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Author Topic:   Introduction To Geology
Pressie
Member
Posts: 1998
From: Pretoria, SA
Joined: 06-18-2010
Member Rating: 2.8


Message 46 of 293 (637294)
10-14-2011 3:13 PM
Reply to: Message 45 by Jazzns
10-14-2011 10:59 AM


Re: Igneous Rocks
I think that the word "basoliths" was a spelling mistake. I missed it, too.
From http://en.wikipedia.org/wiki/Batholith
A batholith (from Greek bathos, depth + lithos, rock) is a large emplacement of igneous intrusive (also called plutonic) rock that forms from cooled magma deep in the Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock-types, such as granite, quartz monzonite, or diorite (see also granite dome).
Plutons include batholiths, dykes, sills, etc. Batholiths are felsic, not mafic. Not all plutons have these characteristics. From http://www.geolsoc.org.uk/...824DBE5109DD29090C9CE8ECC5EDB16
A pluton is any large igneous body that has congealed from magma underground. There are many sorts of pluton including the lens-like and subhorizontal laccoliths and lopoliths, and the vertical or near-vertical sided stocks and batholiths. All these categories of pluton are defined on their overall apparent shape and relationship to the country rock. A batholith is the largest of the pluton types and by definition cover at least 100 square kilometres. A stock is a small discordant pluton, shaped like a batholith but falling below the necessary 100 square km in extent.

Edited by Pressie, : Added source of batholiths


This message is a reply to:
 Message 45 by Jazzns, posted 10-14-2011 10:59 AM Jazzns has not yet responded

    
Pressie
Member
Posts: 1998
From: Pretoria, SA
Joined: 06-18-2010
Member Rating: 2.8


Message 47 of 293 (637301)
10-14-2011 4:10 PM
Reply to: Message 43 by Dr Adequate
10-14-2011 1:59 AM


Dr Adequate, we dont really disagree.
Dr Adequate writes:

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.

We dont even have to infer. We can see them forming today. Same characteristics.
Dr Adequate writes:

This is what distinguishes geology from stamp-collecting.

Geology follows the scientific method. Stamp collecting follows what looks good for making money. If I wanted to make money, I would actually write a statement claiming: Geologist thinks Clarens Formation was deposited in flood! That would be the headline in every religious tract and website in the world for years to come, notwithstanding that none of them would even know what the Clarens Formation looks like. I would appear in every AIG and CMI publication and website and they would would pay a lot of money for that. I, however, wouldnt be able to live with my conscience after doing that.
Dr Adequate writes:

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)

No, it is certainly not historical. We can see it happening today. Look for the same characteristics..voila. We know exactly what happened for those rocks to be deposited.
Dr Adequate writes:

.then I can infer a coastline somewhere between the two, and then I can start looking for nearshore deposits (another historical judgement).

Well, you wont infer that those rocks were poofed into existence.
Dr Adequate writes:

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.

Thats because we can see interference ripples being formed today. We know what they look like.
Dr Adequate writes:

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

Weve got examples of exactly how it works in real life.
This message is a reply to:
 Message 43 by Dr Adequate, posted 10-14-2011 1:59 AM Dr Adequate has responded

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 Message 49 by Dr Adequate, posted 10-14-2011 7:43 PM Pressie has not yet responded

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


Message 48 of 293 (637307)
10-14-2011 6:40 PM
Reply to: Message 45 by Jazzns
10-14-2011 10:59 AM


Re: Igneous Rocks
Is that a typo? Did you mean batholith?

Thanks.

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.

Well, it's puzzling. WP says: "A pluton in geology is a body of intrusive igneous rock (called a plutonic rock) that crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies." That makes sense, in that case a pluton is anything formed from plutonic rock. But then it goes on: "In practice, "pluton" usually refers to a distinctive mass of igneous rock, typically several kilometers in dimension, without a tabular shape like those of dikes and sills."


This message is a reply to:
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Dr Adequate
Member
Posts: 16085
Joined: 07-20-2006
Member Rating: 10.0


Message 49 of 293 (637313)
10-14-2011 7:43 PM
Reply to: Message 47 by Pressie
10-14-2011 4:10 PM


History
We dont even have to infer. We can see them forming today. Same characteristics.

