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Author Topic:   Introduction To Geology
Artemis Entreri 
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Posts: 1194
From: Northern Virginia
Joined: 07-08-2008


Message 106 of 293 (658842)
04-10-2012 10:16 AM
Reply to: Message 103 by Taq
04-09-2012 6:09 PM


Re: On Holiday
Near Pocatello is the Craters of the Moon national park where you can check out massive lava fields:

http://en.wikipedia.org/...on_National_Monument_and_Preserve

I wish i knew that back then. I just remember driving up I-15 (out of salt lake city), and thinking, "dang Idaho is much flatter than I thought (we were in the snake river plain), and a ton of agriculture, Dang that is a lot of hops and where are all the taters?"

thanks for the links.


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RAZD
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Posts: 20326
From: the other end of the sidewalk
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Message 107 of 293 (659014)
04-11-2012 6:18 PM
Reply to: Message 103 by Taq
04-09-2012 6:09 PM


Craters of the Moon
Hi Taq,

Near Pocatello is the Craters of the Moon national park where you can check out massive lava fields:

http://en.wikipedia.org/...on_National_Monument_and_Preserve

Also has some magnificent cinder cones with paths to climb.

(And also a great place to see kestrels hovering and awesomely blue mountain bluebirds)

Enjoy.


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Dr Adequate
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Message 108 of 293 (659586)
04-17-2012 2:12 AM


Reefs
Reefs

Introduction

In this article we shall consider the formation of reefs, and discuss how we can recognize reefs in the geological record.

What is a reef?

A reef, to a geologist, is what you get when macroscopic organisms which secrete skeletal matter (hard corals, for example, or oysters) grow on top of one another, forming a mass of skeletal material in relief from the sea bed.

Since the skeletons of reef-forming organisms are invariably constituted of calcium carbonate, reefs are by definition limestone.

Note that this definition of "reef" is different from that which would be used by sailors, to whom a reef is any submerged hazard.

Note also that the definition does not just include coral, the main group of reef-producing organisms today, but rather includes any organism with this form of growth, including a number of extinct organisms, some of which we shall list below.

Reef-forming organisms

* Archaecyathids are the first known reef-builders: goblet-shaped organisms typically a few centimeters in size which should probably be classed as calcite-secreting sponges. They flourished in the early Cambrian, began to decline by the middle Cambrian, and appear to have become completely extinct by the end of the Cambrian period.

* Stromatoporoids are a group of hard-bodied (i.e. calcium carbonate secreting) sponges which were important reef-formers from the Ordovician to the Silurian periods. While they are not extinct, they have been displaced to marginal habitats by later and more successful groups of reef-forming organisms.

* Rudist bivalves are a group of molluscs that flourished in the Jurassic and Cretaceous periods. During the Cretaceous they replaced corals in many environments as reef-builders, forming reefs sometimes a hundred meters high and hundreds of kilometers long, until they went extinct, like so many other organisms, at the Cretaceous-Triassic boundary.

* Corals, despite their plantlike appearance, are animals closely related to sea anemones. Many of them secrete a skeleton of calcium carbonate, forming the main constituent of most modern reefs.

* Oysters are the bivalves familiar to gourmets and pearl-divers; they are capable of forming reefs, although not so large and spectacular as those formed by coral.

Reefs: how do we know?

It is easy to recognize a former reef in the geological record, since they are still clearly formed from the skeletal remains of coral, bivalves, stromatoporoids, etc. Their biological origin is therefore indisputable.

We might, however, if we were exceptionally cautious, ask ourselves whether the reef-shaped deposits of such remains are really reefs. Conceivably, they are piles of debris transported from elsewhere and then deposited in these formations as what we might call "pseudoreefs".

Like everything else in geology, this question had to be thought about at one point. In the days of Leonardo da Vinci, it was suggested that the oyster reefs and corals discovered inland in his native Italy had been transported there by Noah's Flood (da Vinci disagreed); and if no sensible person today entertains that conjecture, this is only because it has been considered and found to be wrong. For a number of objections occur:

(1) The positions in which the reef-forming organisms are found are the same as they would have in life. Now this is a powerful objection: there is no reason why any sort of transport or deposition forming pseudoreefs should have deposited the skeletal remains in their natural poses.

(2) There is no known mechanism by which water can pile up debris in the shape of a reef rather than spread it over a wider area (let alone give the transported sediment the configurations found in living organisms).

