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Author Topic:   Introduction To Geology
Dr Adequate
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Posts: 16085
Joined: 07-20-2006
Member Rating: 10.0


Message 31 of 293 (635035)
09-26-2011 12:31 AM
Reply to: Message 30 by roxrkool
09-25-2011 12:25 AM


Re: Minerals and rocks: definitions
I've not heard or read anything regarding organic material being classified as a mineraloid, but that can change or has changed(?).

Every list of mineraloids I've seen includes amber, which is organic. As to coal, for example this guy in his lecture notes classes coal as a mineraloid. On the other hand, some people mention jet but not coal, which is odd, because surely there's no qualitative difference.

I think the best thing is if I join the people who call coal a mineraloid, appart from anything else that saves one from havving to add "... or coal" to the definition of a rock.

Clays, such as illite, chlorite, etc. are minerals, but "clay" is also a particle size. So clay minerals, as well as clay-sized particles (of other minerals), comprise shale.

Yes, but as you note the other things that are clay (by particle size) are minerals... OK, maybe with a bit of organic material thrown in, but to a very large extent shales and mudstones are made of minerals, so I stand pat.

We'll get round to discussing these details when we look at sedimentary rocks in particular.

Iron, the metal, may have a higher melting point than magnesium metal, but Mg-rich forsterite is a far more stable mineral under higher temps and pressures (as present in the mantle) than the Fe-rich fayalite. Therefore, forsterite will crystallize first from higher temp ultramafic/mafic melts, rather than fayalite.

Oh yes, I know you're right --- I looked it up. I just thought it was an interesting example of counter-intuitiveness in science.


This message is a reply to:
 Message 30 by roxrkool, posted 09-25-2011 12:25 AM roxrkool has responded

Replies to this message:
 Message 32 by roxrkool, posted 09-26-2011 8:35 AM Dr Adequate has responded

  
roxrkool
Member (Idle past 970 days)
Posts: 1493
From: Nevada
Joined: 03-23-2003


Message 32 of 293 (635058)
09-26-2011 8:35 AM
Reply to: Message 31 by Dr Adequate
09-26-2011 12:31 AM


Re: Minerals and rocks: definitions
Ah yes, I see the inclusion of organic material. Hmmmm... not sure I agree, but not really worth arguing it. As for the clays, I was simply elaborating a bit for the readers. Not everyone knows clays can also be minerals.
This message is a reply to:
 Message 31 by Dr Adequate, posted 09-26-2011 12:31 AM Dr Adequate has responded

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edge
Member
Posts: 4460
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 3.8


Message 33 of 293 (635115)
09-26-2011 9:08 PM
Reply to: Message 32 by roxrkool
09-26-2011 8:35 AM


Re: Minerals and rocks: definitions
Ah yes, I see the inclusion of organic material. Hmmmm... not sure I agree, but not really worth arguing it. As for the clays, I was simply elaborating a bit for the readers. Not everyone knows clays can also be minerals.

Hey, Rox, long time no talk...

Well, I may be old-school, but while amber may be a mineraloid it cannot be a mineral. It does not have a regular, repeating crystal structure.


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Pressie
Member
Posts: 1998
From: Pretoria, SA
Joined: 06-18-2010
Member Rating: 3.7


Message 34 of 293 (635142)
09-27-2011 7:05 AM
Reply to: Message 33 by edge
09-26-2011 9:08 PM


Re: Minerals and rocks: definitions
Hi guys

This is making a very difficult thread a lot more difficult.

I think we should provide the "official" geological explanations of the words crystal and mineral.

Crystal: A homogenous, solid body of chemical element, compound, or isomorphous mixture, having a regularly repeating atomic arrangement that may be outwardly expressed by plane phases.

Isomorphous mixture: Two or more crystalline substances to have similar chemical composition, axial ratios, and crystal forms, and to crystallize in the same crystal class. Such substances form an isomorphous series.

Mineral: (a) a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties. Those who include the requirement of crystalline form in the definition would consider an amorphous compound such as opal to be a mineraloid.
(b) Any naturally formed inorganic material, i.e. member of the mineral kingdom as opposed to the plant and animal kingdoms.

From Bates, R.L. and Jackson, J.A. (Editors), 1980. Glossary of Geology, Second Edition. American Geological Institute. Falls Church, Virginia.

This makes organic material such as amber a mineraloid, not a mineral. Any organic material, such as coal, therefore is not a mineral (although used as such colloquially).

Dr Adequate has a very difficult time at hand for a basic course in geology. Lets not make it more difficult for him!