That's what makes the inference particularly easy. It's an inference on the level of "If it looks like a duck, and it quacks like a duck, it's a duck". But it is still an inference, and what the inference is about is how the sediment of which the rock consists was laid down in the past.

No, it is certainly not historical. We can see it happening today. Look for the same characteristics..voila. We know exactly what happened for those rocks to be deposited.

And the word "deposited" is in what tense?

Geology follows the scientific method. Stamp collecting follows what looks good for making money.

What I meant was, geologists don't merely collect and classify. They have a theory. This theory has predictive and explanatory power.

---

Take aeolian sandstone as an example.

Theory: such-and-such sedimentary structures are aeolian.

Prediction: when we find fresh sediment having these structures, it will be on dry land; when we watch it forming in real time we shall see the wind forming the structures.

Observation: consistent with the prediction.

Explanatory inference: when we find these structures in sandstone, we should infer that it was formed in the same way.

Another prediction: hence if we find sandstone with these structures, then if we find fossils in it they should be terrestrial and not marine.

Observation: We do, hurrah!

Explanatory inference: but if we find no fossils, we should come to the same conclusion, since the theory works every time we can test its predictions.

It may be bleedin' obvious, but it is still an example of a scientific theory which we establish as true by testing its predictions and from which we can then make inferences which rest on the theory rather than on direct observation.

Now it is precisely this aspect of geology which I wish to emphasize.


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Dr Adequate
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Joined: 07-20-2006
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(1)
Message 50 of 293 (638588)
10-24-2011 4:03 AM


Sedimentary Rocks
Sedimentary rocks

Introduction

The article constitutes a brief introduction to the various kinds of sedimentary rocks. Further information about the sources of sediment, its transport, and it 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 physically 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 we 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 we have said, sedimentary rocks are formed from broken pieces ("clasts") of pre-existing rocks.

* Sediment: rounded gravel. Rock: conglomerate.

Gravel is defined as clasts with diameter 2mm or more. Conglomerate is formed of rounded gravel. 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: angular gravel. Rock: breccia.

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: sand. Rock: sandstone

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

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.

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 small fragments) 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.

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

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: bedded mud or clay. Rock: shale.

Most mudstone and claystone is bedded (see the entry above on sandstone for a definition of this term). When it is, it is referred to as shale.

Chemical sedimentary rocks

* Sediment: salt. Rock: halite.

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: hydrated calcium sulfate. Rock: gypsum.

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: silica. Rock: chert.

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: calcium carbonate. Rock: limestone.

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.

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

Biochemical sedimentary rocks

* Sediment: shells of calcium carbonate. Rock: limestone.

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 a 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 silica. Rock: chert.

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: peat. Rock: coal.

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.

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 below shows a particularly large-scale example of cross-bedding in sandstone. (To give you an idea of the scale, the small patches of green visible in the top right corner are not moss, as you might guess, but bushes.)

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 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 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). 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 associate 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, we 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.

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 51 of 293 (639572)
11-02-2011 4:42 AM


Metamorphic rocks
Metamorphic rocks

Introduction

Metamorphic rocks are those in which a pre-existing rock (the parent rock) has been altered chemically or texturally by heat and/or pressure.

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 300C) to low (300C - 500C) to medium (500C - 600C) to high (600C 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 below show some particularly large garnets embedded in metamorphic rock.

We see this same sequence arranged geologically 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 above shows a close-up view of skarn, a very distinctive product of metasomatism. Dark gray intrusive igneous rock has mixed with white recrystalized limestone; index minerals have also been produced, such as garnet (reddish-brown) and epidote (green).

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, the 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 below 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 shale 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?