While the absence of a mechanism is not always a fatal blow to a hypothesis, it is certainly a point against it, especially as we have a perfectly good mechanism of reef formation to back up the theory that the things that look like reefs are in fact reefs.

(3) This hypothetical mechanism would have to be curiously selective in its action. We find apparent reefs built from oysters and corals, which we know from direct observation build reefs. We do not find "pseudoreefs" built out of (for example) the shells of crabs, or the cuttlebones of cephalopods. That is, we never find that this hypothetical mechanism has built reefs out of any organism known not to build reefs. This must cast doubt on the existence of such a mechanism.

A puzzle and a solution

Most varieties of coral are constrained by their biology to be shallow-water organisms, which cannot survive if they are more than a few meters below the surface of the sea. For this reason, corals of this type are invariably found, not at the bottom of the ocean, but at the margins of continents or islands.

But what initially presents a paradox is this: sometime the islands on which shallow-water coral reefs grow are themselves made of shallow-water coral, and to a very great depth. For example, when the U.S. Atomic Energy Commission drilled at Eniwetok Atoll they found 1405 meters of shallow-water coral reef before striking basalt.

This seems, on the face of it, biologically impossible. These kinds of coral cannot grow at such depths, and therefore shouldn't be there. This puzzle was solved by a young man named Charles Darwin, later to achieve greater eminence in the field of biology. His solution seems obvious in retrospect: the islands must have originally been either above the surface of the sea, or within a few meters of it, and must then have sunk beneath the sea at a rate slower than that at which coral can grow. This seems obvious now, as I say, but it was less obvious in Darwin's time, when the idea that geological phenomena were produced by gradual changes over long periods was a new and revolutionary concept.

In our articles on plate tectonics we shall explain why we would expect oceanic islands to sink gradually beneath the surface; for now we shall merely observe that by all appearances Darwin was correct.

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.


Replies to this message:
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 Message 115 by Pollux, posted 04-20-2012 9:15 AM Dr Adequate has responded

  
Taq
Member
Posts: 8207
Joined: 03-06-2009
Member Rating: 3.9


Message 109 of 293 (659611)
04-17-2012 11:43 AM
Reply to: Message 108 by Dr Adequate
04-17-2012 2:12 AM


Re: Reefs
Corals are constrained by their biology to be shallow-water organisms. Some can grow as deep as 50 meters, some will perish in depths greater than 1 meter, but all require relatively shallow water.

There are deep water corals:

http://en.wikipedia.org/wiki/Deep-water_coral

However, they are different from shallow water reefs.


This message is a reply to:
 Message 108 by Dr Adequate, posted 04-17-2012 2:12 AM Dr Adequate has responded

Replies to this message:
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Dr Adequate
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Posts: 16107
Joined: 07-20-2006
Member Rating: 8.3


Message 110 of 293 (659616)
04-17-2012 12:28 PM
Reply to: Message 109 by Taq
04-17-2012 11:43 AM


Re: Reefs
You're quite right. I can't think how I managed to avoid finding that out. I shall amend the article accordingly. Thank you.

This message is a reply to:
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Taq
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Posts: 8207
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Member Rating: 3.9


Message 111 of 293 (659630)
04-17-2012 2:57 PM
Reply to: Message 110 by Dr Adequate
04-17-2012 12:28 PM


Re: Reefs
You're quite right. I can't think how I managed to avoid finding that out. I shall amend the article accordingly. Thank you.

It is true that shallow water corals do depend on sunlight because they carry endosymbionts called zooxanthellae. These are photosynthetic flaggelated protozoans that provide the host with energy (e.g. glucose). However, corals are in the phylum Cnideria which also includes jellyfish, hydrozoa, and sea anemones. They are capable of producing energy of their own, and that is exactly what deep sea corals do by feeding on zooplankton and small invertebrates. Like jellyfish and hydrozoa, they have stinging nematocysts that they use to stun their prey.

It would be accurate to say that specific species of coral can only live within the daylit portion of the ocean due to their dependence on photosynthesis.


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Dr Adequate
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Posts: 16107
Joined: 07-20-2006
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(1)
Message 112 of 293 (659692)
04-18-2012 4:48 AM


Ooids And Oolite
Ooids and oolite

Introduction

In this article we will discuss the formation of ooids, and how to recognize oolitic limestone in the geological record.