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Dr Adequate
Member
Posts: 16085
Joined: 07-20-2006
Member Rating: 10.0


Message 35 of 293 (635159)
09-27-2011 12:46 PM
Reply to: Message 32 by roxrkool
09-26-2011 8:35 AM


Re: Minerals and rocks: definitions
I was simply elaborating a bit for the readers.

Well, please don't. So far I have explained the distinction between a mineral and a rock. This is no place to talk about the distinction between clay as a mineral and clay as a particle size; or about the melting points of olivine endmembers. This will all come in time.

Edited by Dr Adequate, : No reason given.


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Dr Adequate
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Posts: 16085
Joined: 07-20-2006
Member Rating: 10.0


(1)
Message 36 of 293 (635965)
10-03-2011 11:51 AM


Silicate Minerals
Silicate minerals

Silicate structures

By a silicate tetrahedron we shall mean an atom of silicon bonded with four equally spaced atoms of oxygen forming the four corners of a pyramid having a triangular base.

Each tetrahedron can share each one of its oxygen atoms with one other tetrahedron, so that two tetrahedra can join together corner-to-corner (but not edge-to-edge or face to face). Hence each tetrahedron can be linked with up to four other tetrahedra, one for each corner of the tetrahedron; or three, two, one, or none. This gives us a wide variety of structures that can be built out of tetrahedra: three-dimensional lattices; two dimensional sheets, chains, double chains, rings, et cetera; so silicate minerals can be classified according to their silicate structure as lattice silicates, sheet silicates, chain silicates, and so forth. The diagram below shows some of the possible structures.

Key: (a) an isolated tetrahedron; (b) a pair; (c) a six-member ring; (d) a chain; (e) a double chain; (f) a sheet.

Note that the chain and double chain can be extended indefinitely in one direction and the sheet in two directions.

A silicate mineral is a mineral containing silicate structures. Note that with the exception of quartz and its polymorphs, a silicate mineral will not consist entirely of such structures. Atoms of other elements must necessarily be involved so that the rings, chains, sheets or whatever form part of a three-dimensional crystal.

Most silicate structures can be described either by a descriptive English name such as "sheet silicate" or by a name which describes the same thing in Greek such as "phyllosilicate". Where a plain English term exists, I shall employ it in this text. The table below shows how the structures relate to the English and Greek names and gives examples of minerals important to this course.

Structure English name Greek Important examples
Three-dimensional Lattice silicates Tectosilicates Quartz; feldspars
Parallel sheets Sheet silicates Phyllosilicates Micas; clays; serpentine; chlorite
Single or double chains Chain silicates Inosilicates Pyroxenes; amphiboles
Three, four, or six-membered rings Ring silicates Cyclosilicates
Pairs Sorosilicates Epidotes
Isolated tetrahedra Nesosilicates or orthosilicates Olivine

Because tetrahedra link together by sharing the oxygen atoms at their corners, the structure formed by the tetrahedra is reflected in the chemical formula of a silicate. For example, quartz consists of nothing but tetrahedra linked together in a three-dimensional lattice. This means that every tetrahedron is linked to another at all four corners; which means that every oxygen atom is shared by two silicon atoms; which means that quartz has the formula SiO2. Similarly, if you look at the formula for zircon (ZrSiO4) you can see that it must be a neosilicate.

Aluminosilicates

In some silicates, the structure is based not just on silicate tetrahedra but also on tetrahedra with a central atom of aluminum rather than silicon. Silicates which incorporate these aluminum-based tetrahedra as well as silicate tetrahedra are known as alumnosilicates.

Aluminum-based tetrahedra have a different charge from silicate tetrahedra. This means that you cannot have an aluminosilicate which differs from an ordinary silicate only by the substiution of atoms of aluminum for some of the atoms of silicon; there must necessarily be other differences. For example, it is chemically impossible to build a lattice just out of these two kinds of tetrahedra analogous to quartz; other atoms are required to balance the charge of the chemical formula. Hence lattice aluminosilicates such as feldspars do not have the formula (Si,Al)O2, which is impossible, but have formulas such as KAlSi3O8 and CaAl2Si2O8.

Felsic and mafic silicates

Silicate minerals which are high in silicon are called felsic minerals; the opposite of felsic is mafic; minerals which are very mafic are known, sensibly enough, as ultramafic. Note that this term only applies to minerals which are silicate minerals and so contain some silicate tetrahedra; no-one would call calcium carbonate (for example) an ultramafic mineral.