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 excludes 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.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Adminnemooseus, : Add "Metamorphic rocks" subtitle.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


  
Dr Adequate
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(3)
Message 52 of 293 (640486)
11-10-2011 7:37 AM


Mechanical Weathering And Erosion
Mechanical weathering and erosion

Introduction

Geologists make a distinction between weathering and erosion: weathering breaks rocks but leaves them in place; whereas the processes of erosion are capable both of breaking rocks and transporting the broken fragments (clasts). Most, though not all texts will make this distinction. Those that do will still use "erosion" as a catch-all term: that is, when you see a geologist saying that a rock has been "eroded" s/he does not mean to imply that it has not also been weathered.

In this article we shall present a brief overview of erosion and of mechanical weathering. We shall be brief because the erosional processes involved will be discussed at length in subsequent articles, so there is no need to do more than sketch out the topic and its vocabulary.

Mechanical weathering

Mechanical weathering is sometimes referred to as physical weathering. In both cases, the purpose of this nomenclature is to distinguish it from the processes collectively known as chemical weathering, in which chemical action breaks down the rocks. Chemical weathering will be covered in the next article.

Mechanical weathering involves mechanical processes that break up a rock: for example, ice freezing and expanding in cracks in the rock; tree roots growing in similar cracks; expansion and contraction of rock in areas with high daytime and low nighttime temperatures; cracking of rocks in forest fires, and so forth.

Mechanical weathering is probably the least important process we shall mention in this text, in that the history of the Earth, and the resulting geological record, would probably have been very similar if there was no such thing as mechanical weathering.

Mechanical erosion

The main agents of mechanical erosion are: gravity; aeolian processes (i.e. those caused by the wind); ice in the form of glaciers; and water in the form of rivers, waves, turbidity currents, and runoff caused by rainfall.

The reader will be familiar with most of the processes described, but we should provide a brief introduction to the concept of turbidity currents. A current of water that is turbid (that is, which contains a lot of sediment) is denser than clear water, and will flow along the bottom of a lake or the ocean, often over large distances and at high speeds, before failing and dispersing its load; such current occur when a turbid river discharges into the clear waters of a lake, or they can be initiated by a mudslide on a continental shelf. A dust storm may be considered the aeolian equivalent of a turbidity current.

Modes of erosion

Abrasion of rocks is caused by the sediments carried by wind and water: waves, for example, can hurl their seaload of sand and shingle against a cliff; sandstorms can literally sand-blast rocks; the sand and silt carried by rivers or turbidity currents have the same effect.

Attrition is the effect these same forces have on the sediments themselves, breaking them into smaller fragments or rounding the clasts into smooth pebbles or rounded grains of sand. The efficiency of this process can be observed anywhere you can find beach glass, which originates as sharp-edged shards; the process of tumble-polishing semi-precious stones artificially emulates this process and will render most pebbles well-rounded in a matter of days.

The simple mechanical force of water or ice can break off chunks of rock, as when glaciers quarry rocks from the surfaces over they move, or when the pounding of waves hammers against a cliff.

Gravity can break off the overhang of a cliff undercut by abrasion and wave pounding, when the rock at the top of the cliff is unable to bear the mechanical strain. It is also instrumental in causing such things as rockslides and mudflows; such downhill motion is known collectively as mass wasting.

Transport of sediment

Erosional processes, as we have said, are defined by their ability to transport sediment as well as to create the clasts of which it is composed.

Currents of wind or water can transport sediments in three ways: in suspension, where light particles are carried along in the current above the ground, sea bed, or river bed; by saltation, where particles too heavy to be carried in suspension are bounced along the ground (or river-bed, or whatever); and creep, where particles are rolled along the ground. The size of the particles susceptible to these processes will depend, of course, on the velocity of the current.

By contrast, the size of the clasts that can be carried by glaciers is under no such limitation. One of the characteristic results of glacial action is the transport of huge boulders, up to the size of a house, known as erratic boulders. Gravity, too, is obviously under no such limitation; it is possible for entire layers of rock to slide down a hillside.

How do we know?

In this particular case, asking "how do we know?" seems almost superfluous, for the processes involved are neither hidden nor subtle. We can observe a sandstorm; we can see how the head of a waterfall shifts year on year, or how a river shifting its course scours out a new bed; we can see how cliffs crumble and the effects of landslides. The fact that glaciers carry boulders is evident, and the distance they travel each year can be measured; as can the quantity of sediment discharged at the mouth of a river.