Ooids and oolite

An ooid consists of a nucleus (a fragment of shell, a grain of sand, or whatever) around which layers of minerals are deposited to form roughly spherical grains.

These grains are typically between 0.25mm and 2mm in diameter; in fact, some authors use a different term for ooids of different sizes, but in this article we shall use the word "ooid" as a catch-all term.

Although there are a number of minerals which can form ooids, in this article we are interested in ooids formed from calcium carbonate, and from now on we shall confine our discussion to them. Such ooids are typically formed in water rich in calcium carbonate (for obvious reasons) and for preference warm shallow water agitated by waves.

Today ooids are to be found in a number of locations with warm shallow water, including the Bahamas, Shark Bay in Australia, and the Arabian Gulf, all of which are marine sites; but they are also sometimes found in inland waters such as the Great Salt Lake in Utah.

Like other small grains, ooids can be cemented together to form a kind of rock. Rock formed from carbonate ooids is, by definition, a form of limestone, and is known as oolitic limestone or oolite. The term oolith may also be used as a term either for the rock or for an individual ooid.

Oolite: how do we know?

It is very easy to distinguish limestone formed from ooids. Magnified, it looks like it is made of ooids, as shown below. (The limestone in this photograph and the next is from the Carmel Formation in southern Utah.)

This resemblance might be fortuitous, but any such conjecture is laid to rest by looking through a microscope at a cross-section of oolite.

Not only are the grains the right size and shape to be oolites, but in cross-section one can see the nuclei around which they formed and the growth patterns typical of oolites. Plainly, then, what we are looking at is ooids cemented together to form oolite.


  
RAZD
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Posts: 20326
From: the other end of the sidewalk
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Message 113 of 293 (659749)
04-18-2012 12:36 PM
Reply to: Message 108 by Dr Adequate
04-17-2012 2:12 AM


Re: Reefs
Hi Dr Adequate,

Reef-forming organisms

So do the shell deposits of foraminifera and diatoms constitute reefs? (the cliffs of Dover?) or is this a different phenomenon (ooids?)?

Reef-forming organisms

You could also have a combination of sessile organisms in a reef environment (such as barnacles, muscles and brachiopods, yes?

Reefs: how do we know?

Another way would be that upper layers grow attached to lower layers, rather than in loose piles. Not just corals (where this is rather obvious), but in oysters (as you mention) and brachiopods grow attached to a substrate, and in a mature ecology this substrate consists of previous generations of these organisms. Some of the extinct brachiopods found on Mt Everest were fossilized attached to their substrates, including other brachiopods, iirc.

http://en.wikipedia.org/wiki/Brachiopod

quote:
... In a typical brachiopod a stalk-like pedicle projects from an opening in one of the valves, known as the pedicle valve, attaching the animal to the seabed but clear of silt that would obstruct the opening. ...
... Brachiopods live only in the sea, and most species avoid locations with strong currents or waves. Articulate species have larvae that settle in quickly and form dense populations in well-defined areas, ...

These stalks are rather fragile, so fossil preservation of intact stalks indicates that the organisms were not disturbed after death, but buried gradually by natural silting processes and the growth of other generations of organisms.

Enjoy.

Edited by RAZD, : ooids?


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This message is a reply to:
 Message 108 by Dr Adequate, posted 04-17-2012 2:12 AM Dr Adequate has responded

Replies to this message:
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Dr Adequate
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Posts: 16107
Joined: 07-20-2006
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(1)
Message 114 of 293 (659754)
04-18-2012 1:25 PM
Reply to: Message 113 by RAZD
04-18-2012 12:36 PM


Re: Reefs
So do the shell deposits of foraminifera and diatoms constitute reefs? (the cliffs of Dover?) or is this a different phenomenon (ooids?)?

Neither. They're not ooids 'cos of not being ooids, and they're not reefs 'cos they're not macroscopic and they don't stand in relief from the sea bed.

My next article will be about calcareous ooze.

Edited by Dr Adequate, : No reason given.


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Pollux
Member (Idle past 32 days)
Posts: 251
Joined: 11-13-2011


Message 115 of 293 (660007)
04-20-2012 9:15 AM
Reply to: Message 108 by Dr Adequate
04-17-2012 2:12 AM


Re: Reefs
Hi Dr Adequate,
I am appreciating your efforts in this course of instruction.
I note that the report of the core at site 866 does not mention a lot of coral but certainly a lot of shallow water organisms. Does this still qualify as a coral reef? Is it because coral forms the substrate for the other critters to grow on?