In some texts, particularly older texts and British texts, you may see the words acidic, basic, and ultrabasic used instead of felsic, mafic, and ultramafic. These terms refer to an obsolete hypothesis of mineral formation, and I shall not use them; I mention them only for the benefit of those readers who might come across them in further reading.

Some generalizations might be made about the differences between felsic and mafic minerals: as we progress from felsic to mafic the minerals are more dense (because the atoms in them which aren't oxygen or silicon are heavier elements such as magnesium or iron); they have higher melting points; and when they do melt they are less viscous (that is, they flow more easily).

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

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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Posts: 16085
Joined: 07-20-2006
Member Rating: 10.0


Message 37 of 293 (636684)
10-09-2011 3:02 PM


Rocks: Igneous, Sedimentary, And Metamorphic
Rocks: igneous, sedimentary and metamorphic

Introduction

In this article we shall look at the most significant way in which geologists classify rocks. The reader should recall from the article on minerals and rocks that a rock is an aggregate of one or more minerals or mineraloids.

Igneous, sedimentary, and metamorphic

There are all sorts of ways that we might classify rocks. We might, for example, divide them up according to chemistry: and indeed the distinction between silicates and carbonates is a useful one. We might also classify rocks according to whether they contain felsic or mafic minerals, and as we shall see this is a good way to classify certain rocks. But the most fundamental way in which geologists classify rocks is to label them as igneous, sedimentary, or metamorphic.

Igneous rocks are rocks formed by the cooling and setting of molten rock.

Sedimentary rocks are formed by sediment (for example, sand or mud) turning into rock (such as sandstone or mudrock).

Metamorphic rocks are formed when rocks are subjected to heat, to pressure, to chemical reactions, or to any combination of these three, in such a way as to change the properties of the original rock in some way.

The rock cycle

The relation between the various kind of rocks can be summarized by a diagram of the rock cycle. Here is one picture of it which I find more or less satisfactory; I may at some point come back to this and prepare my own.

How do we know?

At this point we are touching on the main theme of this course. For to classify rocks as igneous, sedimentary, or metamorphic is implicitly to classify them not by their directly observable properties such as color or density or chemical composition --- but by their history as inferred from their present-day properties. The reader will, therefore, want to know: how do we know? This question will be answered in separate articles on igneous, sedimentary, and metamorphic rocks.

In the meantime, let us point out how intrinsically historical geology is. After we've got past the most basic of considerations such as defining a mineral and defining a rock, we are plunged into historical considerations. And this in not just because this course is about historical geology: it would be the same if it was an introduction to how to find oil. There is no geology that is not historical, and if we tried to do without historical inferences we might as well classify rocks by how pretty they are for all the good it would do us.

Edited by Admin, : Provide image a light background.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


  
Robert Byers
Member (Idle past 2441 days)
Posts: 640
From: Toronto,canada
Joined: 02-06-2004


Message 38 of 293 (636796)
10-11-2011 2:32 AM


AMEN.
Geology is about history.
it is only the processes that matter as the intimate particles of these rocks did not form themselves together as a independent thought.
Only the processes matter in geology.
The rocks are defined by their history.

Creationists have to insist that geology is from historic actions and so investigation must be about history.
We see a difference between science class and history class.


Replies to this message:
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Pressie
Member
Posts: 1998
From: Pretoria, SA
Joined: 06-18-2010
Member Rating: 3.7


Message 39 of 293 (636901)
10-12-2011 12:33 AM
Reply to: Message 38 by Robert Byers
10-11-2011 2:32 AM


Robert Byers writes:

AMEN.
Geology is about history.

No, geology is the study the earth. The study of rocks is a very important part when studying the earth.
Robert Byers writes:

...it is only the processes that matter.....

No, it's those rocks that matter. That's it.
Robert Byers writes:

... as the intimate particles of these rocks....

I've seen people being "intimate", even done it myself a few times (not enough to my liking). Never seen rock particles being "intimate".
Robert Byers writes:

... did not form themselves together as a independent thought.

Rocks don't think. They exist as rocks.
Robert Byers writes:

... Only the processes matter in geology.

No, only the rocks matter. That's what we study.
Robert Byers writes:

The rocks are defined by their history.

No, they're not. They are defined by their composition.
Robert Byers writes:

Creationists have to insist that geology is from historic actions and so investigation must be about history.

Well, it's been long established that creationists don't do science. They pretend that they do, but geologists are not as stupid creationists think they are. Geologists work on empirical, verifiable evidence. Creationist work on belief. Big difference.
Robert Byers writes:

We see a difference between science class and history class.

