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Author Topic: What is This And Why All The Layers ?  (Read 12686 times)

Offline neilep

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What is This And Why All The Layers ?
« on: 03/09/2008 12:17:49 »
Dear Sirs/Madams/Inbetweeners/Other

See this stripey rock thing ?






Nice eh ?

What's it all about eh ?..what is it ?...why the stripey things ? and why the alternative light and darkness of it all ?

I don't know...I'd like to know and I want ewe to tell me so that I can know !

Thank ewe


hugs and shmishes


Neil
Stripey Stoney Thingy Asker

xxxxxxxxxxxxxxxxxxxxxxx
mwah mwah mwah mwah





 

Offline Bass

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What is This And Why All The Layers ?
« Reply #1 on: 03/09/2008 16:42:22 »
What the schist?

Oh, sorry, gneiss rock specimen.

The layers are formed in a metamorphic rock, probably a schist.  Original rock (and layering) probably shale with sandy layers.  As the rock is heated and subjected to intense pressure, the minerals are changed, or metamorphosed.  In a schist, the clay minerals that make up the original mudstone are changed into mica (most commonly muscovite or biotite) and the sandy layers are changed to quartzite.  The minerals form in a preferred direction- especially the mica, which will be perpendicular to the direction of greatest pressure.  Schist can be recognized by the abundance of shiny mica.
With more heat and pressure, the minerals begin to segregate into bands of light and dark material.  The alignment of minerals causes foliation, or metamorphic layering, which may or may not be the same as the original sedimantary layering in the rock.
The folding of the layers makes it obvious that the rock has been subjected to some intense pressure.  The thick white band in the lower center is probably quartz, again indicating heat and pressure.
Another possibility is metamorphosed muddy limestones, with the white bands being marble segregations- but to me the bottom of the specimen looks micaceous, which suggest schist or gneiss.
 

Offline RD

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« Reply #2 on: 03/09/2008 17:52:03 »
The light stripes were originally the skeletons of microscopic creatures (algae), sinking to the bottom of ocean or lake.
Changes in the concentration of these creatures over time has produced different thicknesses of white stripes,
 e.g. in response to cyclical changes in climate.
The thicker brown (sand) stripes could represent longer periods when the algae were absent,
 or cataclysmic events when more sand than usual was deposited, e.g. tsunami. 
« Last Edit: 03/09/2008 17:56:21 by RD »
 

Offline LeeE

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What is This And Why All The Layers ?
« Reply #3 on: 03/09/2008 18:49:18 »
It's a bit of prehistoric sea-side rock (candy) dating from the time before the wheel and other round things were invented, and before they learned how to put the name of the town through the middle.
 

Offline neilep

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What is This And Why All The Layers ?
« Reply #4 on: 03/09/2008 21:02:45 »
What the schist?

Oh, sorry, gneiss rock specimen.

The layers are formed in a metamorphic rock, probably a schist.  Original rock (and layering) probably shale with sandy layers.  As the rock is heated and subjected to intense pressure, the minerals are changed, or metamorphosed.  In a schist, the clay minerals that make up the original mudstone are changed into mica (most commonly muscovite or biotite) and the sandy layers are changed to quartzite.  The minerals form in a preferred direction- especially the mica, which will be perpendicular to the direction of greatest pressure.  Schist can be recognized by the abundance of shiny mica.
With more heat and pressure, the minerals begin to segregate into bands of light and dark material.  The alignment of minerals causes foliation, or metamorphic layering, which may or may not be the same as the original sedimantary layering in the rock.
The folding of the layers makes it obvious that the rock has been subjected to some intense pressure.  The thick white band in the lower center is probably quartz, again indicating heat and pressure.
Another possibility is metamorphosed muddy limestones, with the white bands being marble segregations- but to me the bottom of the specimen looks micaceous, which suggest schist or gneiss.



Woooo !!

Many many thanks indeed for all this wonderful information.

So much more knowedgebale than that other geologist here, who, lets face it, is probably as old as the rock itself !

Thanks Bass, I appreciate your detailed information.
 

Offline neilep

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What is This And Why All The Layers ?
« Reply #5 on: 03/09/2008 21:08:40 »
The light stripes were originally the skeletons of microscopic creatures (algae), sinking to the bottom of ocean or lake.
Changes in the concentration of these creatures over time has produced different thicknesses of white stripes,
 e.g. in response to cyclical changes in climate.
The thicker brown (sand) stripes could represent longer periods when the algae were absent,
 or cataclysmic events when more sand than usual was deposited, e.g. tsunami. 

Wonderful. First rate... many thanks RD

Very interesting regarding the skeletal algae and the lack of defining the darker stripes.

