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Author Topic:   How do geologist know what they are looking at really is what they say it is?
jar
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Message 1 of 88 (790248)
08-28-2016 10:54 AM


I would like to suggest are slow baby step at a time look at geology.

How do geologists really know what a sample is and how it came to be?

Too often we seem to see very general terms used that then get qualified as a different term but without an explanation of what the differences are and why they are made. I'd like to see if by my asking question those good folk who actually know what they are talking about can 'splain it to me.

Hopefully we can move slowly enough to avoid gross generalities but still keep things simply enough that even I can understand them.

The discussion should move from general terms like sedimentary rock to more specific points like the basic methods sedimentary rocks get produced; to how specific designations are identified and what original materials produce specific final types of rock.

First I like to explore just sedimentary rocks before attempting to identify igneous or metamorphic rocks.

To begin, are all sedimentary rocks produced the same way?

Please, let's go slow and until I understand something put off new questions.

Geology & the Great Flood maybe?


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jar
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Posts: 33957
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 5 of 88 (790266)
08-28-2016 4:27 PM
Reply to: Message 4 by edge
08-28-2016 4:03 PM


uniformitarianism is nice but...
...specifically are all sedimentary rocks produced the same way?

My Sister's Website: Rose Hill Studios My Website

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jar
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Posts: 33957
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 7 of 88 (790273)
08-28-2016 5:17 PM
Reply to: Message 6 by edge
08-28-2016 4:57 PM


great start.
How can I tell if a sample is clastic instead of chemical or biological?

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jar
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Posts: 33957
From: Texas!!
Joined: 04-20-2004
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Message 9 of 88 (790277)
08-28-2016 10:45 PM
Reply to: Message 8 by edge
08-28-2016 10:12 PM


fragments from older rocks
edge writes:

You would look at the grains that make up the rock. Clastic rocks would consist of rock and mineral fragments from older rocks.

I have several question but the first is about "rock and mineral fragments from older rocks".

From that I get a sense that the first thing needed is older rocks of most any kind that get weathered and eroded and that material forms the basic components of clastic sedimentary rocks.

Is that correct?


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jar
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Posts: 33957
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 15 of 88 (790296)
08-29-2016 8:39 AM


can we hold on coal for just a little while.
Before we move on to the biological sedimentary rocks can we spend a little more time at clastics? I know I'm slow but I still have a few more questions about identifying clastics before we move on to chemical and biological sedimentary rocks.

Hopefully we will get there though.


My Sister's Website: Rose Hill Studios My Website

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jar
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Posts: 33957
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 16 of 88 (790298)
08-29-2016 8:57 AM
Reply to: Message 10 by Dr Adequate
08-29-2016 1:26 AM


Re: fragments from older rocks
I'm glad you mentioned mud.

So clastic sedimentary rocks begin as other rocks and before they can exist there needs to be time enough to first weather and erode other rocks to make the small rock and mineral fragments that have been eroded and transported to a depositional center that edge mentions in Message 6.

That brings up a couple other points.

First it seems from what edge said we need two processes. We need weathering and erosion and then the pieces parts need to be transported to some spot where the stop and accumulate. To end up as a layer in the geological column of a given location the pieces parts need to stay in that location long enough for the individual pieces parts to turn back into another big rock.

And back towards mud. It seems that the size of pieces parts also plays some part in determining what the final clastic rock will become.

Is that correct and if so what types of clastic rocks are made from the different sized pieces parts?

How can someone tell the various resulting rocks apart?


My Sister's Website: Rose Hill Studios My Website

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jar
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From: Texas!!
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Member Rating: 2.3


Message 20 of 88 (790319)
08-29-2016 11:24 AM
Reply to: Message 19 by edge
08-29-2016 11:06 AM


Great so far but I'm slow so humor me by expanding some.
edge writes:

Gravel ---> conglomerate
Sand ----> sandstone
Silt ----> siltstone
Mud ----> mudstone
Clay ----> claystone

...

We can make it more difficult by giving them modifiers like 'calcareous' or 'organic'; or we can combine the terms to things like 'sandy carbonaceous mudstone' (which may not actually exist, but you get the idea).

I think so but as usual, a few questions. I've seen things described as mudstone and siltstone but also as shale.