Geology is a natural science. History is not. Big difference between the two.

Edited by Pressie, : No reason given.


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Replies to this message:
 Message 40 by Dr Adequate, posted 10-12-2011 5:42 AM Pressie has responded
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Dr Adequate
Member
Posts: 16085
Joined: 07-20-2006
Member Rating: 10.0


Message 40 of 293 (636909)
10-12-2011 5:42 AM
Reply to: Message 39 by Pressie
10-12-2011 12:33 AM


Well, I have to disagree with you. In fact, I have already disagreed with you, since by over-reacting to Robert you are disagreeing with what I wrote in my previous post.

If we are studying historical geology, which is what we're meant to be doing here, then the history, the process, is the thing being studied. It is the underlying reality of which the rocks are merely the outward and visible sign.

But, as I pointed out, even if we're not doing historical geology, but have a more practical aim, it is still necessary to talk about it in terms that refer to process and history --- igneous, sedimentary, and metamorphic rocks, marine sedimentary rocks, aeolian sandstone, rhythmites, pedogenesis, faulting, folding, mineralization ... and if you tried to rewrite geology to get away from such terms, it would appear as a set of arbitrary rules, like chemistry without the concept of valence. Consequently it would be impossible to think about geology in such terms; you would have to think about it in historical terms and then translate the practical upshot into ahistoric terms.

For example, I can tell you easily in historical terms why oil is to be found in certain nearshore deposits. Now, it is possible to translate this into a method for looking for oil which can be expressed purely in ahistoric terms, in which we refer exclusively to the we-can't-call-it-"sedimentary"-any-more structure of rocks. But it would be impossible to say why any of this should be true --- the instructions would be as arbitrary as a magic spell: "Two nights after the dark of the moon, walk three time widdershins round the enchanted glade, muttering thrice the words 'Grimblethorpe, Mulcifer, Pellicle', and behold, the fairy gold will appear."

The same, of course, is true of Robert Byers' bullshit "history", 'cos of its wild and ludicrous inaccuracy. Someone trying to be ahistorical couldn't talk about the characteristic sedimentary deposits formed by the action of the tide because that refers to history and process, and Robert Byers can't talk about it because that refers to real history and process and not to imaginary impossibilities recounted in a disgusting and ridiculous myth.

Edited by Dr Adequate, : No reason given.


This message is a reply to:
 Message 39 by Pressie, posted 10-12-2011 12:33 AM Pressie has responded

Replies to this message:
 Message 42 by Pressie, posted 10-14-2011 1:37 AM Dr Adequate has responded

  
Robert Byers
Member (Idle past 2441 days)
Posts: 640
From: Toronto,canada
Joined: 02-06-2004


Message 41 of 293 (637174)
10-14-2011 12:49 AM
Reply to: Message 39 by Pressie
10-12-2011 12:33 AM


Thanks Doc.
Keep a eye on the slower students.
its very important to remember geology etc is about processes that happened before the present.
History.
This message is a reply to:
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Pressie
Member
Posts: 1998
From: Pretoria, SA
Joined: 06-18-2010
Member Rating: 3.7


Message 42 of 293 (637184)
10-14-2011 1:37 AM
Reply to: Message 40 by Dr Adequate
10-12-2011 5:42 AM


Sorry about over-reacting to Robert. I tend to overreact to novices who think they know everything and also think that all those hundreds and thousands of real geologists all are wrong. To me this is a sign of complete delusion and it would be safer for humanity if such a person is treated at some mental institution. Still too many sky-scrapers and planes around to feel safe with so many deluded people walking the streets.

I disagree with you. The rocks are and were studied. The processes forming rocks are and were also studied. After that the processes by which the rocks under investigation were formed can be deducted as we can study the processes by which such rocks are formed today.

Those different rocks are and were classified after studying them. The study of rocks lead to classifications such as Igneous, Sedimentary and Metamorphic. We can study those rocks very systematically.

For example: After studying the Clarens formation of the Karoo Sequence, it has been found that said formation has all the characteristics of Aeolian deposits, therefore it was deposited by the working of wind. Thats geology.

As far as the oil deposits go, no magic. The Mississippi Delta exists. To simplify: to find oil we start looking for similar environments.

Edited by Pressie, : No reason given.