Thanks for the link too.

 

Offline Karen W.

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« Reply #6 on: 03/09/2008 21:09:34 »
WOW I would have said Petrified wood.. how does one tell the difference Bass?
 

Offline neilep

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« Reply #7 on: 03/09/2008 21:10:08 »
It's a bit of prehistoric sea-side rock (candy) dating from the time before the wheel and other round things were invented, and before they learned how to put the name of the town through the middle.


Well...I'm convinced !..Finally someone who makes some real sense !

It's obvious now I think about it !

thanks LeeE
 

Offline Bass

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« Reply #8 on: 04/09/2008 00:31:22 »
WOW I would have said Petrified wood.. how does one tell the difference Bass?

Certainly looks like wood- and seeing a 2 dimensional picture makes it difficult to tell. 

Petrified wood is wood that has been replaced by silica, usually found in volcanic areas.  (wood can also be replaced by other minerials- but this is much less common)  The original texture of the wood is preserved, often including rough bark.  Tree rings rarely end, since the entire tree grows- but notice how several of the layers are discontinuous.  Also note on the upper end of the specimen that the white layers appear to be harder and more resistant that the dark layers- usually petrified wood is uniform (that is, it doesn't just replace select thin layers)

I suspect the layers are made of different material.  Notice how the layers are folded, the folds have sharp angular hinges (we call them chevrons) where they are smallest, and become more rounded as you move outward.  Take a small stack of paper and push the edges toward each other while also pushing down on the stack (you may need help for this), and you will develop the same sort of pattern.  More evidence for metamorphism and mineral migration is the concentration of dark minerals in the large fold hinge- as the layers fold, the more competent layers (white) don't deform as easily as the softer dark layers, creating a localized low pressure zone in the hinge, which allows the darker minerals (maybe biotite) to migrate there.

That the rock breaks so readily along the layers, instead of across the layers, suggests it is metamorphic.  Most petrified wood breaks across layers, since the silica is uniform.  This specimen also somewhat resembles stromatolites (algal beds), but they also tend to break across layers.

Lastly, the surface of the bottom of the specimen looks like mica (but can't tell for sure)- layered mica like this only forms in metamorphic rocks.
 

Offline JimBob

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« Reply #9 on: 04/09/2008 03:18:21 »


And now for the real answer. Bass, don't bow to pressure from the rabble. You were right the first time. The chevron folding would not occur unless it were a metamorphic banded gneiss. Neither would the highly deformed area at the bottom to which dashed line leads. That is just too complex of an area of deformation to be of any other origin except tectonic folding. To "cement" the deal, the heavily contorted area at the bottom of the box would not exist if it were sedimentary in origin or petrified wood. Petrified wood occurs in a softer matrix from which it can be relatively easily eroded. Sediments (before becoming metamorphosed) would just become mush.

To support Bass's definitive explanation of chevron folding I have indicated the axis of the chevron with a dashed line and added an arrow for those who have trouble finding dotted lines (and/or boxes.)
 

Offline Karen W.

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« Reply #10 on: 04/09/2008 07:28:27 »
That is very interesting thanks for explaining the differences and how to tell... I would never have known that.. I thought it was wood with a knot in it! Cool explanations Both of you.. Thanks Bass and JimBob!
 

Offline JimBob

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« Reply #11 on: 04/09/2008 21:43:15 »
Addendum for Bass:

gneiss
(nies)  n.
                  1.  a metamorphic rock, generally made up of
                       bands that differ in color and
                       composition, some bands being rich in
                       feldspar and quartz, others rich in
                       hornblende or mica.
             [1750-60; < G Gneis, ult. der. of OHG gneisto spark]
   Derived words
             --gneiss'ic, gneiss'oid, adj.

___________________________________________

schist (shist)  n.
                  1.  any of a class of crystalline metamorphic
                       rocks whose constituent mineral grains
                       have a more or less parallel or foliated
                       arrangement.
             [1775-85; < NL schistus, L (lapis) schistos < Gk
             schistós divided, curdled, divisible, der. of
             schízein to split, with -tos adj. suffix]
________________________________________________________

It is normal terminology that schist be assigned to metamorphic rocks that are more uniform in nature than the one that is the subject of this discussion. in general, the crystalline structure is visible with the naked eye. Banding is a very good possibility but is not clear-cut or well defined.

A gneiss, on the other hand, whill have distinct banding and the layers are well differentiated.
_______________________________________

It is really difficult assuming the role of the geological martyr that I seem to be cast into at times. Ah, the responsibilities of age and wisdom.


 

Offline JimBob

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« Reply #12 on: 04/09/2008 21:46:38 »
Having said the above, a gneiss can belong to the greater group of schistoid metamorphic rocks.
 