Particle size is pretty clear but what produces the different sized particles. Why does something end up as silt or mud or clay?

Where does shale fit in? What is it and why is it different? And then there is slate???????


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jar
Member
Posts: 33957
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


(1)
Message 24 of 88 (790325)
08-29-2016 11:56 AM
Reply to: Message 23 by edge
08-29-2016 11:48 AM


Re: Great so far but I'm slow so humor me by expanding some.
edge writes:

Yes, shale is defined as a mudstone with a lot of irregular partings due to bedding.

Okay but jargon alert. Help please. What does irregular partings mean and what does bedding mean?


My Sister's Website: Rose Hill Studios My Website

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jar
Member
Posts: 33957
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 27 of 88 (790330)
08-29-2016 12:20 PM
Reply to: Message 26 by Dr Adequate
08-29-2016 12:07 PM


Re: Great so far but I'm slow so humor me by expanding some.
DrA writes:

Oh, I missed this. Slate is a metamorphic rock, and comes apart into layers because it has undergone foliation. To explain this, consider that the silicate minerals in a rock have shapes: they can come in strings or sheets. So when a rock is heated and compressed, these minerals get pushed so that they lie perpendicular to the direction of the compression.

So far we have discussed two kinds of rock that both are found to create layers, shale and slate. The later has undergone metamorphose and it was that process through crystal alignment that produced the layering.

Are the layering found is shale produced a different way?


My Sister's Website: Rose Hill Studios My Website

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jar
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Posts: 33957
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 29 of 88 (790332)
08-29-2016 12:54 PM
Reply to: Message 28 by Dr Adequate
08-29-2016 12:42 PM


great stuff so far, now three sheets to the wind.
That makes sense.

So you are telling us that sometimes more than one process contributes to create a final sample? Is that correct?

And would the form of things like mica and asbestos have similar origins?


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jar
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Posts: 33957
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 34 of 88 (790351)
08-29-2016 4:16 PM


so short summary so far.
For clastic sedimentary rocks to form there first need to be other rocks and those prior rocks need to get weathered and eroded into small pieces parts. The final material will be determined by what the original material was and particles size as well as crystalline structure. Some materials will also change form should they undergo metamorphose.

Back in Message 6 edge mentioned a second family of processes, chemical sediments that precipitate out of water such as chert, or evaporites, or travertine, or various types of iron formations. I imagine that for things to precipitate out they must first be suspended in water and so once again they must exist before they can be precipitated out.

Is that correct?

What is chert and travertine.

Would limestone and chalk fall in the clastic category?

Are the mineral salts examples of precipitates?

AbE:

edge's reply while I was writing this adds another question.

What is the difference between lithification and metamorphism.

Edited by jar, : see AbE:


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jar
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Posts: 33957
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 36 of 88 (790374)
08-29-2016 6:46 PM
Reply to: Message 35 by Dr Adequate
08-29-2016 6:32 PM


Re: so short summary so far.
DrA writes:

Lithification is the process of turning into stone; metamorphosis changes the properties of a stone.

Thanks. So neither term actually says how hard the stone will be or anything else specific unless the exact materials are also specified?


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jar
Member
Posts: 33957
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 41 of 88 (790448)
08-30-2016 11:50 AM


What are examples of biological rocks and how are they identified?
Back in Message 1 edge mentions a third type of sedimentary rocks and that was accumulations of biological materials such as coral reefs coal and other types of bioherms.

What are bioherms and how are they identified?

What are other examples of biological material rocks?


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jar
Member
Posts: 33957
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 42 of 88 (790525)
08-31-2016 9:56 AM
Reply to: Message 37 by Dr Adequate
08-29-2016 7:23 PM


Return to lithification for a moment.
So from what has been said lithification is basically compaction and cementation.

From that I would guess that the deeper something is buried the greater the compaction. Also the longer something is buried the greater the compaction and cementation.

For cementation though I assume what is needed would be water and minerals leached from the overburden. Is that correct?


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jar
Member
Posts: 33957
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.3


Message 46 of 88 (790619)
09-01-2016 7:49 PM
Reply to: Message 45 by edge
08-31-2016 6:43 PM


Re: What are examples of biological rocks and how are they identified?
Good info. Can limestone and similar products can also get incorporated into clastic sediments?

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