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Author Topic: Geology Question of the Week  (Read 166946 times)

Offline Bass

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Re: Geology Question of the Week
« Reply #50 on: 13/01/2006 05:08:05 »
Rocks can develop planar features for several reasons- the most common are bedding planes.  The next most common is pictured here- in the top photo, bedding is obvious and aligned with the blue pencil, the second planar feature cuts across the bedding and is aligned with the black pencil.  In the bottom photo, bedding is the crinkled colored layers while the second planar feature is almost vertical (aligned with knife):




GQOTW:  Name this planar feature?

Bonus:  How does this planar feature form?  What does it tell us about the history of the rock?


Subduction causes orogeny.
 

Offline Bass

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Re: Geology Question of the Week
« Reply #51 on: 22/01/2006 01:46:06 »
Hint:

Wonderbra -
what most men first notice about a woman wearing a wonderbra.

Subduction causes orogeny.
 

Offline Bass

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Re: Geology Question of the Week
« Reply #52 on: 17/02/2006 22:33:21 »
Cleavage!

More commonly called "rock cleavage" or more properly called either "axial planar cleavage" or "foliation".

Occurs in metamorphic rocks that have been subjected to pressure.  The pressure and heat of metamorphism change the clay minerals in sedimentary rocks into mica (several varieties).  Mica has a distict planar orientation, and it grows in the direction of least pressure- which will be perpendicular to the stress field.



Commonly, bedding planes will fold during metamorphism, also perpendicular to the greatest stress- so the foliation ends up being in the axial plane of the fold, hence "axial planar cleavage".  The amount and orientation of foliation allows the observer to figure out the relative intensity of metamorphism and gives clues as to the true orientation of the original beds.

Subduction causes orogeny.
 

Offline Ray hinton

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Re: Geology Question of the Week
« Reply #53 on: 24/02/2006 12:15:18 »
quote:
Wonderbra -
what most men first notice about a woman wearing a wonderbra.

their not very well blessed ?

RE-HAB IS FOR QUITTERS.
 

Offline JimBob

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Re: Geology Question of the Week
« Reply #54 on: 05/03/2006 01:53:30 »
quote:
Originally posted by Bass



Maybe this belongs in the "Science Photo of the Week" thread, but the reference to cross-beds in this thread made it relevant.

This photo from the Mars Rover Opportunity near the edge of Erebus Crater shows good evidence for the historical presence of water on Mars- note the "festoons" or cross-laminations (curved upward layers) that indicate water-formed ripples.
http://www.spaceref.com/news/viewsr.html?pid=19212


Subduction causes orogeny.



Bass, may I politely disagree. I concur that this is evidence for water on Mars, but the structures to me look to be a series of seasonal dessication cracks that are stacked one upon another. To bad we can't get to the outcrop to determine the accuracy of the interpretation.

And orogeny causes uplift

 

Offline Ophiolite

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Re: Geology Question of the Week
« Reply #55 on: 05/03/2006 02:29:56 »
Interesting. I am seeing a cross section through a finely laminated sequence; no cross bedding; no dessication cracks. It is good to know the art of geological equivocation is still alive and well.
 

Offline JimBob

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Re: Geology Question of the Week
« Reply #56 on: 06/03/2006 01:22:46 »


Alright Ophialite (rock or snake?).

The structure just off of the end of the arrow is not a cleavage related feature. It lacks one side of being a closed square. This suggest dessication to me. There are traces of the same type of feature and one whith a chip "wedged" between the to sides of a upturned bed. But I am not going to make any more of this as we will never know for sure.

What bothers me more is the origin of the pellitoidal thingies that are all over the place. Are they desert, wind-generated concreations? Can you think of another explinaiton? Weatherd out of another rock? Or other?  

I am open to all as I know the laminated are faily well explained.

Jim
 

Offline Ophiolite

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Re: Geology Question of the Week
« Reply #57 on: 06/03/2006 02:57:03 »
[Ophiolite - ocean rocks welded to continental masses by subduction]
If you will imagine swinging the arrow around in a clockwise direction, till it is vertical, you will find it ends in a narrow column of rock, replete with laminations, slightly displaced from the adjacent blocks. I see post depositional collapse structures. Nothing more. However, you are correct: from this single photograph we are unlikely to reach a firm conclusion.

The small spherules I took to be blueberries, which were observed early on by both (?) rovers. These may be formed as concretion within the rock as a result of groundwater activity, then released by normal erosional processes. For example: http://www.spaceflightnow.com/news/n0406/16blueberries/
 

Offline JimBob

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Re: Geology Question of the Week
« Reply #58 on: 06/03/2006 17:12:43 »
I can go along with collapse structurs. Also, the hematite concreation hypothisis was first on my list. I dared not hope for oolites eroded from a less dense carbonate. And I was taught that ophiolte was a mostly mafic igneous rock with a little metamophic thrown in that is metamorphosed in a subduction that probably failed - example: the highlands of Cuba. I am a rock pounder as well, but of a much ealier vintage.

Good to know I am not alone here.

Jim
 

Offline Bass

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Re: Geology Question of the Week
« Reply #59 on: 08/03/2006 20:52:35 »
Since JimBob brought it up-

GQOTW:  What are oolites and how do they form?

Subduction causes orogeny.
 