This message is a reply to:
 Message 40 by Dr Adequate, posted 10-12-2011 5:42 AM Dr Adequate has responded

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Dr Adequate
Member
Posts: 16085
Joined: 07-20-2006
Member Rating: 10.0


Message 43 of 293 (637186)
10-14-2011 1:59 AM
Reply to: Message 42 by Pressie
10-14-2011 1:37 AM


I disagree with you. The rocks are and were studied. The processes forming rocks are and were also studied. After that the processes by which the rocks under investigation were formed can be deducted as we can study the processes by which such rocks are formed today.

I'm not sure that that's disagreeing with me ...

For example: After studying the Clarens formation of the Karoo Sequence, it has been found that said formation has all the characteristics of Aeolian deposits, therefore it was deposited by the working of wind. Thats geology.

It is. But in particular the deduction that the Clarens formation is aeolian is just as much geology as looking at the structure of the rock. And considerably more interesting. Mere observational statements such as "This rock contains pinstripe laminae" without any deduction from it would be no more informative than "This rock is pretty". But the former statement is more interesting if we use it to infer that we're looking at what used to be a desert --- a notion which has predictive and explanatory power. This is what distinguishes geology from stamp-collecting.

As far as the oil deposits go, no magic. The Mississippi Delta exists. To simplify: to find oil we start looking for similar environments.

OK, but how do you do that? For example, suppose I find aeolian sandstone (which is a deduction about its history) over here, and I find deep marine sediment (which is a deduction about its history) over there, and if I find that they are the same age (obviously a historical judgement) then I can infer a coastline somewhere between the two, and then I can start looking for nearshore deposits (another historical judgement) somewhere in the line between them, and if I find, for example, interference ripples, I would infer that they were formed by the tide and know that I was getting close to the sort of structures I'm actually looking for.

Now one could express how to do this and similar things in terms of a mechanical procedure that just involves looking at rocks and making certain measurements of isotope ratios, but only by expunging any clue as to how it actually works.

Edited by Dr Adequate, : No reason given.


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 Message 42 by Pressie, posted 10-14-2011 1:37 AM Pressie has responded

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 Message 47 by Pressie, posted 10-14-2011 4:10 PM Dr Adequate has responded

  
Dr Adequate
Member
Posts: 16085
Joined: 07-20-2006
Member Rating: 10.0


(1)
Message 44 of 293 (637187)
10-14-2011 2:09 AM


Igneous Rocks
Igneous rocks

Introduction

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 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 than 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 may be represented by the diagram below. 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.

We have divided 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

In the diagram below we show some of the structures formed by igneous rocks. The black represents igneous rock; the other colors represent sedimentary rocks.

Key: (1) Volcanic ash. (2) A volcano. (3) A volcanic conduit. (4) A fissure. (5) A lava flow. (6) A lacolith. (7) Dikes. (8) Sills. (9) Stocks. (10) A batholith.

As this is a two-dimensional 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 never 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 below 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; again, I do not intend to use these terms, as they are obsolete and misleading.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


Replies to this message:
 Message 45 by Jazzns, posted 10-14-2011 10:59 AM Dr Adequate has responded
 Message 293 by Phat, posted 11-20-2017 10:57 PM Dr Adequate has not yet responded

  
Jazzns
Member (Idle past 1984 days)
Posts: 2657
From: A Better America
Joined: 07-23-2004


Message 45 of 293 (637249)
10-14-2011 10:59 AM
Reply to: Message 44 by Dr Adequate
10-14-2011 2:09 AM


Re: Igneous Rocks
(10) A basolith

Is that a typo? Did you mean batholith?

Also, since you seem to be okay with mentioning other vocabulary, aren't batholiths also called plutons sometimes? I seem to remember that it may just be when they are small they are called plutons but it has been awhile.


BUT if objects for gratitude and admiration are our desire, do they not present themselves every hour to our eyes? Do we not see a fair creation prepared to receive us the instant we are born --a world furnished to our hands, that cost us nothing? Is it we that light up the sun; that pour down the rain; and fill the earth with abundance? Whether we sleep or wake, the vast machinery of the universe still goes on. Are these things, and the blessings they indicate in future, nothing to, us? Can our gross feelings be excited by no other subjects than tragedy and suicide? Or is the gloomy pride of man become so intolerable, that nothing can flatter it but a sacrifice of the Creator? --Thomas Paine

This message is a reply to:
 Message 44 by Dr Adequate, posted 10-14-2011 2:09 AM Dr Adequate has responded

Replies to this message:
 Message 46 by Pressie, posted 10-14-2011 3:13 PM Jazzns has not yet responded
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