A more interesting question is, how do we know that these processes have happened when the agency that caused them is no longer present? How, for example, do we identify the courses of glaciers long since melted, or of rivers that have dried up or shifted their beds? These are questions that we shall review in subsequent articles.

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


(17)
Message 53 of 293 (640630)
11-11-2011 5:47 AM


Does This Still Have Your Attention?
BTW, is anyone still reading this? You could indicate as much by "liking" this post. Pressie and I were expecting more feedback.

Edited by Dr Adequate, : No reason given.


Replies to this message:
 Message 54 by jar, posted 11-11-2011 8:24 AM Dr Adequate has not yet responded
 Message 55 by bluescat48, posted 11-11-2011 8:07 PM Dr Adequate has not yet responded
 Message 56 by Pollux, posted 11-13-2011 11:19 PM Dr Adequate has not yet responded
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jar
Member
Posts: 30934
From: Texas!!
Joined: 04-20-2004


Message 54 of 293 (640637)
11-11-2011 8:24 AM
Reply to: Message 53 by Dr Adequate
11-11-2011 5:47 AM


Re: Does This Still Have Your Attention?
I know exactly how you feel. I saw the same thing in my threads on "How to make sand" and "Exploring the Grand Canyon from the bottom up".

But I for one am still following along. It's too bad that those who would really benefit don't seem to be here.


Anyone so limited that they can only spell a word one way is severely handicapped!

This message is a reply to:
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bluescat48
Member (Idle past 2267 days)
Posts: 2347
From: United States
Joined: 10-06-2007


Message 55 of 293 (640695)
11-11-2011 8:07 PM
Reply to: Message 53 by Dr Adequate
11-11-2011 5:47 AM


Re: Does This Still Have Your Attention?
I find it interesting, reacquainting myself to geology and more or less filling in the blanks to my geo knowledge. I haven't said anything since I had nothing to add nor did I have any negative views either. Thus I didn't say anything.

There is no better love between 2 people than mutual respect for each other WT Young, 2002

Who gave anyone the authority to call me an authority on anything. WT Young, 1969

Since Evolution is only ~90% correct it should be thrown out and replaced by Creation which has even a lower % of correctness. W T Young, 2008


This message is a reply to:
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Pollux
Member
Posts: 240
Joined: 11-13-2011


Message 56 of 293 (640878)
11-13-2011 11:19 PM
Reply to: Message 53 by Dr Adequate
11-11-2011 5:47 AM


re : Does This Still Have Your Attention
As a Geology tyro I am reading this with interest. I have been a regular visitor for three years and have learnt a lot, especially enjoying RAZD's thread on Age Correlations. So go for it Dr.A!
This message is a reply to:
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Dr Adequate
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Posts: 16085
Joined: 07-20-2006
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(4)
Message 57 of 293 (641668)
11-21-2011 12:04 PM


Chemical Weathering
Chemical weathering

Introduction

Chemical weathering is the breakdown of rocks into sediment by chemical processes.

The reader who does not appreciate chemistry may skip the chemical formulas in this article and simply note the results of the reactions, as described in the summary section below. The results themselves cannot be skipped over: understanding chemical weathering is essential to answering such basic questions as: "Why is sand mostly made of quartz?" and "Where does clay come from?"

Agents of chemical weathering

The main agents of chemical weathering are:

Water. Some minerals, such as rock salt, will dissolve readily in water; others such as pyroxene, will also do so, though at a much slower rate.

Carbonic acid. Rainwater and groundwater are not pure water; some of the molecules of water react with the carbon dioxide in the atmosphere (in the case of rainwater) or produced by bacteria and plant roots (in the case of groundwater) producing carbonic acid, as follows:

H2O (water) + CO2 (carbon dioxide) -> H2CO3 (carbonic acid)

Oxygen. This is a highly reactive chemical, and the only reason that there's so much of it about in the atmosphere is its constant production by biological processes and a shortage of things that it hasn't reacted with already.