This message is a reply to:
 Message 108 by Dr Adequate, posted 04-17-2012 2:12 AM Dr Adequate has responded

Replies to this message:
 Message 116 by Dr Adequate, posted 04-20-2012 2:10 PM Pollux has responded

  
Dr Adequate
Member
Posts: 16107
Joined: 07-20-2006
Member Rating: 8.3


Message 116 of 293 (660051)
04-20-2012 2:10 PM
Reply to: Message 115 by Pollux
04-20-2012 9:15 AM


Re: Reefs
You're right. That'll teach me to use secondary sources. And misinterpret them, and generally be an idiot. The guyot has corals in it, but as you say it's certainly not one big reef. I shouldn't have gone with a Cretaceous guyot. It anyone would like to suggest a better example, that would be nice.

This message is a reply to:
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Pollux
Member (Idle past 32 days)
Posts: 251
Joined: 11-13-2011


(1)
Message 117 of 293 (660102)
04-20-2012 11:52 PM
Reply to: Message 116 by Dr Adequate
04-20-2012 2:10 PM


Re: Reefs
Daniel Wonderly in "Coral reefs and related carbonate structures as indicators of great age" describes Enewetak as a coral-algal reef so I guess that qualifies. In its 4610 feet of reef structure it has erosion surfaces at 300, 1000,and 2280 feet showing periods when sea level fell faster than subsidence was occurring. To fit all this coral growth, with erosion periods, into the time since the Flood as YEC try requires some imagination!

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Dr Adequate
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Member Rating: 8.3


(3)
Message 118 of 293 (660295)
04-24-2012 1:36 AM


Calcareous Ooze
Calcareous ooze

Introduction

In this article we shall discuss the formation of calcareous ooze by carbonate-secreting organisms; the mechanisms which control the distribution of this ooze (such as the supply of nutrients and the carbonate compensation depth); and we shall look at how rocks are formed from the ooze and how we can recognize such rocks when we see them.

Calcareous ooze: what is it?

Calcareous ooze is a calcium carbonate mud formed from the hard parts (tests) of the bodies of free-floating organisms. Once this mud has been deposited, it can be converted into stone by processes of compaction, cementation, and recrystallization.

The main contributors to the ooze are coccolithophores and foraminifera. Coccolithophores are tiny single-celled organisms which cover themselves with tiny plates of calcite known as coccoliths. Foraminifera are also single-celled organisms. In some species this single cell will grow to be several centimeters in diameter, but most species of foraminifera are less than 1mm in diameter. While some will produce shells by gluing available sediment together, or by secreting shells from silica dissolved in seawater, most produce shells of calcite.

The image below shows, on the left, a (false-color) image of a coccolithophore, and on the right the test of a foraminiferan.

The deposition of foraminifera is generally more common today and in most regions; however, this varies from place to place and from time to time; coccoliths, for example, are more common in the ooze on the floor of the Mediterranean, and are also more common in rocks dating from the early Tertiary period.

Excepting a few odd species of foraminifera, these organisms float or swim near the surface of the ocean. When they die, they sink. Perhaps "sink" is too strong a word: they are sufficiently small that they drift gently down like dustmotes through air, and can take months to hit bottom. The ooze composed of their hard parts accumulates at a rate of about 10mm - 50mm per thousand years, varying from location to location; which doesn't sound like much, but is actually a faster rate than other marine sediments such as siliceous ooze or pelagic clay.

Since calcareous ooze is formed from the hard parts of the bodies of free-floating organisms, this means that unlike ooids, which are nearshore sediments, and unlike reefs, which require shallow water, calcareous ooze can be deposited over vast swathes of the deep ocean floor.

However, calcareous ooze will not accumulate in the very deepest parts of the ocean, even if the surface is teeming with the right sort of organism. The reason for this will be discussed in the next section of this article.

The CCD

Calcium carbonate will dissolve in the presence of carbon dioxide and water, as follows:

CaCO3 + H2O + CO2 -> Ca2+ + 2HCO3-
calcium carbonate + water + carbon dioxide -> one calcium ion and two bicarbonate ions

Readers familiar with chemistry will not be surprised to learn that this is the reverse of the process by which calcium carbonate precipitates. The question of which will happen, dissolution or precipitation, depends on the relative abundance of calcium ions, bicarbonate ions, calcium carbonate, and carbon dioxide present. To cut the chemistry short, we may say that where carbon dioxide is scarce, precipitation will take place, and when it is abundant, calcium carbonate will dissolve.

