking at somewhat older rocks, there will be some organisms that we don't recognize, but they will overlap in time and location with organisms we do recognize; if we can identify those as freshwater organisms, then the unfamiliar organisms that lived in the same place must also have been freshwater organisms. And we can continue this line of reasoning: when we find those organisms in company with still more types of fish or shellfish that we don't know, then they too must be freshwater organisms.

We can also look at the sedimentary environments in which organisms are found; fluvial sediments, for example, are very distinctive, and rivers are freshwater except near their mouths, so organisms found in fluvial sediments are freshwater, and if the same kinds of organisms are found in what on sedimentary grounds we would identify as a freshwater lake, then it's reasonable to conclude that it is a freshwater lake.

Then again, we can also use similar reasoning to identify marine organisms. As aquatic organisms are either marine or freshwater, those that aren't marine are freshwater by a process of elimination.


  
Dr Adequate
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Message 278 of 292 (695997)
04-10-2013 11:27 PM


Appendix On Chemistry
I thought I should include this so that the textbook really can stand alone. Chemistry is not my best subject, all criticisms will be gratefully received.

---

Chemistry for geologists

Introduction

In this article we shall take a look at some of the chemistry necessary to follow this or any other introduction to geology.

Chemistry is of course a large and complicated subject which can hardly be adressed in an article such as this one. I have omitted all inessential details and probably several that a chemist would think are in fact essential; I have recklessly sacrificed accuracy to simplicity in a way that would be quite unacceptable in the main body of this textbook. To get a full and accurate picture of chemistry, the reader will have to turn to a textbook on chemistry; the objective of this appendix is to make sure that when chemical terms, notation, and concepts are introduced in this text, the reader will not find it written in a completely foreign language. More than that I have not attempted.

Some other less basic information on chemistry is introduced within the individual articles in this textbook as and when it becomes relevant.

Structure of an atom

An atom consists of protons, neutrons and electrons. Protons have an electrical charge of +1, neutrons have no charge, and electrons have a charge of -1. The number of protons in an atom is equal to the number of electrons, so the atom as a whole has no charge. The number of protons in an atom is called its atomic number, and atoms are classified into elements according to their atomic numbers: for example the element sodium consists of all atoms having an atomic number of 11.

The protons and neutrons in an atom sit together at the center of the atom in a cluster called the nucleus. The electrons form a cloud surrounding the nucleus. A proton or a neutron has a mass about 1836 times that of an electron. This means that the mass of an atom will be determined almost entirely by the sum of the protons and neutrons it contains; this is called the atomic weight of the atom.

The electrons don't just buzz randomly around the nucleus like flies around a jam jar. Rather, they arrange themselves around the nucleus according to the laws of quantum electrodynamics. The reader will find these discussed in chemistry textbooks; to drag quantum theory into an introductory textbook on geology would be a step too far in reductionist explanation.

Instead, we shall have to content ourselves with the sort of explanation that you would have been offered before the discovery of quantum mechanics, and which is still offered to middle-schoolers; this should be quite sufficient for our purposes.

According to this scheme, the electrons are arranged in electron shells around the nucleus. Two electrons can fit into the innermost shell, eight in the second, eight in the third, eighteen in the fourth, and so on in a pattern which we shall have to leave unexplained since we're not going to talk about quantum theory. The electrons arrange themselves in these shells from the inside working out, so that the first shell must have its full quota of electrons before any will go into the second; the second shell must be full before any electron go into the third shell; and so on.

Chemical bonds and molecules

Metaphorically, we might say that an atom is "happiest" when out of all its electron shells that contain electrons at all, the outermost one contains as many electrons as will fit in the shell.

This means that atoms can come to a mutually beneficial relationship in which they share the electrons in their outer shell; this is known as a covalent bond. For example, consider the diagram below, showing such a bond between two oxygen atoms, with the nucleus represented in red and the electrons in green. Each atom on its own has six electrons in its outer shell, but by sharing electrons they have eight electrons apiece in their outer shells. The two atoms bonded together in this way form a molecule: in this case the molecule O2 (this chemical notation will be explained below).

Alternatively, atoms can form ionic bonds in which instead of sharing electrons, one atom outright gives one or more of its electrons to another. So, for example, a sodium atom, with eleven electrons, would prefer to lose an electron, leaving it with eight electrons in what would then be its outermost shell, whereas a chlorine atom, with 17 electrons, would prefer to add an electron to its outermost shell, filling it up. If the sodium atom gives its unwanted electron to the chlorine atom, both are satisfied.