Chemical weathering of common minerals

In this section we shall look at how some common minerals are affected by chemical weathering. We have arranged the list more or less in order from the minerals most susceptible to chemical weathering to the most resistant.

Halite. Salt, of course, dissolves in water. This is why you are unlikely to see rock salt on the surface except in desert environments.

Gypsum. This, like halite, is soluble in water; similar remarks apply to it.

Calcite. This, you will recall, is the mineral forming limestone and its metamorphic counterpart, marble. It can just dissolve in water; it also reacts with carbonic acid as follows:

CaCO3 (calcite) + H2CO3 (carbonic acid) -> Ca2+ + 2(HCO3)- (dissolved ions of calcium and bicarbonate).

The ease with which limestone dissolves (relative, at any rate, to other minerals) produces the distinct topography of a region built on limestone rocks, with underground caves full of stalactites; sinkholes where the land has subsided; streams disappearing into the ground or rising out of it as springs. This is known as karst topography.

This is also the reason why a marble tombstone, though handsome in appearance, is not a good long-term investment.

Silicate minerals. Of silicate minerals in general we may observe that the more mafic minerals with higher melting points (those to be found to the right of our diagram in the article on igneous rocks) are more susceptible to chemical weathering than felsic low-temperature minerals. This will be reflected in the order in which we list them below.

Olivine. This mafic mineral has the formula (Mg,Fe)2SiO4. Recall that the (Mg,Fe) in the formula means that it is a solid solution in which varying quantities of magnesium or iron can play the same chemical role. It reacts with carbonic acid as follows:

(Mg,Fe)2SiO4 (olivine) + 4H2CO3 (carbonic acid) -> 2(Mg,Fe)2+ + 4HCO3- (dissolved ions of magnesium/iron and bicarbonate) + H4SiO4 (silicic acid).

As with limestone, the constituent parts of the mineral are now entirely dissolved in water, leaving no residual mineral.

On the other hand, iron olivine can react with water and atmospheric oxygen like this:

2Fe2SiO4 + 4H2O + O2 -> 2Fe2O3 + 2H4SiO4

And the hematite can further react with water as follows:

Fe2O3 + H2O -> 2FeO(OH) (goethite)

Hematite and goethite are both very insoluble in water: they remain as residual minerals. It is these iron oxides that give many soils their reddish or yellowish color.

Calcium feldspar. Chemical weathering turns calcium feldspar into clay (a residual mineral, since clay does not dissolve in water) and leached ions, which dissolve in water and which usually will end up contributing to the dissolved mineral content of seawater. The reaction is as follows:

CaAl2Si2O8 (calcium feldspar) + 2H2CO3 (carbonic acid) + H2O (water) -> Al2Si2O5(OH)4 (kaolinite clay) + Ca2+ (calcium ion) + 2HCO3- (bicarbonate ions)

Pyroxene. The typical rock-forming pyroxenes have the formula (Mg,Fe)SO3. This can react with carbonic acid as follows:

(Mg,Fe)SiO3 + H20 + 2H2CO3 -> (Mg,Fe)+2 + 2HCO3- + H4SiO4

Again, as with olivine, the constituent parts of the mineral are dissolved in water. However, as with olivine, iron pyroxene can react with oxygen and water to produce the residual mineral hematite:

2FeSO3 + 4H2O + 2O2 -> 2Fe2O3 + 2H4SiO4

Again, the hematite can turn to goethite.

Mica and amphibole, minor constituents of felsic and intermediate rocks, undergo rather more complicated reactions. (Details may be found here for biotite, a mica, and here for amphibole.)

In summary, the residual minerals produced are clay minerals; iron oxides; and, in the case of biotite, the mineral gibbsite (Al(OH)3), which is usually found in association with clay.

Potassium and sodium feldspars produce residual clay minerals. Here, for example, is the reaction by which potassium feldspar produces kaolinite (plus various dissolved substances):

4KAlSi3O8 (K feldspar) + 4H2CO3 (carbonic acid) + 2H2O (water) -> Al4Si4O10(OH)8 (kaolinite) + 4K+ (potassium ions) + 4HCO3- (bicarbonate ions) + 8SiO2 (dissolved quartz)

As sodium is chemically almost indistinguishable from potassium (since they both have the same valance) potassium feldspar reacts in a similar way.