Offline Ophiolite

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Re: Geology Question of the Week
« Reply #60 on: 08/03/2006 23:10:41 »
Oolites are a component of certain sedimentary rocks. However, I view everything Cambrian and later as superficial drift deposits, so I choose not to answer. Basalts are real rocks, so are eclogites.

JimBob, I will be marginally surprised if you are an earlier vintage than I. For one thing I can remember how warm the summers were in the late Cretaceous! Class of '70. Yourself?
 

Offline JimBob

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Re: Geology Question of the Week
« Reply #61 on: 09/03/2006 01:46:02 »
'66

If I only had a little humility, I'd be perfect.
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Offline Ophiolite

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Re: Geology Question of the Week
« Reply #62 on: 09/03/2006 02:41:44 »
Damn. You don't live in Texas do you? Or, ever worked for Conoco?
 

Offline JimBob

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Re: Geology Question of the Week
« Reply #63 on: 09/03/2006 02:54:21 »
I do. Never worked for Conoco but had three partners that did. Timko, Lindah, Schweirkert. Obviously, working with them I lived in Houston (ICH!) for 12 years until the worldwide consuling job in my hometown came up.

If I only had a little humility, I'd be perfect.
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Offline Bass

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Re: Geology Question of the Week
« Reply #64 on: 22/03/2006 18:11:29 »
Picture of oolites




Subduction causes orogeny.
 

Offline JimBob

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Re: Geology Question of the Week
« Reply #65 on: 22/03/2006 23:32:52 »
Beautiful picture, Bass. Looks as if it cold have come from the broken well bore core obtained from the Smackover Limestone of the Gulf Coast of the US. Is the limestone pictured Upper Jurassic? If so it would be the ABOUT the same age as the Smackover. Below the Smackover is the mainly aeolian, arkosic Norphlet Sand that is then underlain by the Louann "Salt". This is a series of mixed evaporites (the core sitting on my desk is mostly gypsum) that was deposited during the opening of the Gulf basin.

I am intersted to see how this correlates.


If I only had a little humility, I'd be perfect.
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Offline Bass

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Re: Geology Question of the Week
« Reply #66 on: 23/03/2006 21:34:28 »
Upper Jurassic Portland Group.  The photo is not mine, but I remembered the distinctive oolites from a field trip eons ago.

Subduction causes orogeny.
 

Offline The Silurian Prince

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Re: Geology Question of the Week
« Reply #67 on: 19/04/2006 04:09:41 »
The red is oxidation.  The green is likely evaporite deposits from a hypersaline brine.  Likely gypsum or something like that.  This would be a periodic deposition process that produces the green layers you see in these red bodies. Not to sure about the decomposing plants idea.  Sounds a bit far fetched.  The environment was likely pretty dry.

Enjoy diversity.
 

Offline lovelesh

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Re: Geology Question of the Week
« Reply #68 on: 02/05/2006 12:54:26 »
Red Sandstone fairly formed in the oxidation environment. It contains Iron oxide and probably comming out from the chemical weathering of Basic o Ultrabasic rocks. This type of rocks form in backarc basins.
Green colour representing Glauconite.
In India Vindhyan sandstone of Central province have both characteristics

lovelesh
 

Offline Bass

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Re: Geology Question of the Week
« Reply #69 on: 26/07/2006 18:21:48 »
What is a "nick point"?


Subduction causes orogeny.
 

Offline JimBob

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Re: Geology Question of the Week
« Reply #70 on: 27/07/2006 16:23:31 »
According to the AGI Glossary of Geology ...

No, that is cheating. I am above that (right [}:)])



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Offline Bass

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Re: Geology Question of the Week
« Reply #71 on: 26/07/2006 18:21:48 »
What is a "nick point"?


Subduction causes orogeny.
 

Offline JimBob

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Re: Geology Question of the Week
« Reply #72 on: 27/07/2006 16:23:31 »
According to the AGI Glossary of Geology ...

No, that is cheating. I am above that (right [}:)])



The mind is like a parachute. It works best when open.  -- A. Einstein
 

Offline Bass

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Re: Geology Question of the Week
« Reply #73 on: 18/08/2006 05:24:07 »
Here's a hint


Subduction causes orogeny.
 

Offline JimBob

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Re: Geology Question of the Week
« Reply #74 on: 21/08/2006 15:03:18 »
Magnificent picture, Bass. I would like to be there now - temp was 103 F yesterday and more of the sasme today.

Since no one else seems inclinded to answere this, let's keep it amoung ourselves, the pros.

A nick point in geology is derived from the same term in mathematics that denotes the single point where a curve changes slope abruptly.

This term is also applied to the gradient of a stream. If streams flowed over only one homogenious substratum, the gradient curve for the stream would be steep at the head of the stream and shallow at the mouth of the stream. This can occur over a very short distance within a stream's profile.

BUT, since there is a distance longer than a few hundred or thousands of feet in a streams, the substratum varies from soft to hard, ususally by sharp geologic contatcts. (The soft-hard rock interface is relative to each other.)

As pictured, the hard up-stream rock is in contact with a softer down-stream rock and a water fall is produced. The top of the waterfall is a nick point - the inflection point in the stream's gradient.

Streams can go from softer rock formations to harder rick formations: these gradient changes often result in rapids.




The mind is like a parachute. It works best when open.  -- A. Einstein
 

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Re: Geology Question of the Week
« Reply #74 on: 21/08/2006 15:03:18 »

 

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