This leaves the sodium atom with a charge of +1, because it has one more proton than it has electrons, and the chlorine atom has one more electron than it has protons, giving it a charge of -1.

Chemical notation

Abbreviations for elements

Each element is represented either by a single capital letter (e.g. H for hydrogen, K for potassium, W for tungsten) or by a capital letter followed by a lower case letter (e.g. Cl for chlorine, Hg for mercury, Na for sodium).

Chemical formulas

We can use these abbreviations to describe the composition of molecules: for example the combination of sodium and chlorine described above can be written as NaCl. (By convention Na is written first because the sodium has a positive charge.)

When there is more than one atom of a given element in a molecule, this is written by means of a number written in subscript to the right of the symbol for the element. So, for example, a molecule of water contains two atoms of hydrogen (H) and one of oxygen (O), and is written H2O; a molecule of methane has one atom of carbon (C) and four of hydrogen (H), and is written CH4.

Chemical reactions

We can write chemical reactions, in which molecules form, break apart, and recombine, by using chemical formulas and the addition of a couple more symbols. For example, we can write:

Na + Cl → NaCl

to indicate that sodium and chlorine, to the left of the arrow, will combine to produce NaCl, on the right.

Note that the same elements, in the same quantities, appear on both sides of the arrow; in a chemical reaction no atoms are created or destroyed, only the nature of their relationship with one another are changed.

Another convention used in describing chemical reactions is to use full-sized numbers in front of the name of an atom or molecule to indicate how many of them enter into the reaction. So for example we can write:

2HCl + Mg → MgCl2 + H2

Here the full-sized 2 indicates that there are two HCl molecules on the left-hand side. Note that the 2 refers to the entire formula HCl to the right of it, not just to the H immediately to the right of it.

Why don't we just write H2Cl2 instead of 2HCl? Because H2Cl2 is not a molecule; rather we have two molecules of HCl, and the notation reflects that. On the other hand we write H2 on the right hand side rather than 2H because H2 is a molecule, and it would be inaccurate to write 2H.

Ions

If an atom or a collection of atoms has lost an electron, as in the formation of ionic bonds, then it will have a positive charge, and is said to be a positively charged ion. We can represent this fact by a plus sign written as a superscript to the right. If instead it had gained and electron it would have a negative charge and would be described as a negative ion; we represent this by a minus sign written in the same place, above and to the right of the symbol for the element.

So for example we could if we wished write the combination of sodium and chlorine as Na+Cl-.

If an ion has lost or gained more than one electron, then we can represent this with a number written before the sign of the charge: so, for example, we could write Ca2+ to indicate an atom of calcium which has lost both of the electrons in its outer shell, giving it a charge of +2.

It is not compulsory to indicate the ionization. For example, no chemist would actually bother to write Na+Cl-, because any chemist would know that this is how sodium and chlorine must be ionized in order to form a molecule. However it is useful to have this notation as an option.

Chemical names

There are also conventional ways of giving molecules reasonably pronounceable names: so for example a chemist would look at the formula CaSO4 and know to call it calcium sulfate. For our purposes it is not necessary to explain the details of this system: in the articles in this textbook I have simply presented the chemical name alongside the chemical formula.

The periodic table

If two elements have a similar situation in their outermost shells, then chemically one will behave much the same as the other. For example, sodium and potassium both have one electron in their outermost shells. This means that just as one atom of sodium can combine with one atom of chlorine to make NaCl, so can one atom of potassium combine with one atom of fluorine to make KCl.

Similarly fluorine (F) is in the same situation as chlorine: each of them is short one electron in their outer shells. This means that fluorine will combine with sodium or potassium just the same as chlorine will, forming NaF or KF, respectively.

This information about elements can be summarized by arranging the elements in the periodic table, shown below.

Elements in the same column are said to belong to the same group, and have similar situations in their outer electron shells. For example, the elements in the far left-hand column all have a single electron in their outer shells, which they are anxious to give away; meanwhile those in the far right-hand column all have full outer electron shells, and so will not form chemical bonds, since they are perfectly happy the way they are.

The facts summarized in the periodic table are important to our understanding of various aspects of geology; for example in our discussion of paleoclimatology we shall make use of the fact that both magnesium and strontium can substitute for calcium; and in discussing U-Pb radiometric dating, it is important to know that uranium can substitute for zirconium in the formula ZrSiO4 but that lead cannot.