Now, colder and deeper water contains more carbon dioxide than shallower and warmer water. The calcium carbonate compensation depth (or carbonate compensation depth, or CCD) is the depth at which the concentration of carbon dioxide is sufficiently high that calcium carbonate is dissolved faster than it can settle.

To speak of "the" CCD as though it was one specific depth in the ocean is rather misleading: there are other factors besides depth which affect this issue. First, there is temperature: cold water will hold more carbon dioxide than warm water, and so the CCD will be deeper in warm water. And secondly, there is the fertility of the water. For the reader should always bear in mind that the carbonate compensation depth is not the depth at which calcium carbonate dissolves; rather, it is the depth at which calcium carbonate dissolves faster than it is deposited. We should also note that as aragonite is more unstable than calcite, it dissolves rather more readily, so the type of calcium carbonate being deposited also plays a role, and we should properly distinguish between the calcite compensation depth and the aragonite compensation depth.

These caveats aside, we may say that the CCD is about 4500 meters down, give or take a few hundred meters either way.

The fact that this form of chemical weathering takes place has been confirmed experimentally, by scientists who took perfectly machined spheres of calcium carbonate and left them for a year at various depths on the ocean floor. Those in shallow waters showed no signs of weathering; those that were left in deeper waters were found to be pitted and corroded as a result.

The existence of the CCD explains the rather curious pattern of deposition of calcareous ooze on the ocean floor, as seen in the map below, where areas where calcareous ooze predominates are marked in yellow. Three things are required for this to be the main sediment: first, there must be sufficient nutrients for calcite-forming organisms to flourish; second, the ocean floor must be above the carbonate compensation depth; third, there must not be the right conditions for other sediments to swamp the deposition of calcareous ooze.

There is one point that we should emphasize: calcium carbonate below the CCD will not dissolve immediately, like an Alka-Selzer tablet fizzing away in water. The rate at which it dissolves is rather slow. It doesn't need to be fast, it just needs to be faster than the rate at which calcium carbonate is deposited. This point will be significant when we consider the evidence for plate tectonics in a later article.

Rocks from calcareous ooze: how do we know?

We should first sort out a small matter of vocabulary, Chalk might be defined as a stone which is, under a microscope, visibly composed of the tests of microorganisms. It differs from calcareous ooze itself by a degree of lithification and cementation that converts it from mud to rock.

From the nature of its composition, it is by definition limestone. However, many experts on marine carbonate sediments will distinguish between chalk and what they are pleased to call "limestone", by which they mean a rock which has undergone more extensive recrystallization so that its origin as tests has been largely or completely obscured. We shall continue to regard chalk as a form of limestone, but anyone who wishes to read further on the subject should be aware that the distinction may be made.

If we ask, then, how we know that marine limestone is formed from calcareous ooze, half the question is already answered: we know that chalk is formed from calcareous ooze because it is still visibly formed from tests.

In more completely recrystallized limestone, however, such visible tests may be few and far between. Are we really entitled to say in such cases that the parent material was chiefly calcareous ooze?

The answer is yes. First of all, consider the question of mechanism: we expect time and burial at depth to produce recrystallization in chalk; and we have no alternative mechanism that would explain the production of such limestone.

This is somewhat of a negative argument. A more positive argument is produced by deep sea drilling. Geologists have taken core samples which progress from loose calcareous ooze at the top through "stiff", compacted ooze, to chalk, with progressively greater dissolution, recrystallization and filling of pore spaces, to limestone in which "all detailed nannofossil morphology is lost near the base as sediment becomes almost totally recrystallized". (See here for further details.)

We can therefore suppose either that the transition upwards from limestone to ooze represents a gradual change in the process of deposition, from a process as yet undiscovered to the observable deposition of tests; or that the process of deposition was the same throughout but that the more deeply buried sediments have been affected to a greater degree by known processes, namely compaction, dissolution, and recrystallization. The latter hypothesis, being more parsimonious, is clearly to be preferred.

Edited by Dr Adequate, : No reason given.


  
Dr Adequate
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(3)
Message 119 of 293 (661099)
05-02-2012 4:40 AM


Siliceous Ooze
Siliceous Ooze

Introduction

In this article we shall consider the origin, deposition and lithification of siliceous ooze.