Offline Bass

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« Reply #13 on: 05/09/2008 22:09:48 »
Addendum for Bass:

gneiss
(nies)  n.
                  1.  a metamorphic rock, generally made up of
                       bands that differ in color and
                       composition, some bands being rich in
                       feldspar and quartz, others rich in
                       hornblende or mica.
             [1750-60; < G Gneis, ult. der. of OHG gneisto spark]
   Derived words
             --gneiss'ic, gneiss'oid, adj.

___________________________________________

From The Glossary of Geologic Terms:

Gneiss:
A coarse-grained regional metamorphic rock that shows compositional banding and parallel alignment of minerals.

(emphasis added)

Coarse grained, JimBob?  You might want to schedule an appointment with your optometrist. ;D

Based on the photo, I would still favor schist (on the verge of becoming gneissic):
       1.  Breaks along well developed micaceous layer
       2.  No visible crystals in laminations- suspect that laminations are original texture instead of chemical segregations of minerals

Quote
It is really difficult assuming the role of the geological martyr that I seem to be cast into at times. Ah, the responsibilities of age and wisdom.

Who said anything about wisdom?
 

Offline JimBob

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« Reply #14 on: 06/09/2008 03:05:38 »
Duh - I don't no

Besides, anyone who thinks that rocks with a hardness of more than 6 are, in general, the only type of rock worth looking are already unteachable.

BUT ...


Geology.com Gneiss:
A coarse-grained, foliated rock produced by regional metamorphism. The mineral grains within gneiss are elongated due to pressure and the rock has a compositional banding due to chemical activity.

Iowa State University - gneiss
A coarse, foliated metamorphic rock in which bands of granular minerals (commonly quartz and feldspars) alternate with bands of flaky or elongate minerals (e.g., micas, pyroxenes). Generally less than 50% of the minerals are aligned in a parallel orientation.

gneiss - GeologyLink Glossary -- http://college.cengage.com/geology/resources/geologylink/glossary/g.html
    A coarse-grained, foliated metamorphic rock marked by bands of light-colored minerals such as quartz and feldspar that alternate with bands of dark-colored minerals. This alternation develops through metamorphic differentiation.


Iowa State - schist A strongly foliated, coarsely crystalline metamorphic rock, produced during regional metamorphism, that can readily be split into slabs or flakes because more than 50% of its mineral grains are parallel to each other.

schist - GeologyLink Glossary
    A coarse-grained, strongly foliated metamorphic rock that develops from phyllite and splits easily into flat, parallel slabs ...

Wikipedia


Gneiss - Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes ...

SCHIST The schists form a group of medium-grade metamorphic rocks ...

NOW, BASS

If you can show that the rock that started this discussion was the result of high-grade regional metamorphic processes, resulting in metamorphic differentiation, I'll be very glad to cede the point.

« Last Edit: 06/09/2008 15:41:48 by JimBob »
 

Offline Bass

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« Reply #15 on: 07/09/2008 03:23:17 »
JimBob, when you get done at the optometrist, schedule an appointment with a neurologist to check your short term memory!



Based on the photo, I would still favor schist (on the verge of becoming gneissic):
       1.  Breaks along well developed micaceous layer
       2.  No visible crystals in laminations- suspect that laminations are original texture instead of chemical segregations of minerals

Having always called this rock schist, I'm happy to accept your concession that it is not gneiss.  Though I am somewhat concerned by your sudden flip-flop.   [:0]    I've got it, maybe you're running for public office JimBob??

Gneiss - Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes ...

SCHIST The schists form a group of medium-grade metamorphic rocks ...

NOW, BASS

If you can show that the rock that started this discussion was the result of high-grade regional metamorphic processes, resulting in metamorphic differentiation, I'll be very glad to cede the point.
 

Offline JimBob

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« Reply #16 on: 07/09/2008 16:26:21 »
And I will not raise your taxes!
 

Offline Bass

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« Reply #17 on: 12/09/2008 00:35:42 »
While out looking at a worthless prospect the other day, I came across a whole mountain of schist similar to Neil's rock (the mountain is being delivered to Neil on Tuesday, COD of course).

This is Prichard formation, laminated muds and sands that have been metamorphosed into schist.  (Note the fine faux-leather red vinyl background)

« Last Edit: 12/09/2008 00:37:14 by Bass »
 

Offline JimBob

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« Reply #18 on: 12/09/2008 02:32:41 »
(Must be his living room set. Redneck, you know)
 

Offline Bass

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« Reply #19 on: 12/09/2008 03:09:00 »
Hey!  I resemble that remark.
 

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