The reader will note by looking at the periodic table that in that last example uranium (U) does not exactly lie in the same column as zirconium (Zr). However, for reasons which can hardly be explained without reference to quantum theory, there is more flexibility for elements colored pink in the table to substitute for one another: so uranium can stand in for zirconium, but (for example) calcium could not substitute for sodium despite the fact that they're closer together in the table than uranium is to zirconium.

Solvents and solutes

A substance (a solute) is said to be dissolved in another substance (a solvent) if it is mixed with it in such a way as to acquire the phase of the solvent (i.e. whether it is solid, liquid or gas, and its crystal structure if it is a solid.)

Compare, for example, what happens when we add to water a substance which is not soluble in water, such as sand, and a substance which is soluble in water, such as table salt (NaCl). The sand remains a solid; all we have done is mix a solid with a liquid. What's more, it will not be evenly distributed in the water, but will sink to the bottom, being more dense than water; if it was less dense, it would float to the top.

By contrast, salt does dissolve in water, so what we get is not a solid (salt) in a liquid (water), but rather is simply a liquid, salty water; and the salt rapidly becomes evenly distributed throughout the water rather than sinking to the bottom.

Salt (NaCl) is an example of a molecule held together by an ionic bond, as we have noted above. Dissolved in water, however, the bond is also dissolved, as the positively charged sodium is attracted to the negative charge on the oxygen atoms in the water molecules, and the chlorine atoms are attracted to the positive charge on the hydrogen atoms. Instead of describing the dissolved salt as NaCl, it is more accurate to describe it as Na+ + Cl-; this is one occasion on which the notation for ions is useful.

Isotopes

Because the chemical behavior of atoms is determined by the interaction between their electrons, the key figure that determines the chemical properties of an atom is its atomic number; this tells us how many electrons it has, and the number of electrons determines their arrangement in the electron shells. This is why to chemists the right way to classify atoms is into elements according to their atomic number.

However, we can make a finer distinction between atoms: two atoms which have the same number of electrons (and therefore protons) can have different numbers of neutrons and therefore different atomic weights. So for example we can distinguish between uranium-235 (uranium with 143 neutrons, 92 protons, 92 electrons, and an atomic weight of 235) and uranium-238 (uranium with 146 neutrons, 92 protons, 92 electrons, and an atomic weight of 238).

The notation used for isotopes is to write the atomic weight as a superscript to the left of the symbol for the element: so for example uranium-235 would be written as 235U. (Some texts also write the atomic number as a subscript to the left of the atomic symbol; in this textbook I have not followed this convention.)

Because two different isotopes of the same element still are the same element, with the same arrangement of electrons, they have the same chemical properties. So just as one element can substitute for another if they are in the same group, it is even more the case that two isotopes of the same element will be chemically interchangeable. This fact is particularly useful to geologists, as many forms of radiometric dating would not be possible without it.

At this point it would be usual to explain nuclear decay and radioactivity; however, this textbook contains an article exclusively devoted to the subject in the section on absolute dating.

Edited by Dr Adequate, : No reason given.


  
petrophysics1
Member
Posts: 343
From: Boulder, Wy
Joined: 04-05-2006


Message 279 of 292 (697126)
04-21-2013 7:21 PM


Quiz?
So are you going to do a Quiz or would you like me to make one?

You probably will not like mine, since it will be about doing geology instead of reading about it, but it could be enlightening.


    
RAZD
Member
Posts: 18241
From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 3.0


Message 280 of 292 (697134)
04-21-2013 8:19 PM
Reply to: Message 266 by Dr Adequate
03-06-2013 1:32 AM


Re: Ice Cores
Excellent as usual.

Another visual of ice layers comes from South America:

Ice layers introduction on Age Correlations thread

quote:
http://www.ncdc.noaa.gov/paleo/slides/slideset/index20.htm (3)

quote:
...

(Slide 3) The Quelccaya cap terminates abruptly and spectacularly in a 55 m ice cliff. The annual accumulation layers clearly visible in the photograph are an average of .75 m thick. While snow can fall during any season on the altiplano, most of it (80-90%) arrives between the months of November and April. The distinct seasonality of precipitation at Quelccaya results in the deposition of the dry season dust bands seen in the ice cliff. ...