Siliceous ooze

As with calcareous ooze, siliceous ooze is formed from the tests of microorganisms; in the case of siliceous ooze, the organisms come in two varieties, diatoms and radiolarians. In both cases the silica forming the tests is in the amorphous form known technically as opal. To be classed as siliceous ooze, sediment needs to be composed at least 30% of this material.

The below are photomicrographs of, on the left, a diatom, and on the right a radiolarian. Neither should be considered a typical representative of the type, since there is really no such thing: both diatoms and radiolarians exhibit a wide variety of forms.

The map below shows the distribution of siliceous ooze in green.

The tropical siliceous ooze is dominated by radiolarians; the bands at higher latitudes are dominated by diatoms.

It is estimated that the very small tests of these organisms would take 20 to 50 years to drift down to the sea floor; however, they can descend more rapidly in the form of the fecal pellets of the organisms which browse on diatoms and radiolarians. The rate of deposition of siliceous ooze is about 10 mm per thousand years.

The opal compensation depth

As with calcareous ooze, there is a depth below which siliceous ooze will be dissolved faster than it can be deposited, known as the opal compensation depth. However, silica is more resistant to dissolution than calcium carbonate, and the depth is correspondingly deeper: approximately 6000 meters.

Rocks from siliceous ooze: how do we know?

In some cases it is very easy to relate siliceous rocks to siliceous ooze. Diatomite, for example, when viewed under a microscope, is quite clearly made of diatoms; it might be described as the siliceous equivalent of chalk. The photomicrograph below shows crumbled fragments of diatomite: it is unmistakably composed of diatoms.

Marine chert, however, is at first glance more enigmatic: its structure consists of very fine crystals of silica, and hints of its biological origin have largely been lost, raising the question of how we know that it did in fact have a biological origin. The question (and indeed the answer) is similar to the question we raised about marine limestone in the previous article. In this section we shall briefly review the evidence suggesting that the silica of which marine chert is composed has its origins as siliceous ooze.

First of all, note that chert is just what we would expect to get if time, pressure, dissolution and reprecipitation caused recrystalization of silicious ooze. Marine chert requires a source of silica; siliceous ooze provides a source of silica. Even if we had no other relevant evidence, we should preferably ascribe the origin of bedded chert to siliceous ooze rather than hypothesizing some other origin for the silica as yet undiscovered.

More direct clues are revealed by the circumstances under which chert is found. Taking samples of rock from the same site at varying depths (an example is given here on p. 575) then we may for instance find layers of clay; of clay rich in radiolarians; of radiolarians filled and cemented with silica; and of true chert. Now it would take a stretch of the imagination to suppose that the silica composing the chert has quite a different origin from the silica in the other layers; especially as there is no good hypothesis as to what that origin could be.

Furthermore, in some cases when chert reveals no visible organic structures, treatment of the chert with hydrofluoric acid reveals radiolarian structures in the chert, presumably because the silica of the matrix has a rather more soluble structure than that of the radiolarians (see here for further details). Now, it would be possible to argue that these radolarians are "incidental": that they just happened to get buried in some siliceous sediment other than siliceous ooze. However, in the light of the other considerations we have mentioned, this seems unlikely.

Some have argued that there must be at least some alternative origin for chert, on the grounds that Precambian cherts are known, and these precede the evolution of radiolarians and diatoms. However, study of these Precambian cherts reveals microscopic spherical ornamented structures (see here for further details). If some of these are the shells of extinct silica-secreting organisms then we might think it probable that the Precambrian cherts were formed from siliceous ooze composed of these tests.

On the other hand, we should note that if there were no organisms in the Precambrian seas that secreted silica, then the oceans would have had a much higher concentration of silica than they do today, and perhaps under these conditions non-biological deposition of silica might have been possible by processes which could not operate in modern conditions.

However this may be, there seems little to disturb the consensus that more recent cherts have a biological origin. This is not to say that the topic of chert is without its controversies, but these fall outside the scope of an introductory article such as this one.


  
Dr Adequate
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Posts: 16107
Joined: 07-20-2006
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Message 120 of 293 (664367)
05-31-2012 12:53 PM


Sorry For The Delay
Pressie hasn't been about for a couple of weeks to review my articles. I know that sometimes his geological duties force him to go and drill things. Hopefully he'll be back soon.

In the meantime, here's a picture of Fly Geyser.

Edited by Dr Adequate, : No reason given.


  
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