Quartz is the most stable of the silicate minerals. This is why quartz sand is so common as a sediment; when all the other constituents of igneous rocks have either dissolved or been converted to clay, grains of quartz will remain. This is why there is no such thing as a large grain of sand: the maximum size of such grains is limited by the size of the quartz crystals that form in granite and similar rocks.

Note, however, that quartz sandstone is vulnerable to chemical weathering, because although the grains of quartz themselves are resistant, the minerals cementing them together may not be.

Clay minerals are very resistant to chemical weathering, because they are, as we have seen, a product of chemical weathering, and, like all minerals, they are stable under the conditions under which they were formed.

Iron oxides. These, as we have seen, are a product of chemical weathering of iron-bearing forms of such mafic minerals as olivine and pyroxene. We have noted that hematite can be converted to goethite by oxidation:

Fe2O3 + H2O -> 2FeO(OH)

Once iron oxide has formed, there is very little that can happen to it except conversion to another sort of iron oxide; these are regarded as the most stable of all common classes of minerals.

Summary

The residual minerals left after chemical weathering has done its work are quartz, clay minerals, a scattering of ion oxide, and sometimes a little gibbsite. The other constituents of minerals are dissolved; their usual fate is to be carried by rivers to the sea, where they contribute to the dissolved mineral content of seawater.

We may note that most land-formed sediments are in fact quartz sand, clay, or a mixture of the two. This demonstrates the predominance of chemical weathering over mechanical weathering and erosion. If sand or mud were produced simply by mechanical crushing of granite, then they would be 60% feldspar; but they are not. When we find any appreciable amount of feldspar in sand (as in arkose sandstone), we may infer that there has been a higher than usual ratio of mechanical to chemical processes.

How do we know?

The processes of chemical weathering are sufficiently slow that it is reasonable to wonder how we know that they take place at all.

In the first place, we know that they ought to take place. According to the theory of chemistry, in the chemical equations given above, under the conditions in which chemical weathering takes place, the situation described by the right-hand side of the equation are more thermodynamically stable than the situations described on the left hand side; so the reactions described should happen.

We can speed up reactions which, in nature, involve weak, highly-diluted carbonic acid, and instead use something stronger, such as hydrochloric acid (HCl). In principle the only difference this should make (besides, obviously, the substitution of chloride ions for bicarbonate ions) is that as hydrochloric acid gives up its hydrogen ions more readily, the reaction will go faster. So, for example, we can use hydrochloric acid to convert potassium feldspar to clay as follows:

4KAlSi3O8 (K feldspar) + 4HCl (hydrochloric acid) + 2H2O (water) -> Al4Si4O10(OH)8 (kaolinite) + 4K+ (potassium ions) + 4Cl- (chloride ions) + 8SiO2 (quartz)

And this happens fast enough for us to observe it happening. Alternatively, to experiment under more natural conditions, you can bury samples of minerals in a nice moist acidic soil, where chemical weathering occurs fastest, leave them for a few years, and see what's happened to them; they will not weather completely, but if the process involves conversion of one mineral to another, rather than just dissolution, then the chemical changes are observable on the surface of the mineral.

Without any of these artifices, we can find naturally occurring rocks which appear to be in the process of weathering: for example, the exterior of such mafic rocks as basalt can often be seen to be rusty, as a result of the iron pyroxene being converted to iron oxides. In the same way, granite boulders can be found with a weathering rind, where on the outside of the boulder the feldspar on the outside has been partly converted to clay, while the feldspar on the inside is still relatively intact. In tropical soils, it is possible to find granite boulders in which the feldspar has become so "rotten" with clay that it is literally possible to kick the boulder to pieces. Under a microscope, the feldspar crystals will appear corroded and pitted. Or if a road cut or railroad cut goes through a hill (an event which always delights geologists) then since the rocks near the top are more weathered, we can take a series of samples going up through the vertical section from unaltered rock to completely weathered rock (saprolite) as in this study of the weathering of biotite.