Note that they are talking about correlating layers with climate information provided by d18O. We'll also come across this in other measurement systems. This is the proportion of a "heavy" isotope of oxygen in the atmosphere (16O is "normal" weight oxygen)

see http://en.wikipedia.org/wiki/Oxygen-16 (1)
and http://en.wikipedia.org/wiki/Oxygen-18 (2)

While this series of layers only date back to ~500AD they are important for a couple of reasons: they show visible layers, and they allow calibration of the oxygen isotope ratio (d18O) as a measure of layers and of climate. These layers also show a period of sever weather that is known from history (the Little Ice A


And there is more about other ice layer data sources and how the atmosphere isotopes can then be used to estimate ages of deeper layers when they become hard to distinguish.

It saddens me to think that we may be losing the Greenland and Antarctic ice fields and the immense bank of data that has not yet been tapped.

Enjoy


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petrophysics1
Member
Posts: 343
From: Boulder, Wy
Joined: 04-05-2006


Message 281 of 292 (697868)
05-01-2013 9:43 AM


General Quiz
So here is a rather good quiz that you should do well on if you have read and understand what Dr.A has written here.

19 topics with 10 questions each about geology. I regard a test/quiz not as a test to see what you know but as a way to point you to what you don't understand so you can find out about it.

http://homepage.smc.edu/...richard/rocktest/physical_geology

I think this quiz is good and covers a lot of what Dr.A has talked about. It is, from my viewpoint as a working geologist, very simple, but I think if you can get around 70% on the quizes Dr.A has done a good job.

What do you think?

{I have taken this message and created a new topic with it. Please do any responses to the message at that new topic - "Introduction To Geology quiz".- Adminnemooseus}

Edited by Adminnemooseus, : Note in red.


    
Pollux
Member
Posts: 184
Joined: 11-13-2011


Message 282 of 292 (715466)
01-05-2014 6:47 PM


Bowen Reaction Series
I'm not sure where this question for the geologists should go so I am putting it here.
I have recently purchased "The Illustrated Encyclopedia of Minerals Rocks and fossils of the World" written by John Farndon and Steve Parker. In discussing how igneous rocks form they mention the BRS and list the crystallization of minerals from high to low temperature as olivine, pyroxine, amphibole, biotite mica, quartz, muscovite mica, K-feldspar and plagioclase feldspar. From my reading of the BRS I think I have a reasonable understanding of it, and it seems that they have gone backwards up the right hand part of the Y after quartz to high temperature. Am I right?

Also I note that the melting point of quartz is 1713 deg C but it comes out of a magma at about 800. Is this difference because of the difference between pure quartz and a complex mixture?


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Dr Adequate
Member
Posts: 15472
Joined: 07-20-2006
Member Rating: 3.9


(1)
Message 283 of 292 (720151)
02-20-2014 1:04 PM


Wikibook
I don't think I've mentioned generally that I've turned this thread into a wikibook.

It appears to get quite a high rank if one googles on the words historical geology. However, I am aware that google rankings are tailored to suit the preferences of the googler (in my case, me) so I would be interested to know where it appears in other people's google searches. Anyone who'd like to tell me should try it before clicking on the link below.

So for those who'd like to try it, what you should do is put historical geology into the google search field, and see how far down you have to look to see a hit titled "Historical Geology - Wikibooks, open books for an open world". Thank you.

Several errors have been corrected in the wikibook that remain in the thread, so this should be taken as the definitive version. The other major difference is that the pictures are often different, as the wiki foundation is scrupulous about copyright. Also, there are now many internal links, and the introduction has been rewritten for a broader audience.

So, anyway, the thing can be found here: Historical Geology.

Thank you again for all your help and suggestions, and thanks again in particular to Pressie.


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Coyote
Member
Posts: 5540
Joined: 01-12-2008
Member Rating: 2.5


Message 284 of 292 (720153)
02-20-2014 1:16 PM
Reply to: Message 283 by Dr Adequate
02-20-2014 1:04 PM


Re: Wikibook
#7
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Tanypteryx
Member
Posts: 1324
From: Oregon, USA
Joined: 08-27-2006
Member Rating: 7.0


Message 285 of 292 (720154)
02-20-2014 1:25 PM
Reply to: Message 283 by Dr Adequate
02-20-2014 1:04 PM


Re: Wikibook
#7 for me also.

Well done Dr. A


What if Eleanor Roosevelt had wings? -- Monty Python

One important characteristic of a theory is that is has survived repeated attempts to falsify it. Contrary to your understanding, all available evidence confirms it. --Subbie

If evolution is shown to be false, it will be at the hands of things that are true, not made up. --percy


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