We can observe the effect of weathering on old tombstones or on dressed stones used for building; as you would expect, this is most noticeable in the case of limestone or marble. In such cases we can usually be quite certain that the stones in question have suffered little from merely physical processes such as abrasion by sandstorms, tidal action, transport in rivers, and so forth.

Finally, we may note that the processes we have described explain the nature of sediment: they explain why so much of it is quartz sand and clay; they explain, as we have seen, the size of sand grains; they explain the origin of the minerals that cement together the grains in sandstone; and they explain the origins of the minerals found in sea water as dissolved ions.

In summary: these processes ought to happen; we can simulate them happening; and what we see in nature is just what we should see if they did happen.

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.


Replies to this message:
 Message 58 by Pressie, posted 11-22-2011 1:49 AM Dr Adequate has responded

  
Pressie
Member
Posts: 1998
From: Pretoria, SA
Joined: 06-18-2010
Member Rating: 2.8


Message 58 of 293 (641726)
11-22-2011 1:49 AM
Reply to: Message 57 by Dr Adequate
11-21-2011 12:04 PM


Re: Chemical Weathering
Dr Adequate,

This is a very difficult subject, but you summerized it very well.

Of course there are so many chemical reactions happening in "rocks" coming into contact with phenomena like acids, other "rocks", fluids, heat, pressure, etc. , that it is impossible to summarize them all in one post. The lesson to learn, I think, is that chemical weathering happens according to chemistry. We can study the "rocks" and the chemical reactions they part take in and we can study the products of said chemical reactions.

Thank you.


This message is a reply to:
 Message 57 by Dr Adequate, posted 11-21-2011 12:04 PM Dr Adequate has responded

Replies to this message:
 Message 60 by Dr Adequate, posted 11-22-2011 12:50 PM Pressie has not yet responded

    
Larni
Member
Posts: 3975
From: Liverpool
Joined: 09-16-2005


Message 59 of 293 (641744)
11-22-2011 6:54 AM
Reply to: Message 53 by Dr Adequate
11-11-2011 5:47 AM


Re: Does This Still Have Your Attention?
I've got nothing to contribute but it's really interesting. Takes my right back to doing A Level Geoggers!

The above ontological example models the zero premise to BB theory. It does so by applying the relative uniformity assumption that the alleged zero event eventually ontologically progressed from the compressed alleged sub-microscopic chaos to bloom/expand into all of the present observable order, more than it models the Biblical record evidence for the existence of Jehovah, the maximal Biblical god designer.
-Attributed to Buzsaw Message 53

Moreover that view is a blatantly anti-relativistic one. I'm rather inclined to think that space being relative to time and time relative to location should make such a naive hankering to pin-point an ultimate origin of anything, an aspiration that is not even wrong.

Well, Larni, let's say I much better know what I don't want to say than how exactly say what I do.


This message is a reply to:
 Message 53 by Dr Adequate, posted 11-11-2011 5:47 AM Dr Adequate has not yet responded

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


Message 60 of 293 (641804)
11-22-2011 12:50 PM
Reply to: Message 58 by Pressie
11-22-2011 1:49 AM


Re: Chemical Weathering
Of course there are so many chemical reactions happening in "rocks" coming into contact with phenomena like acids, other "rocks", fluids, heat, pressure, etc. , that it is impossible to summarize them all in one post. The lesson to learn, I think, is that chemical weathering happens according to chemistry. We can study the "rocks" and the chemical reactions they part take in and we can study the products of said chemical reactions.

Quite so. I have in fact gone into much more detail than is usual in an introductory textbook. I feel that this is in line with my intention to explain "how do we know". I could have been much shorter, at the cost of making it seem that primary rocks turned into clay and quartz sand by magic --- but I think it's important to explain why this is so.

Thank you.

Here Pressie is being needlessly modest: I know, and the readers of this thread should know, that he worked very hard behind the scenes to help check the facts for this particular article.


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Replies to this message:
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